July 30, 2024

Barometer and Radio Altitude Settings (EFIS) - How To Use Them

July 30, 2024/ Flaps2Approach

Topolev TU 154 M (https://commons.wikimedia.org/w/index.php?title=File:Polish_Air_Force_Tupolev_Tu-154M_Lebeda.jpg&oldid=485072709)

On 10 April 2010, a Tupolev Tu-154 aircraft operating Polish Air Force Flight 101 crashed near the Russian city of Smolensk, killing all 96 people on board.

The pilots were attempting to land at Smolensk North Airport — a former military airbase — in thick fog, with visibility reduced to about 500 metres (1,600 ft). The aircraft descended far below the normal approach path until it struck trees, rolled, inverted and crashed into the ground, coming to rest in a wooded area a short distance from the runway.

The terrain on approach to Smolensk airport is uneven and locally much lower than the runway level and the use of the radio altimeter would be unusual in such a location. Notwithstanding, as the aircraft approached 300 metres (980 ft), the navigator began calling out the radar altimeter's reading.

This is not standard practice for a non-precision approach, as the radio altimeter does not take into account the contour of the terrain around the airport. Standard practice would entail calling out the readings on the pressure altimeter, which is set according to atmospheric pressure and thereby references to the elevation of the actual airport.

Whatever the cause for the accident, the use of the radio altimeter was not considered to be standard practice and its use no doubt contributed to the confusion leading up to the accident.

This snippet has been used in part from a more in-depth article from Wikipedia (copyright @ Wikipedia).

737-800 electronic Flight Information System (EFIS) panel

In this article, I will explain the functionality and operation of the two rotary knobs located on the Electronic Flight Information System (EFIS) panel.

The knob on the left is the minimums selector that enables the minimums to be changed to either barometric altitude (BARO) or radio altitude (RA) . The knob on the right is the barometric reference selector that enables the barometer to be set either in inches (imperial) or hectopascals (metric).

Each knob comprises an outer and inner knob.

The outer knobs are selectors (left or right) and the inner knobs are spring-loaded, and when rotated and released, self-center with the descriptor label resetting to the horizontal position. Each of the two inner knobs can be pressed to either reset the minimums, or to change the barometer setting from QNH to standard (STD) and vice versa.

The inner knobs have two speeds: a slight turn left or right will alter the single digits, while holding the encoder left or right for a longer period of time will change the double digits, and cause the digits to change at a higher rate of speed.

Both selectors display their respective readings on the Primary Flight Display (PFD) on the Captain and First-officer side.

The radio barometric and radio altitude are sometimes referred to as the barometric altimeter and radio altimeter.

Important Definitions

Before going into greater detail, it is important to understand some terminology prior to using the barometric reference selector, in particular the terms: QNH, QFE, STD, transition altitude and transition level.

QNH and QFE

QNH and QFE are not recognised aviation acronyms, although the later is sometimes referred to as ‘field elevation’. The Q-codes were developed by the British Government immediately after the First World War to enable aviators at the time to set their altimeters against a specific reference. This ensured that all aircraft were flying at a specific altitude in relation to each other (when flying at or above a specified height from the ground).

QNH is the altimeter setting that corresponds to mean sea level (MSL) at the location that the pressure has been recorded. Therefore, if you landed on the ocean the altimeter will read zero. If QNH is set to the surrounding air pressure, the aircraft’s altimeter will read zero (or near to) on the runway (unless the runway is located below sea level. For example, Rotterdam Airport (EHRD) and Schiphol Airport Amsterdam (EHAM)).

QFE on the other hand, is the surface pressure at a set reference point (airport that you are landing or departing from). With the barometric setting set to this pressure, the aircraft’s altimeter will read zero, and at other altitudes will read the height above airfield elevation. However, it must be stressed that this barometric setting will only be accurate for that specific location (and time). If the location or pressure changes, then the setting will be incorrect.

For the most part, airline operations always use QNH and some airlines ban the use of QFE.

STD

STD is an acronym for standard pressure (also known as standard altimeter setting (SAS) and is the internationally recognised air pressure that all aircraft must use when reaching a predefined altitude. Using STD sets the aircraft’s altimeter to a pressure based on a set datum, in this case 29.92 in/1013.2 hPa (this being the air pressure at sea level in the International Standard Atmosphere (ISA)). This ensures adequate separation between aircraft as all aircraft have the same pressure set on their altimeters. Failure to reset the barometer to STD at the transition altitude/level will cause the information that is sent to the altimeter to be incorrect.

Transition Altitude

Transition altitude is a ‘fixed’ altitude used when an aircraft is departing an airport and climbing. Transition altitude is the highest altitude that an aircraft can fly with QNH set. Below the transition altitude the altimeter should be set to QNH and this setting should be changed to STD (standard pressure) when the aircraft reaches transition altitude. The STD pressure is 29.92 in/1013.2 hPa.

When an aircraft reaches the transition altitude, the altitude is referred to as a flight level (FL).

At and above the transition altitude, the local pressure has no bearing, and importance is placed upon each aircraft flying with the same barometer reference datum.

The transition altitude will differ from region to region and country to country. In Australia it is 10,000 feet, in parts of Asia 11,000 feet, and in the US 18,000 feet. In some parts of Europe the altitude changes again, and in England the transition altitude is 3,000 feet. Then again, in certain countries in Latin America it depends on terminal airspace.

The transition altitude is pre-selected from the Control Display Unit (Perf Init (1/2)/ Trans Alt).

Transition Level

Transition level is the opposite of transition altitude, and is used for aircraft descending to arrive at an airport. It occurs during the descent and is the lowest altitude that an aircraft can fly having standard pressure (STD) set. When the aircraft reaches or travels below the transition level, the barometer is changed from STD to QNH.

The transition level is more often than not assigned to the aircraft by Air Traffic Control (ATC) and as such is a variable altitude level. This is because the pressure on any particular day will be different, and will not be a fixed value. Often ATC will assign a transition layer that is in between two altitudes (usually with a difference of 1000 feet).

If a transition level is not assigned by ATC, the ‘fixed’ transition altitude is used (fixed meaning the altitude that has been established for that particular country. For example, Australia is 10,000 feet).

Mnemonic

To avoid confusion a basic mnemonic can be used:

Transition Altitude (going up) = Ascent (letter A associates with Ascent).

Transition Level (going down) = Lower (letter L associates with lower or descent)

Important Points:

At transition altitude (going up) the barometer must be changed from QNH to STD.
At transition level (going down) the barometer must be changed from STD to QNH.

Minimums Reference Selector Knob (BARO/MINS/RA)

The selector knob establishes whether barometric pressure or radio altitude is used as a reference point for minimums. The selector knob has three functions:

The outer knob selects either barometric altitude or radio altitude.
The inner knob adjusts the barometric reference height or radio altitude.
By pressing the inner knob marked RST the following occurs:

The radio height alert is inhibited (call-out);
The radio altitude minimums alert display (displayed in the PFD in white) is blanked out; and,
The reference altitude marker on the altimeter (green carrot) is reset to zero.

If the inner knob is rotated left or right and held for longer than a few seconds, the speed that the digits change will increase to a higher speed (slew mode).

Radio altitude is the actual height that the aircraft is flying over the ground (terrain). The height is measured by a transducer located on the underside of the aircraft. This height will alter depending upon whether the aircraft flies over a small hill or shallow valley. The former will decrease the height while the later will increase the height.

When the selector knob is turned to RA and the inner knob rotated, the radio altitude display can be adjusted.

BARO

The barometric altitude measures the atmospheric pressure above sea level and converts this to a height above sea level. This height is then displayed in the PFD and on the altitude tape.

When the selector knob is turned to BARO and the inner knob rotated, the barometric pressure can be adjusted.

MINS

Minimums (MINS) refers to the minimum altitude (and visibility requirements) that must be met for a flight crew to land the aircraft safely. Minimums can vary based on several factors, including the type of approach, the specific airport, weather conditions, and the pilot's qualifications. A go-around is mandatory if the requirements stipulated for the approach type are not met by the time the aircraft reaches minimums.

A future article will discuss minimums and visibility requirements in more detail.

minimums alert display (baro / 6700) and green reference altitude marker (ProSim737)

RST button

The main use of the RST button is comparatively simple: it is to remove (blank out) the minimums alert display and reference altitude marker when minimums are not used; thereby, removing non-essential information displayed on the PFD. By pressing the RST button the Baro and RA displays are blanked out (removed).

Barometric Reference Selector Knob (IN/HPA)

The barometric reference selector knob changes the barometer altitude setting that is used by the avionics as a reference point. It has three functions:

The outer knob selects inches (IN) or hectopascals (HPA).
The inner knob enables the barometric altitude reference on the altitude tape to be changed.
If the inner knob marked STD is pressed, the preselected barometer reference can then be changed. Pressing the knob will display the letters STD on the PFD.
By pressing the inner knob marked STD, the following occurs:

The standard barometric setting of 29.92 in/1013.2 hPa is selected;
If STD has already been selected (and is displayed) it opens the lower window beneath STD to enable the barometric setting to be changed (STD will be coloured green and the reference characters will be displayed in white); or,
If there is no pre-selected barometric reference, the display will show the last value before STD was selected

The inner knob has two speeds: a slight turn left or right will alter the single digits, while holding the encoder left or right for a longer period of time will change the double digits, and cause the digits to change at a higher rate of speed (slew mode).

Important Point:

• Pressing the STD button switches between QNH and standard air pressure.

Colours

The barometer reference display on the PFD is displayed in one of three colours: green, white, and amber.

Green: The display will be coloured green when the aircraft is on the ground, or when STD has been pressed on the barometer reference selector.

White: When the inner knob of the barometer reference selector is pressed (STD), the reference characters (in the lower right window of the PFD) will be displayed in white.

Amber (boxed): Indicates the aircraft is climbing above the transition altitude, or if STD is displayed, the aircraft is descending below the transition altitude. Amber is a caution alert, and if displayed, action should be taken to rectify the situation by pressing the button marked STD on the barometer reference selector.

The altitude at which the amber caution is displayed is determined by the transition height that has been set in the CDU.

Safety Feature

By default the reference will always display (29.92 in/1013.2 hPa). This is a safety feature that has been designed into the system. If a random QNH setting was allowed, for example the last QNH used, there is a possibility that the crew will not notice the incorrect setting. A crew at the beginning of a flight tends to notice the 29.92 in/1013.2 hPa reading as it is what they expect to be displayed.

Which To Use – BARO or RA

It’s not unusual for trainee pilots to become confused concerning whether BARO or RA is used for minimums. I think much of this confusion is generated from web references which try to make the topic more in-depth to what it actually is. Certainly, the different approach types can be confusing, as can the various visibility requirements, but not when to use BARO or RA.

The decision to use barometric or radio altitude as a minimums reference is determined by the type of approach that is being flown, and the information published on the approach chart for the runway in question.

Radio Altitude (RA) is typically used for CAT II/III approaches and those that have a published RA stated on the approach chart (note that most CAT III subsets are flown with autoland).

With regard to CAT III approaches, where a specified failure occurs, the radio altitude is used to indicate the alert height. The alert height is the height above the runway at which an approach must be aborted and a missed approach initiated. The alert height for all Boeing aircraft is 200 feet AGL.

Barometric Altitude is used for CAT I and Non Precision Approaches (NPA). For example, GLS, ILS, IAN, VOR & RNAV approaches.

Simply stated, always use barometric altitude unless the minimums on the approach chart states to use radio altitude (RA).

Important Point:

Except for visual landings when minimums are not used, the minimums height, and whether it is BARO or RA, will be annotated on the approach chart for the approach type and airport. In some instances, at specific airports the airline may have a policy dictating whether BARO or RA is used. The pilot does not have a personal preference.

The below video, taken inside the flight deck of a 737-800 aircraft shows the operation of the barometric and radio altitude selector knobs.

Operation of Barometric and Radio Altitude selector knobs (OEM 737-800). Courtesy Shrike 200

Final Call

The correct use of the minimums and barometric reference selectors is important, in so far as their importance comes into being when landing in inclement weather, as demonstrated in the accident of Polish Air Force Flight 101.

The most important points are to consult the approach chart to determine whether BARO or RA is used, and to change the barometric pressure reading when the aircraft reaches transition altitude, and to remember that if a transition level has not been assigned by ATC, to use the established transition altitude for that particular country.

This article has been proof read for accuracy by a third party.

April 07, 2024

737-800 Landing Procedure

April 07, 2024/ Flaps2Approach

737-800 Transocean Air on finals Komatsu (RJNK) Japan © redlegsfan21 from Vandalia, OH, United States, JA8991 (24643740539), CC BY-SA 2.0

In this article I will discuss the techniques used to land the 737-800 aircraft.

The choice of landing approach is often influenced by considerations such as the specific criteria required for the approach, the desired level of automation, and the individual pilot's preference and technique. Regardless, the method used to actually land the aircraft is similar in all approach types.

The first part of the article discusses techniques used in the approach, descent and landing. This is followed by a short recap regarding situational awareness, which is critical in any approach and landing. At the end there is a downloadable step guide explaining the procedure to land the 737-800.

Discussing landing technique without addressing the approach is counter intuitive. As such, a generic style approach has been ‘loosely’ used to provide a frame of reference. Furthermore, in an effort to ensure clarity and provide sufficient context, certain information discussed in previous articles may have been reiterated. I purposely have not discussed the requirements for a specific approach type, nor have I included, or discussed detailed checklists.

I have attempted to include as much information as possible which, can have a tendency to make the subject appear complicated; it is not complicated. Carefully read the information and note that:

There is a considerable variability in how the 737 is flown. Certainly there are wrong ways to do things, however, there is no single right way to do it; and,
Airline policy often dictates how an approach is flown based on whether it is a Precision Approach or a Non Precision Approach.

Generally speaking, an approach can be segregated into three segments:

The initial approach;
The landing approach (descent phase); and,
The final approach (landing phase).

Discussion

Initial Approach

Technically, the approach starts when entering the traffic pattern, terminal airspace or at the Initial Approach Fix (IAF), which is published on the approach chart. However, not all approaches have an IAF, and some require that the airplane be vectored to the final approach course by Air Traffic Control. Even if there is an IAF, ATC may still decide to vector a plane to the final approach course to make more efficient use of airspace.

Prior to reaching the IAF, or receiving vectors to final, the flight crew should have prepared the aircraft for approach, briefed the crew, and begun to slow the aircraft. Workload increases considerably during the descent; therefore, it is sensible to complete whatever can be completed prior to the descent point. Descent planning and preparation is usually completed before the initial approach segment begins, which is approximately 25 miles from the runway.

Important Points:

Approach planning should be completed prior to the descent point; preferably completed before reaching the IAF.
In general, unless indicated otherwise, a flight crew will want the aircraft at approximately 3000 ft AGL no less than 10 NM from the runway.

Landing Approach

The landing approach begins at the Final Approach Fix (FAF). However, the terminology will differ depending upon the type of approach being flown. For the purposes of this article, I will use the term Final Approach Fix (FAF) to indicate the decent point.

Precision and non precision approaches will have the required descent point indicated on the approach chart, which will differ depending upon the approach type selected.

When reaching the FAF, the aircraft will in all probability be controlled by the autopilot with guidance being controlled by LNAV and VNAV (or another pitch/roll mode).

Depending on the type of approach chosen, the aircraft will be transitioning from level flight to either a step-down approach (SDA) or a continuous descent approach (CDA). Step-down approaches are rarely used today; continuous descent approaches are more the norm. A CDA, unless otherwise stated on the approach chart, uses a 3 degree glide path.

If you examine the two approach charts (click to enlarge) you will note that the VOR 06 approach shows the descent point at HERAI at 1455 ft AGL. The point is marked by a Maltese Cross and is also shown as the FAF (Final Approach Fix) in the distance legend. Also note that both a step down and a continuous approach is displayed on the chart. In the second chart (ILS 06) the descent point is shown as a LOC (localizer) at 1964 ft AGL and the FAF is noted in the distance legend. Note the chart is also annotated IF (Initial Fix). Different charts will display different annotations.

The reason for showing these two charts, is to demonstrate that the descent point and distance from the runway to begin the descent, will change depending upon the approach type selected from the FMC (assuming an approach from the FMC is used).

‘Loose’ Recommendation

As I have already mentioned, there are multiple ways to approach and land the 737; ask several pilots and each opinion will be slightly different. Generally speaking, without alternate guidance from Air Traffic Control or an approach chart, the following recommendations should be adhered to. The aircraft should begin descent to the runway at:

Approximately 10 NM from the runway;
At approximately 3000 ft AFE;
Have flaps 1 extended; and,
Be flying at as airspeed no greater than 200 kias.

If the aircraft is following the ILS approach course, it is better to intercept the ILS glideslope slightly from below rather than above. Intercepting the glideslope from below enables greater control of airspeed.

Speed Management

Speed management is probably the most critical factor during any approach. A common saying is ‘you have to slow down to get down’. This said, it is a bit of a conundrum. The airline wants its pilots to optimise the aircraft’s airspeed for as long as possible, because this means less fuel use, less noise, and lower engine operation times.

Slowing the 737-800 aircraft is not easy when the aircraft is descending, so it is a good idea to begin to reduce the airspeed when the aircraft is in level flight prior to beginning the descent. The thrust levers should be brought to idle (idle thrust or near to) and the airspeed allowed to decay to the flaps UP maneuvering speed. The flaps UP indication is displayed on the speed tape in the PFD. If speed reduction is initiated before reaching the IAF, the airspeed will decay naturally without use of the speedbrake.

Important Points:

It requires approximately 25 seconds and 2 NM to decelerate the 737-800 from 280 kias to 250 kias, and it will take a little longer decelerating from 250 kias to 210 kias. More simply written, it takes approximately 1 NM to decrease airspeed by 10 kias in level flight.
The aircraft should begin slowing at 15 NM from the airport to be at 10 NM at 3000 ft AFE at a speed of approximately 190-200 kias with flaps 1 extended.
The aircraft’s airspeed should be reduced to flaps UP maneuvering speed no later than the IAF.

Speedbrake and Flaps Use

The transition from level flight to descent will be much easier, with less need to use the speedbrake, if the aircraft is already at a lower airspeed prior to the descent. If the speedbrake must be used, try to minimise its use at and beyond flaps 5. With flaps 15 extended the speedbrake should be retracted. The speedbrake should not be used below 1000 ft AGL.

Although the speedbrake is designed to slow the aircraft, its use causes increased inside cabin buffeting and noise, decreases fuel efficiency, and can lead to unnecessary spooling of the engines; these factors are exacerbated if the aircraft is descending and travelling at a slower speed. If the speedbrake is to be used during the descent, lower the speedbrake (clean configuration) before adding thrust, otherwise thrust settings will need to be adjusted.

It must be stressed that using the flaps to slow down by creating more drag is not good technique and is frowned upon. Additionally, continual use of the flaps to slow an aircraft can cause damage to the flaps mechanism over a period of time - adhere to the flaps extension schedule (discussed shortly).

If the aircraft’s speed is too high and the approach is too fast, lowering the landing gear early is an excellent way to slow the aircraft, but bear in mind that this will also increase drag, generate noise, and increase fuel consumption. This should only be done as a last resort.

Important Points:

Whenever the speedbrake is used, the pilot flying should keep his hand on the speedbrake lever. This helps to prevent inadvertently leaving the speedbrake lever extended.
Flaps, in principle, are not designed to slow the aircraft (although their drag does, by default, slow the aircraft); the aircraft’s pitch, thrust, and the use of the speedbrake do this.

Flaps Extension Schedule

All to often novice virtual flyers do not adhere to the flaps extension schedule. Extending the flaps at the incorrect airspeed can cause high aircraft attitudes, unnecessary spooling of engines, excessive noise, and increased fuel consumption which can lead to an unstable approach. If the flaps are extended at the correct airspeed, the transition will be relatively smooth with minimal engine spooling.

The correct method to extend the flaps is to extend the next flaps increment when the airspeed passes through the previous flaps increment. For example, when the airspeed passes through the flaps 1 indication, displayed on the speed tape in the PFD, select flaps 2.

The 737 has 8 flap positions excluding flaps UP. It is not necessary to use all of them. Flight crews will often miss flaps 2 going from flaps 1 to flaps 5. Similarly, flaps 10 may not be extended going from flaps 5 directly to flaps 15 and flaps 25 maybe jumped over selecting flaps 30. Flaps 30 in the norm for most landings with flaps 40 being reserved for short-field landings or when there is minimum landing distance. In the case of using flaps 40, flaps 25 is normally extended.

My preference is to use flaps 25 as it makes the approach a little more stable. However, if you are conducting a delayed flaps approach, selecting flaps 25 may not give you enough time to extend flaps 30 or 40 and complete the landing checklist before transitioning below ~ 1500 feet AGL.

Flaps 40

The use of flaps 40 should not be underestimated, as aircraft roll out is significantly reduced and better visibility is afforded over the nose of the aircraft (because of a lower nose-up attitude). Because the landing point is more visible, some flight crews regularly use flaps 40 in low visibility approaches (CAT II & III). If the aircraft’s weight is high, the runway is wet, or there is a tailwind, flaps 40 is beneficial. A drawback to using flaps 40, however, is the very slow airspeed (less maneuverability) and higher thrust required. For this reason, if there are gusting winds it is better to use flaps 30.

Advantages

Less roll out;
Better visibility over the nose of the aircraft due to lower nose-up attitude;
Less wear and tear to brakes as the brakes are generating less heat (faster turn around times);
Less chance of a tail strike because of slightly lower nose-up attitude during flare;
More latent energy available for reverse thrust (see note); and,
Helpful when there is a tailwind, runway is wet, or aircraft weight is high.

Disadvantages

Increased fuel consumption (negligible unless flaps 40 are extended some distance from runway);
Increased drag equating to increased noise (flaps 40 generates ~10% additional thrust); and,
Less maneuvering ability.

NOTE: When the aircraft has flaps 40 extended, the drag is greater requiring a higher %N1 to maintain airspeed. This higher N1 takes longer to spool down when the thrust levers are brought to idle during the flare; this enables more energy to be initially transferred to reverse thrust. Therefore, during a flaps 40 landing more energy is available to be directed to reverse thrust, as opposed to a flaps 30 landing.

Important Point:

Correct management of the flaps is selecting the next lower speed as the additional drag of the flaps begins to take effect.

**TABLE 1:** Flaps Extension Table. The table does not include flaps 2, 10 & 25. © JAL-V

Maneuvering Margin

The maneuvering margin refers to the airspeed safety envelope in which the aircraft can be easily maneuvered. This is pertinent during descent, as when the aircraft slows down its ability to maneuver is less than optimal. An adequate margin of safety exists when the airspeed is at, or slightly above the speed required with the flaps extended. This is displayed as a white carrot in the speed tape in the PFD.

Procedure Turns

A procedure turn (PT) is a maneuver to perform a course reversal to establish the aircraft inbound on an intermediate or final approach course. They are often used when flying a VOR approach. If carrying out a procedure turn to intercept the localizer and FAF, try to be at flaps 5 maneuvering speed, with flaps 5 extended, prior to localizer capture and descent.

Pitch and Power Settings (Fly By The Numbers)

Whenever the aircraft is flown by hand (manual flight), pitch and power settings become important. A common method used by experienced pilots is to fly by the numbers.

The term fly by the numbers is when the pilot positions the thrust levers commensurate to a desired %N1 pursuant with the aircraft’s attitude, configuration and speed. The %N1 is based on aircraft weight and is displayed in the EICAS. If the published figures are not available, a reasonable baseline %N1 to begin with is around 55%N1. Aircraft with heavier weights will require higher thrust settings while lower thrust settings will be needed for lighter weights. The thrust setting is arbitrary and %N1 will need be fine-tuned with small adjustments.

Once the thrust has been set, always allow the thrust to stabilise for a few seconds and ensure that both thrust levers display an identical %N1. If you fail to do this, and the thrust settings are slightly offset (despite the thrust levers being beside each other) the aircraft will turn in the direction of least thrust (asymmetric thrust).

During the descent, %N1 may be close to idle thrust, however, as the flaps are extended and the landing gear is lowered, the %N1 will need to be increased to counter the effects of drag. The approximate figure of %55N1 should be set immediately prior to the landing gear being lowered.

Important Points:

The %N1 is a baseline figure, the correct %N1 will depend on the weight of the aircraft and any wind component.
Set %N1 immediately prior to lowering the landing gear and extending flaps 15.

It is almost miraculous that once the correct thrust has been set, the others numbers that relate to airspeed and rate of descent fall into place, and the aircraft will only require small incremental adjustments to maintain a 3 degree glide path.

Recommendation:

In order to gauge how the aircraft reacts during an approach, fly several automated approaches (the easiest to fly is the ILS Approach). Observe the thrust settings (%N1) as you extend the flaps and lower the landing gear. Note the numbers for the particular weight of the aircraft.

Reaching the Initial Approach Fix (IAF)

As discussed above, the IAF will differ between approach types. The two most important aspects that should be completed just prior to reaching the IAF are:

The landing briefing and tasks completed; and,
The aircraft’s airspeed should be at flaps UP maneuvering speed, or at flaps 1 maneuvering speed.

Reaching the Descent Point (FAF)

Ideally the aircraft will be at flaps UP maneuvering speed no later than the IAF. If this is done, the transition from level flight to descent will be much easier. At the very latest, plan to be at, or just before the FAF at flaps UP or flaps 1 maneuvering speed. If concerned that the airspeed is too fast, slow the aircraft to a speed that corresponds to the flaps 1 or flaps 2 indications displayed on the speed tape. The airspeed will usually fall between 210-190 kias

The point at which the aircraft descends will depend on the approach type used, but If the aircraft’s airspeed has been managed appropriately, initiating the descent at the FAF is relatively straightforward. During descent the aircraft should:

Have the thrust levers set to idle thrust (or near to);
Have an attitude of approximately 5 degrees nose-up;
Maintain a constant rate of descent (sink rate) between ~600-800 ft/min;
Be on a constant 3 degree glide path; and,
Not have a descent rate greater than 1000 ft/min.

Important Points:

In some situations (for example, whether the aircraft is in level flight or is descending) to initiate the descent, it may be necessary to lower the attitude to below ~5 degrees nose-up, and then increase the attitude to counter any initial airspeed increase, until the appropriate rate of descent and glide path is established.
To aid in passenger comfort, steep descents with the aircraft’s nose below the horizon and pointing downwards should be avoided.

If you are uncertain to the glide path being flown, refer to the Flight Path Vector (FPV) in the PFD.

During the initial descent phase:

Speed is controlled by pitch; and,
Rate of descent is controlled by thrust.

As you transition to the final approach phase, this changes and:

Speed is controlled by thrust; and,
Rate of descent is controlled by pitch.

Model aircraft is used to visualise various approach and landing attitudes

The above dot points confuse many virtual flyers and trainees alike. Rather than attempting to visualise this in your mind, use a small model airplane and position the model in a particular flight phase with the correct attitude. After a while it will make sense and become second nature.

Descent

After initiating the descent in idle thrust and with the aircraft’s attitude set to approximately 5 degrees nose-up, the aircraft’s airspeed will slowly decay. As the aircraft slows, match the airspeed to the flap indications on the speed tape. The maximum airspeed during the descent should not exceed Vref +20 or the landing placard speed minus 5 knots – whichever is lower (Boeing FCTM, 2023).

Vref +20 is indicated by the white carrot on the speed tape, which is displayed when Vref is selected in the CDU.

Lowering the Landing Gear (General Rule)

A rule of thumb used by many flight crews in favourable weather conditions is to lower the landing gear and select flaps 15 at ~7 NM from the runway threshold. At this distance, the aircraft’s altitude is ~2500-2000 ft AGL, and then, prior to reaching 1500 ft AGL, select landing flaps (25, 30 and/or 40). This enables ample time to ensure that the aircraft is stabilised, and to complete the landing tasks and landing checklist.

As an aid, flight crews typically will place a ring, displayed on the Navigation Display, at the distance that the landing gear is to be lowered. The ring, created in the CDU, provides a visual reference as to when to lower the landing gear. A ring is also often added at the IAF, or at 10 NM from the runway threshold.

Delayed Flaps Approach

Some airlines and pilots use less conservative distances, thereby minimising the time that the aircraft is flying with the landing gear lowered and flaps extended. A delayed flaps approach or minimum noise approach, will usually have the landing gear lowered and flaps 15 extended at 4 NM from the runway. Landing flaps will then be extended very soon after.

Delayed Flaps Approach - Caution

While lowering the landing gear and extending the landing flaps close to the runway threshold has positive benefits to the airline, and does limit the noise generated, it is not without its problems. Potential problems are:

If there is a landing gear or flaps failure, the aircraft is very close to the ground;
The landing checklist must be done quickly when concentration may be needed elsewhere (landing);
If the aircraft’s airspeed is too high, slowing down is difficult at this late time; and,
If windshear or other weather related events occur the aircraft is very close to the ground with minimal room to escape.

When the landing gear is lowered and the landing flaps are extended, the aerodynamics of the aircraft are significantly changed. The pilot must be prepared to adjust the flight controls (pitch and thrust) to maintain control; this is especially so when hand-flying the aircraft. Being in close proximity to the ground at this stage can amplify the risk of a ground strike should the pilot have difficulty adapting to the altered aerodynamics.

Lowering the landing gear and extending the flaps, at a distance of 7-5 nautical miles from the runway, provides additional time and a crucial safety buffer for the pilot to acclimate to the new aerodynamic conditions.

Important Points:

The 737-800 is renown for being slippery and difficult to slow down, which is why it is recommended to slow the aircraft prior to the FAF.
A Rule of Thumb often used is: It takes approximately 3 NM to loose 1000 ft of altitude (assuming flaps UP maneuvering speed).
A delayed flaps landing should be attempted only in optimal weather conditions.

If you slow the aircraft prior to reaching the IAF, maintain the correct thrust settings to aircraft weight, and extend the flaps at their correct speeds, the descent and approach will usually be within acceptable limits. You will also not have to use the speedbrake.

Stabilised Approach

During the final approach the aircraft must be stabilised; if the approach becomes unstable and the aircraft descends below 1000 feet AFE in IMC, or 500 feet AFE in VMC, an immediate go around must be initiated.

An approach is considered stable when the following parameters are not exceeded:

The aircraft is on the correct flight path;
Only small changes in heading and path are needed to maintain the correct flight path;
The power settings for the engines are appropriate to the aircraft’s configuration;
The aircraft’s airspeed is no more than Vref +20 kias and not less than Vref (plus wind component); and,
The descent rate of the aircraft is no greater than 1000 ft/min (no special briefing).

Stability during an approach is made considerably easier if the aircraft:

Is travelling at the correct airspeed;
Is trimmed correctly for neutral stick.
The flaps are extended at the correct flaps/speed ratio;
The attitude (pitch) is correct; and,
The thrust settings are commensurate with the desired airspeed and rate of descent.

My preference is to have the aircraft stabilised with the landing check list completed by 1500 feet AFE. At this point, the autopilot and autothrottle are disengaged and the aircraft is flown manually. Although the handoff can be done later, doing it at ~1500 feet enables enough time to take control of the aircraft and make any final adjustments from automated to manual flight.

Important Point:

The height that an aircraft must be stabised is often dictated by airline policy. The height typically is between 1500-1000 feet AFE, but can vary between operators.

Final Approach

The final approach, flare and touchdown occurs very quickly.

At 500 ft AGL, the pilot should begin to include the outside environment in their scan. This adjustment allows for better situational awareness and helps in preparing for a smooth landing.

As the aircraft descends further to 200 ft AGL, the approach becomes predominantly visual. During this phase, the pilot relies heavily on external visual references to maintain proper alignment of the aircraft (runway cues, approach lighting, and other visual references).

Select a part of the runway where you want to the land (use the runway aiming markers) and adjust the attitude of the aircraft so that it is aimed at this location. For guidance, the runway centerline should be running between your legs.

As the aircraft flies over the runway threshold (piano keys) and when you hear the fifty call-out, adjust your viewpoint from the aiming point to approximately 3/4s down the runway. I find looking at the end of the runway works well, as I can see the horizon which aids in determining if the wings are level and in determining the sink rate.

Flare and Touchdown

The flare is a term used to describe the raising of the aircraft’s nose, by approximately 2-3 degrees nose-up (from whatever attitude the aircraft is in), to slow the aircraft to a speed suitable for landing (Vref).

The aircraft should pass over the threshold of the runway (piano keys) at ~50 ft RA. Then at ~15 ft RA the flare is instigated by raising of the aircraft’s nose to an angle of ~2-5 degrees nose-up. This attitude is maintained (held with minimal adjustments) with constant back pressure on the control column, and no trim inputs, until the main landing gear makes contact with the runway (touchdown). At the same time the thrust levers are slowly and smoothly retarded to idle, and if done correctly, the landing gear will touchdown as the thrust levers reach idle.

The reason the thrust levers are retarded slowly is to help prevent any unwanted nose-down pitch that naturally occurs when thrust is reduced. If the thrust is cut suddenly, the nose of the aircraft has a tendency to drop.

The duration of the flare ranges from 4-8 seconds and the flare distance, the distance that the aircraft has travelled beyond the runway threshold, is between of ~1000-2000 feet. The difference in the duration of the flare is dependent upon the aircraft’s airspeed when it crosses the runway threshold.

A common mnemonic to remember during the flare is Check/Close/Hold (CCH). Check the attitude, close the thrust levers, and hold the attitude position.

Important Points:

Pilots during the flare and landing are more concerned with the attitude (pitch) of the aircraft than the descent rate. If the attitude is correct, the descent rate will be within acceptable bounds.
There is space of time between when the throttles are retarded and the %N1 is commensurate with idle thrust.

**Diagram 1**: Runway aiming point and distances from threshold

Call-outs

Immediately prior to and during the flare it is important to carefully listen to the radio altitude call-outs; the speed at which these occur indicate the rate of descent. When the twenty call-out out is heard the flare should begin, as there will be a delay between hearing the call-out out and applying the required control input to initiate the flare (which will be at 15 ft RA). If the flare is delayed until after the twenty call-out out there is a strong possibly that the landing will have too high a descent rate.

Important Points:

The flare can make or break a good landing. It is important to have a thorough understanding of the concept.
Do not trim the aircraft when below 500 ft RA.
Remember, the pilot flying controls the aircraft. The aircraft does not control the pilot.

Flare Problems

A successful flare to land involves several tasks that are done almost simultaneously. If the final approach has not gone according to plan, or the pilot is not vigilant, two problems that can occur are:

If the flare attitude is too steep, or the thrust not at idle, the aircraft may go into ground effect and begin to float down the runway. Floating is to be avoided at all costs; the aircraft should be flown onto the runway.
If the height that the flare is instigated is misjudged (too high) the flare distance will be prolonged leading to a possible tail strike. If on the other hand the flare is begun too low, the rate of descent will be high causing a very firm landing with possible damage to the landing gear.

In situations such as this, a go around should be carried out.

Interestingly, during the flare there is a natural tendency to pull back on the control column further than necessary. This can be quite common with new pilots (at least initially). Bear in mind this can easily occur and be vigilant so it does not occur.

Some pilots prolong the duration of the flare, or minimise the flare attitude in an attempt to slide the aircraft onto the runway with an almost zero descent rate (often called a greaser, slider or kiss). Whilst ego-inspiring, attempting to do this should be avoided.

Important Points:

An aircraft in ground effect is difficult to land, because the air pressure keeps the aircraft airborne. Eventually, the airspeed will decay to a point where the effect ceases, resulting in a heavier than normal landing. Additionally, ground effect causes the aircraft to consume more runway length than usual.
Do not prolong the flare in the hope of a zero descent rate touchdown (slider) A slider style touchdown is not the criteria for a safe landing.
Do not prolong the flare, trim, or hold the nose wheel off the runway after landing (for example, trying to slow the aircraft because of a higher than normal airspeed), as this may lead to a tail strike.

Landing Descent Rate

A landing (touchdown) occurs when the main landing gear makes contact with the runway (not the nose wheel). Ideally, a descent rate between ~ 60-200 ft/min is desired for passenger comfort. This said, Boeing aircraft can tolerate reasonably high descent rates in the order of 600 ft/min.

Speaking with line pilots regarding what constitutes a hard landing will garner innumerable responses, but most agree that a hard landing is in excess of 250 ft/min.

Slider style landing can cause a shimmy to occur to the landing gear

Interestingly, a slider style landing can be detrimental to the landing gear by causing the wheels to shimmy (left and right vibration), leading to increased wheel maintenance. This is because the landing gear is designed to land on the runway with a certain amount of inertia. Also, a slider style landing in wet conditions can lead to aircraft skidding. In wet and icy conditions, it is desirable to have a firm landing to aid in tyre adhesion to the runway.

If the aircraft is travelling at the correct airspeed, has the correct attitude, and the thrust levers are reduced to idle at the correct time, the aircraft will land at a reasonable descent rate.

Things to Consider (situational awareness)

During the approach and landing phase of flight, maintaining situational awareness is crucial. Pilots must be fully aware of the aircraft's altitude, and position in relation to the runway, terrain, and other aircraft in the vicinity. This level of awareness, often referred to as situational or positional awareness, is essential for safe and efficient landing operations.

Important Point:

It is important to take advantage of electronic aids to assist in situational awareness.

The following (at a minimum) is recommended to increase situational awareness:

Create distance rings from the runway threshold. For example, a ring at 10 miles and a ring a 7 miles (CDU);
Select an appropriate approach type from the FMC (ILS, RNAV, VOR, IAN, etc);
Set the Navigation Display (ND) to Map mode;
Turn on the various navigation display aids for the ND (waypoints, station, airports, range rings, etc) by selecting them on the EFIS;
Select the Vertical Situation Display (VSD);
Display the Flight Path Vector (FPV) on the PFD by pressing FPV on the EFIS;
Display range rings by pressing the EFIS knob;
Turn on TCAS on the by pressing the TFC button on the EFIS; and,
Set the EFIS to terrain.

Another aid frequently forgotten about is the Vertical Bearing Indicator (VBI). The VBI is an ideal way to determine the correct rate of descent to a known point. The VBI can be accessed from the descent page in the CDU.

Depending on the approach type selected from the FMC, the PFD will display critical information relevant to the chosen approach. The pilot can either use automation to fly the approach, or if hand flying follow the pitch and roll guidance markers. The Navigation Display (ND) in MAP mode, displays a clear overview of the aircraft's lateral and vertical position in relation to the designated navigation aids.

The information that is available is impressive, but sometimes too much information is not a good thing; a cluttered display can cause confusion and a time delay understanding the data displayed. Nearly all flight crews use the Captain and First Officer ND to display different snippets of information depending upon who is flying the aircraft and how they want to view the information.

Additional information: Approach Tools.

Auto Brakes and Reverse Thrust

The auto brakes should be disarmed as the aircraft approaches 60 knots ground speed and prior to reverse thrust being reduced. This reduces the jolt that can occur when the auto brakes are disarmed.

Reverse thrust should be engaged, without delay, when the aircraft’s main wheels land on the runway. Typically, maximum reverse is not used, but whether maximum reverse thrust is used or not will depend on environmental factors, runway length, aircraft speeds, and other variables.

Reverse trust should be maintained until approaching 60 knots, then following the 60 knots call-out, reverse thrust should be slowly reduced to reverse idle. if done correctly, the thrust will be at reverse idle when you reach taxi speed. Wait until the generated reverse thrust has bleed off, then slowly close the reversers and place them in the stow position

Addition information: Reverse Thrust Procedure.

Control Column Movements - how much is too much

It is evident from various discussions on forums, that a number of virtual pilots do not understand how much movement of the control column is considered normal. This is exacerbated by U-Tube videos of pilots aggressively moving the yoke in real aircraft at low altitudes. Often this leads to these individuals re-calibrating their controls in flight simulator to mimic what they have seen in various videos.

Understandably, many virtual pilots have not piloted a real 737; many have flown light aircraft, however, the control movements in a light aircraft such as Cessna are completely different to those in the 737.

First, many of the U-Tube videos do not provide any input to what the crosswind and gust component was during the landings in question. In windy conditions, control movements (that also include the rudder) may require a more heavy handed approach, however, without this information gauging technique is impossible.

Second, there are three types of individuals: those that at excel their chosen profession, those that get by, and those that should not be in the profession at all. Which type of individual is flying the aircraft in the U-Tube videos ? If an approach is moderately unstable, and the aircraft is piloted by a below average pilot, then they may be moving the control column erratically as they try to bring the aircraft back onto station.

Many of the U-Tube videos are uploaded to generate clicks - not to teach correct technique, and erratically moving the control column may, in their mind, instill excitement that the approach is difficult but manageable. In other words, excitement brings clicks… I have not even touched upon the ‘look at what I can do’ philosophy.

Moving the control column when flying the 737 should be done smoothly, and during the approach the movements should be relatively minor with incremental adjustments to pitch and roll. The more aggressive the movement, the more the aircraft will alter its position, requiring yet further adjustment to bring the aircraft back into line (yo-yo effect).

If you are needing to make large movements of the control column to keep the aircraft on course (minimal crosswind), then there is a strong possibility that the calibration of the control column is not correct, or the control column has not been correctly calibrated in Windows.

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Download Step Guide

Step Guide To Landing the 737-800

To Land - Summary

To land the 737-800, the general idea is to gradually slow the aircraft to an airspeed which at the beginning of the descent will, at idle thrust, enable the aircraft to descend on a 3 degree glide path to the runway.

As the airspeed decays, the flaps are extended as per the flaps extension schedule. The thrust levers, rather than being continually adjusted, which can cause engine spooling, are set to approximately 55%N1 with the ultimate aim of airspeed not exceeding (going under) Vref+20.

At approximately 7 NM from the runway, the landing gear is lowered and flaps 15 extended. Flaps 30 and/or flaps 40 are extended as the aircraft’s airspeed decays to Vref +5. At this point the landing checklist is completed; the aircraft should be stabilised by 1500 ft AGL.

After crossing the runway threshold at 50 ft RA, the aircraft is flared at approximately 15 ft RA by raising the aircraft’s nose 2-5 degrees nose-up and simultaneously bringing the thrust levers to idle.

Notes:

Please read the discussion article prior to reading this document as it will make the guide easier to understand.
This guide primarily discusses the landing of the 737-800. A generic style approach has been ‘loosely’ used to provide context. In this guide, the Final Approach Fix (FAF) has been used to signify the descent point.
There are numerous ways to fly the 737 aircraft, however, the landing technique has little room for variation.

Important Points:

This guide assumes manual flight (hand flying). If using full or part automation, disconnect the autopilot and autothrottle at ~1500-1000 ft AGL and land manually.
Speed Check refers to a possible adjustment of pitch or thrust following a change in the aerodynamics of the aircraft. For example, extending flaps or lowering the landing gear.

Prior to Initial Approach Fix (IAF)

1. Aim to be at 10,000 feet (250 kias) at 30 miles from runway.

2. Complete initial descent briefing prior to the IAF and configure the aircraft’s avionics and instruments for the chosen approach.

The IAF (location, distance from runway, and altitude) is printed on the approach chart.

3. Reduce airspeed to the flaps UP indication on the speed tape (usually approximately 210 kias) prior to reaching the IAF.

Reduce airspeed in level flight.
Bring the thrust levers to thrust idle; the aircraft’s airspeed will slowly decay.
When reaching or passing through flaps UP select flaps 1.
Correct flap procedure is to extend the next flap increment at, or passing through the previous flap increment.

4. Reduce airspeed to ~190 kias and extend appropriate flaps increment as per the flaps extension schedule (usually flaps 1).

Note that the above airspeeds may differ slightly depending on the weight of the aircraft.

Beginning Descent

1. Complete the approach checklist.

The approach chart will indicate at what point you should begin a descent. In the absence of an approach chart, then an approximate altitude and distance to begin descent is ~ 4000-3000 feet AGL ~ 12-10 NM from the runway threshold (use rule of thumb: 3 NM/1000 ft loss in altitude).

2. At the FAF, reduce thrust to idle (or near to) and raise the aircraft’s nose to an attitude of ~5 degrees nose-up.

Note that the attitude may differ depending upon circumstance).

3. If not already at, extend flaps 5 (flaps 1 to flaps 5 jumping flaps 2). Airspeed will be approximately 190 kias.

Speed Check.
The pitch may need to be adjusted to maintain desired airspeed.
If the aircraft is travelling too fast, or ATC have advised to slow down, consider slowing the airspeed to ~180 kias and extending flaps 10. If necessary, increase thrust to maintain descent rate.
For a step-down approach, use the same procedure as mentioned above, with the added step that you must anticipate what the aircraft will do when you level off at the end of the step-down. At the level off, you will need to adjust pitch for level flight and probably need to increase thrust. In both scenarios, the Flight Path Vector (FPV) can be very helpful in determining the attitude of the aircraft.

4. During the descent, try to maintain a descent rate of 600-800 ft/min.

Do not exceed 1000 ft/min (unless a special briefing has been carried out for a non-standard approach).

5. The aircraft should descend on a 3 degree glide path.

Use the speedbrake sparingly, especially after beginning your descent.
Adhere to the flaps extension schedule. Correct management of the flaps is selecting the next lower speed as the additional drag of the flaps begins to take effect. This minimises engine spooling and increases passenger comfort in addition to making the flaps transition smooth.
Anticipate what the aircraft will do when you extend the flaps. The flaps will cause increased drag which, assuming you want to maintain the same airspeed and rate of descent, will either require a decrease in pitch or an increase in thrust.
During the descent, the aircraft’s airspeed will decay. As the airspeed passes through the flap indications on the speed tape extend the next flaps increment.

6. Do not exceed (go under) Vref +20.

Vref +20 is displayed as a white carrot on the speed tape in the PFD (displayed after setting Vref in the CDU).

7. As the aircraft nears the outer marker, or is ~ 8-7 NM from the runway, idle thrust should be increased to ~55%N1.

If a delayed flaps approach is being carried out, the distance will be 5-4 NM).
Note that actual %N1 may differ slightly due to aircraft weight.
Increasing %N1 is to counter the effect of drag from the flaps and soon to be the lowered landing gear. Allow thrust to stabilise for a few seconds.
It is a balancing act (based on aircraft weight, airspeed, and drag) to what %N1 is set. Start with 55%N1 and adjust from here.
The thrust setting that has been set should be enough to compensate for the increased drag from the flaps and landing gear, however, you may need to adjust the thrust setting slightly to maintain the desired airspeed and rate of descent. Think ahead and factor this into your pitch and thrust settings.
Increase the thrust immediately prior to lowering the landing gear and extending flaps 15.

8. At the outer marker, or at ~7 NM from the runway threshold, or between 2400-2000 feet AGL, lower the landing gear.

There is no absolute rule as to when to lower the landing gear. The longer you delay, the less noise and fuel will be used. I find that anywhere between 7-5 NM works well (weather dependent).

If you are carrying out a delayed flaps approach, then the landing gear is usually lowered at 5-4 NM. (distance may change depending upon pilot preference and airline policy). In this case, the increase %N1 should occur immediately before lowering the landing gear.

9. Immediately after lowering the landing gear, extend flaps 15.

Speed Check.
The drag will increase dramatically after lowering the landing gear and extending flaps 15. Plan ahead and if necessary decrease pitch and/or increase thrust.

10. Arm the speedbrake.

11. Set the Missed Approach Altitude in the altitude window of the MCP.

12. Complete the landing checklist.

Final Approach

1. At ~ 5-4 NM from the runway threshold, and at an altitude greater than 1500 feet AGL, extend landing flaps.

Extend flaps 30 jumping flaps 25 unless flaps 40 is being used, in which case you would extend flaps 25.
Speed Check.

2. At this point the aircraft’s airspeed will be very close to Vref +5 and the aircraft will be closing rapidly on the runway threshold.

Add wing/gust component if necessary to Vref +5.

3. Raise the aircraft’s nose to an attitude of ~2.5 degrees nose-up.

4. Decrease the aircraft’s descent rate to ~ 500-600 ft/min.

This will aid in the transition to the flare by slightly increasing the nose-up attitude.
At 1500 ft RA each pilot’s deviation alerting system self tests upon becoming armed. The test will display on the PFD an amber coloured localizer deviation that will intermittently flash for 2 seconds.
Depending upon airline policy, the aircraft must be stabilised between 1500-1000 ft AFE.

For example, QANTAS state that the aircraft must be stable by 1000 ft RA with a attitude pitch of 1-3 degrees nose-up.

Landing, Flare and Reverse Thrust

1. Select a part of the runway where you want to the land (use the runway aiming markers).

2. Adjust the attitude of the aircraft so that it is aimed at this location.

For guidance, the runway centerline should be running between your legs.

2. As the aircraft passes over the runway threshold (piano keys), adjust your aiming point to approximately 3/4 down the runway.

When crossing the runway threshold and beginning the flare, focus your eyes on the end of the runway and watch the horizon. This helps to gauge whether the aircraft wings are level.

3. The height that the aircraft should be at when crossing the runway threshold is ~ 50 feet AGL.

4. At ~15 feet RA, initiate the flare and increase the aircraft’s attitude ~ 2-3 degrees nose-up.

Listen for the RA call-outs. At the RA 20 call-out begin the flare (this is because by the time your brain has processed the call-out and you have moved the control column, the aircraft will be at RA 15 ft.
Maintain back pressure on the control column to keep the attitude constant until the aircraft’s main gear touches down. If the flare has been done correctly, the main gear will touchdown simultaneously with the thrust levers reaching idle.
When initiating the flare, the increased attitude will decay the +5 kias plus any gust correction that was added to Vref. The aircraft’s main gear should touchdown at Vref.
During the flare smoothly bring the thrust levers to idle. Do not suddenly chop the thrust.

5. Ideally the aircraft’s descent rate, when landing, will be 200 ft/min or less.

6. Lower the nose wheel without delay by smoothly flying the nose wheel onto the runway.

Control column movement forward of neutral should not be required.

7. Engage reverse thrust and check that spoilers have engaged.

8. Verify that speedbrake lever is down.

9. Disarm the auto brakes as the aircraft approaches 60 knots ground speed.

10. Approaching 60 knots ground speed, and only after hearing the 60 knots call, begin to slowly retard reverse thrust.

The reversers should be at reverse idle as you reach taxi speed. Maintain reverse idle for a few seconds to enable the reverse thrust to fully dissipate. Close and stow the reversers.

11. Apply manual braking as required.

Important Points:

Below ~ 200 feet AGL the landing is primarily visual.
To assist in gauging the flare, focus your eyes nearer to the end of the runway and watch the horizon (which should be horizontal).
A go around (TOGA) can be instigated at anytime prior to landing touchdown.

Final Call

Although approach types differ, the technique of landing the 737 is identical in each approach. By far the most critical elements of a successful approach and landing are speed management, extending the flaps on schedule, thrust settings and using the correct attitude during the flare. Despite a number of variables occurring in quick succession, with experience, you can easily maintain a constant speed, attitude and descent rate as you fly down the 3 degree glide path.

Related Articles

Glossary

AFE – Above Field Elevation
AGL – Above Ground Level
Attitude – Synonymous with pitch. The angle that airflow hits the wing.
DFA – Delayed Flap Approach
DH (A) – Decision Height (or Decision Altitude). If not visual, the approach cannot continue (Precision Approach)
EFIS – Electronic Flight Instrument System
ILS – Instrument Landing System
IMC – Instrument Meteorological Conditions
KIAS – Knots Indicated Airspeed
MAP – Map display (forms part of Navigation Display)
MAA - Missed Approach Altitude
MDA - Minimum Decent Altitude. If not visual, the aircraft cannot descend lower than this altitude (Non Precision Approach)
ND – Navigation Display
NM - Nautical Miles
PFD - Primary Flight Display
Pitch – Synonymous with attitude. The direction of the aircraft relative to the horizon.
RA – Radio Altitude
VMC – Visual Meteorological Conditions
~ Symbol for approximate

Review and Updates

09 April 2024 - review and release of .pdf.
19 May 2024 - partial rewrite to improve clarity.

January 13, 2024

Scale ID Annunciation (RW/APP CRS Error)

January 13, 2024/ Flaps 2 Approach

Scale ID Annunciation display in upper left hand corner of the Primary Flight Display

The Scale ID annunciation (often called the approach reference), displayed in the upper left of the Primary Flight Display (PFD), is one of a suite of displays that comprise the PFD Navigation Performance Scales (NPS) Indications.

In the image a runway approach course error (RW/APP CRS Error) is being displayed. The airport is Hobart, Tasmania and the ILS approach is to runway 12. The error has been generated because the CRS window in the MCP has the incorrect approach course (140 degrees). If the approach course was correct, the display would be coloured white - not amber with a strike-through line.

The Scale ID Annunciation display provides, the for the selected approach type, the following approach reference information:

Airport identifier;
Runway approach course;
Distance to the runway threshold; and,
Approach type.

The display also indicates whether a runway approach course error (RW/APP CRS) has occurred.

Possible approach type displays include:

LNAV/VNAV (LNAV and VNAV deviations).
LOC/VNAV (Localiser with VNAV deviation).
FAC/VNAV (IAN final approach course with VNAV deviation).
LNAV/G/S (LNAV deviation and glideslope).
LNAV G/P (LNAV deviation with IAN glidepath).
ILS (ILS approach).
FMC (IAN approach).
GLS (GLS approach).

Airport Identifier and Display Colour

The airport identifier comprises the identifier and airport name (abbreviated). The identifier will change depending upon the approach type. For an ILS (and IAN approach) the identifier will be the letter I followed by the airport abbreviation. For example, Hobart airport is YMHB. In this case for an ILS approach the airport identifier will be IHB.

The identifier is displayed in two colours: white and amber; amber being cautionary. The later also incorporates a strike-through line (this line dissects the airport identifier and approach course).

White indicates that all the parameters required for the approach have been completed correctly. An amber colour/strike-through indicates that one or more of the required parameters have not been met.

Colour Combinations

The following colour combinations can be observed (further information is discussed later in the article).

Frequency and approach course displayed in white:

When the navigation radio is tuned to the ILS frequency, the identifier will initially display the ILS frequency (109.90) for the approach. The frequency will then change to display the airport identifier (IHB). Whether the colour displayed remains white or changes to amber will depend on whether both navigation radios and CRS course windows are set to the correct ILS approach.

If either display is coloured amber it indicates a RW/APP CRS error has occurred.

Airport Identifier displayed in amber:

One navigation radio is tuned to the ILS frequency. Tuning the second radio to the same frequency will cause the display to change from amber to white.

Approach course displayed in amber:

One or both courses in the CRS course windows (MCP) is not set to the correct ILS approach course.

DME and approach type:

The DME and approach type (ILS) are always displayed in white. The DME will display the distance to the runway when the glideslope is captured by the aircraft.

Pre-Approach Tasks

Prior to commencing an approach, the following should be carried out:

The correct frequency entered into to the navigation radios (NAV 1 & NAV 2);
The correct approach course (for the runway selected) entered into the Captain and First Officer side CRS course windows in the MCP;
An appropriate approach selected from the FMS database (depends on the approach type being used); and,
The approach course for the runway entered into the heading window in the MCP.

Delay

The logic controlling the scale ID annunciation periodically interrogates that data entered into the navigation radios and MCP. This means that a delay is often observed between the annunciation changing colour from white to amber or back again. I am unsure of the timing.

Discussion

The indication that a RW/AP CRS error has been triggered doesn’t alwasy preclude an approach from being carried out (although it’s not recommended). The annunciation indicates that, for the selected approach, something hasn’t been completed with regard to the configuration of the avionics. It's rarely the case that the frequency hasn't been correctly entered into to the navigation radio; more often than not the cause of the annunciation is a CRS course discrepancy, or failure to configure the second navigation radio to the same frequency as the controlling navigation radio.

Using the ILS approach as an example. To correctly configure the instruments for an ILS approach and not receive a cautionary warning, the following must be completed:

Enter the correct ILS frequency into the BOTH navigation radios; and
Enter the correct approach course into BOTH the CRS course windows in the MCP.

It’s also recommended, but not mandatory to:

Enter the approach course into the heading window in the MCP; and
Enter an appropriate approach into the CDU/FMC.

If you enter the ILS frequency into the controlling navigation radio, and enter a different frequency into the other navigation radio, an amber-coloured RW/APP CRS annunciation will be generated. Likewise, a caution will occur if the Captain-side and First Officer side CRS windows don’t display the identical ILS approach course.

**IMAGE A-1:** ILS approach into runway 12 for Hobart, Tasmania (IHB). The approach course for this approach is 120 degrees. The controlling navigation radio (Captain-side/not shown) has been set to the correct ILS frequency (109.90). The heading that the aircraft is flying is 120 degrees, and the compass rose is offset to the course direction that is displayed in the Captain-side CRS window (140 degrees)

Example (Hobart, Tasmania IHB)

Image A-1 shows an ILS approach into runway 12 for Hobart, Tasmania (IHB). The approach course for this approach is 120 degrees. The controlling navigation radio (Captain-side/not shown) has been set to the correct ILS frequency (109.90). The heading that the aircraft is flying is 120 degrees, and the compass rose is offset to the course direction that is displayed in the Captain-side CRS window (140 degrees).

In the example, a RW/APP CRS annunciation has been triggered for an ILS approach. The airport identifier and approach course are coloured amber with a strike-through line. The DME is 9.4 miles and is coloured white (correct data).

This approach can be flown despite the discrepancy between the four courses (120, 180, 130 & 140 degrees) and a RW/APP CRS annunciation. This is because the ILS approach course (120 degrees) is coupled to the ILS frequency set in the controlling navigation radio – not the course as indicated in the CRS windows in the MCP.

In the example you can see that the localiser has been captured (this is identified by the magenta-coloured course deviation line being centered/in-line with the course pointer) despite the CRS window displaying a course of 140 degrees. Once the aircraft has captured the localizer it will fly the localiser heading no matter what course is displayed in the CRS window (provided it does not exceed 90 degrees).

While this example holds true for an ILS approach other approach types may behave differently.

Important Points:

The scale ID annunciation is an amber-coloured display that annunciates when the avionics have not been correctly configured for the selected approach. The display is a cautionary.

The approach cannot be flown If the CRS course discrepancy is greater than 90 degrees from the ILS approach course. This is because the aircraft will follow the direction of the course set in the CRS window (if greater than 90 degrees).

ProSim-TS

The ProSim737 avionics suite replicates the RW/APP CRS logic used in the real aircraft.

Database Inconsistencies

In some instances the annunciation is displayed despite entering the correct information. A possible reason for this is a scenery navigation database inconsistency.

In older scenery designs the physical location of the localiser beacons was part of the scenery file and this information is what the simulator referred to. With the advent of up-to-date navigational points (supplied by Navigraph) the simulator now refers to a navigational database rather than a scenery database. An inconsistency will occur if there is a discrepancy between the location of the localiser beacons in the scenery and the information recorded in the navigational database.

Final Call

The RW/APP CRS annunciation, although confusing to the uninitiated, does not necessarily mean that an approach cannot be carried out. However, it’s prudent before flying the approach to understand why the RW/APP CRS error has been displayed.

In more cases than not, the reason for the cautionary annunciation is a failure to configure the navigation radios to the same frequency and/or enter the same ILS approach course into both the CRS course windows in the MCP.

September 09, 2022

Reverse Thrust Procedure

September 09, 2022/ Flaps 2 Approach

The reverse thrust levers are clearly visible in the first detent position. OEM throttle quadrant converted for flight simulator use

Pilots tend to be numbers-orientated individuals. They like concise instructions and do not like ambiguity. Nor do they like being presented with something that is in ‘shades of grey’ rather than ‘black and white’.

When, how, and for how long to deploy the reverse thrust (reversers) falls into the 'grey area'.

In this article, I will endeavour to unravel some of the uncertainties as to when and how to use reverse thrust. I will also briefly discuss the relationship between the use of the autobrake and reverse thrust.

I am not going to delve deeply into every environmental consideration that needs to be analysed prior to the use of reverse thrust; this information is more than readily available from the Flight Crew Operations Manual (FCOM), Flight Crew Training Manual (FCTM) and other specific airline policy documentation.

Reverse Thrust Basics

Reverse thrust (reversers) is used only for ground operations and is used after touchdown to slow the aircraft; it is used to reduce the stopping distance, minimise brake temperatures and decrease wear and tear.

Reverse thrust comprises four détentes and an interlock position, that are engaged by moving the thrust levers from the stowed down position through to the fully up position.

No reverse thrust (thrust levers are closed / stowed position).
Detent 1 (idle reverse / thrust levers are at first position).
Detent 2 (thrust levers are at second position).
Full maximum reverse thrust (thrust levers are at fully upward position).

Between detent 1 and full maximum reverse thrust there is scope for the thrust levers to be positioned part way; thereby, altering the amount of thrust generated.

Schematic showing various positions for the thrust reverser levers

The interlock mechanism is felt when the reverse thrust levers are advanced to detent 1. The purpose of the interlock is to restrict movement of the reverse thrust lever until the reverser sleeves have approached the deployed position.

The procedure to use reverse thrust is very straightforward, however, questions arise as to whether to use detent 2 or full maximum reverse thrust, and when to begin reducing thrust and for how long.

Procedure

Following touchdown, without delay, move the reverse thrust levers to the interlock position and hold light pressure until the interlocks release (as the sleeves move rearwards).

For most landings, detent 1 and detent 2 will usually provide adequate reverse thrust (for normal operations). If additional reverse thrust is needed (wet, slippery or short field landing), full maximum reverse thrust can be selected by raising the thrust levers past detent 2 to full maximum reverse thrust.

To come out of reverse, the reverse thrust levers are returned to the detent 1 position, the engine allowed to spool down, and the levers then returned to the stow position.

Practically speaking, after touchdown maintain reverse thrust as required up to maximum thrust until the airspeed approaches 60 knots. Reverse thrust is then slowly reduced to detent 1 and then to reverse idle by taxi speed. Wait until the generated reverse thrust has bleed off, then slowly close the reversers and place them in the stow position.

Bringing the reverse thrust levers to detent 1 is important because it prevents engine exhaust re-ingestion and minimises the risk of foreign object debris (FOD) ingestion. Idle thrust also bleeds off forward thrust from the engines.

The autobrake is disarmed when a safe stop of the aircraft is assured, or when the aircraft reaches taxi speed.

Important Point:

If transitioning from using the autobrake to manual braking, use reverse thrust as required until reaching taxi speed and then disarm the autobrake.

Disarming the autobrake before closing reverse thrust provides a relativity seamless transition which increases passenger comfort (there is no aircraft jolt).

Conditions Required To Engage Reverse Thrust

The reversers can be deployed when either of the following conditions occur:

The radio altimeter senses less than 10 feet altitude;
When the air/ground sensor is in ground mode; and,
When the forward thrust levers are in the idle position.

Until these conditions occur, the movement of the reverse thrust levers is mechanically restricted and the levers cannot be moved into the aft position.

It is important to always deploy reverse thrust as soon as possible following touchdown. Do not wait for the nose wheel to touch down, but engage reverse thrust when the main wheels are on the runway. Timely deployment will increase stopping power; thereby, increasing safety and reducing heat build-up in the brake system.

A study determined that there was roughly a 17 second difference in stopping time when reverse thrust was deployed immediately the landing gear was on the runway as opposed to waiting several seconds for the nose gear to also be on the runway - reverse thrust is most effectual at high airspeeds and its effect decays on a linear scale as forward airspeed decreases.

Important Points:

Reverse thrust should always be used with the autobrake, unless the runway is exceptionally long without a possibility of runway overrun (the reason for this will be explained shortly).
When closing the reversers, always pause at detent 1. Monitor the REV thrust output on the Primary Engine Display (center panel) and stow the reversers only after reverse thrust has dissipated.

Call-outs

The pilot monitoring usually makes the following call-outs:

‘60 knots’;
‘Reversers normal’ - when both REV indications are green;
‘No reverser engine No: 1’ - if no REV indication or colour is amber; or,
‘No reverser engine No: 2’ - if no REV indication or colour is amber; or,
‘No reversers’ - if no REV indications or colour is amber.

NOTE: Annunciators and displays are discussed later in the article.

During landing, the pilot monitoring (PM) should call out 60 knots to advise the pilot flying (PF) in scheduling the reduction of reverse thrust.

When landings are in conditions that are suboptimal (heavy rain, snow, slush, etc), some operators stipulate that the PM operate and control the reverse thrust . This enables the PF to concentrate solely on the landing roll out rather than having the extra responsibility of also controlling the reverse thrust.

This said, although this procedure may lower pilot workload, it can cause problems when the PF is landing on a slippery runway or in marginal crosswind conditions. At these times, the PF may wish to use the reverse thrust in conjunction with the brakes and there is little time to call out instructions to the PM.

Technical Aspects (basic operation)

Each engine on the Boeing 737 Next Generation is equipped with an hydraulically operated thrust reverser, consisting of left and right translating (moving) sleeves. Aft movement of the reverser sleeves cause blocker doors to deflect fan discharge air forward, through fixed cascade vanes, producing reverse thrust.

Hydraulic pressure for the operation of the thrust reversers comes from hydraulic systems A and B, respectively. If hydraulic system A and/or B fails, alternate operation for the affected thrust reverser is available through the standby hydraulic system. When the standby hydraulic system is used, the affected thrust reverser deploys and retracts at a slower rate and some thrust symmetry can be anticipated.

When reverse thrust is selected an electro-mechanical lock is released. This causes the isolation valve to open which results in the thrust reverser control valve moving to the deploy position, allowing hydraulic pressure to unlock and deploy the reverser system.

The system is designed in such a way that an interlock mechanism restricts movement of the reverse thrust lever until the reverser sleeves are in the deployed position.

Closing the thrust levers past detent 1 to the stow position initiates the command to stow the reverser. When the lever reaches the full down position, the control valve moves to the stow position allowing hydraulic pressure to stow and lock the reverser sleeves. After the thrust reverser is stowed, the isolation valve closes and the electro-mechanical lock engages.

Relationship with Flaps

There is an interesting relationship between the use of reverse thrust and flaps 40.

When the aircraft has flaps 40 extended, the drag is greater requiring a higher %N1 to maintain airspeed. This higher N1 takes longer to spool down when the thrust levers are brought to idle during the flare; this enables more energy to be initially transferred to reverse thrust.

Therefore, during a flaps 40 landing more energy is available to be directed to reverse thrust, as opposed to a flaps 30 landing.

Annunciators and Displays

Thrust reverse indicators are displayed in the Primary Engine Display located in the center panel slightly above the No: 1 and No: 2 %N1 indicators. When reverse thrust is commanded, REV will be displayed initially in amber followed by green dependent upon the position of the thrust reverse levers.

Amber: Thrust reverser has been deployed from the stowed position and both sleeves have travelled ~10-90% to the deployed position.
Green: Thrust reverser has been deployed from the stowed position and both sleeves have travelled greater than 90% to the deployed position.

When either reverser sleeve moves from the stowed position, the amber REV indication annunciator, located on the upper display will illuminate. As the thrust reverser reaches the deployed position, the REV indication illuminates green and the reverse thrust lever can be raised to detent 2.

Electronic Engine Control (EEC) panel (AFT overhead). ProSim737 avionics suite virtual display

Additional reverse thrust annunciators are located on the aft overhead panel in the Electronic Engine Control (ECC) panel. These annunciators are triggered by the retraction of the reverse thrust levers to the stow position.

The annunciators will illuminate during a normal reverse thrust / stow operation for 10 seconds and then extinguish 10 seconds later when the isolation valve closes.

A system malfunction has occurred if the reverser (REV) annunciator illuminates at any other time, or illuminates for more than approximately 12 seconds (in the later instance, the master caution and ENG system annunciator will also illuminate).

Possible reasons for a system malfunction are that the isolation valve, thrust reverser control valve, or one or both of the thrust reverser sleeves are not in their correct position.

Autobrake and Reverse Thrust Use (the grey area)

Both the autobrake and timely application of reverse thrust can be used to slow the aircraft, however, both come at a cost.

Using the autobrake generates considerable heat in the braking system, translating to increased expenditure in maintenance and possible delays in turn around times (waiting for brakes to cool to operational temperature). Conversely, reverse thrust consumes excess fuel. Clearly there is a middle point where each will cancel out the other.

The immediate initiation of reverse thrust at main gear touchdown, and use of maximum reverse thrust, enable the autobrake system to reduce brake pressure to the minimum level – this is because the autobrake system senses deceleration and modulates brake pressure accordingly. Therefore, the proper application of reverse thrust results in reduced braking and less heat generation for a large portion of the landing roll.

Based on this premise, it stands to reason that this is why Boeing recommend to use the autobrake in conjunction with reverse thrust.

Boeing states in the FCTM that: ‘After touchdown, with the thrust levers at idle, rapidly raise the reverse thrust levers up and aft to the interlock position, then to reverse thrust detent 2. Conditions permitting, limit reverse thrust to detent 2’.

It appears to be Boeing’s intention to use reverse thrust as the major force to stop the aircraft, and as the use of maximum reverse thrust further minimises brake system heating, it would appear to be a preferred choice, despite the FCTM stating detent 2 is the preferred position for normal operations.

The official literature does not satisfactorily address this ‘grey area’ The result being that many 737 pilots use differing techniques when deploying and stowing the reversers.

Various Methods

If you observe how other pilots use the reversers, you will discover that there are several variations that follow the same theme.

1. A pilot will, when the aircraft passes through 60 knots, close reverse thrust by lowering the reverse thrust levers through detent 1 to the stow position without stopping at detent 1;

2. Try to locate detent 1 by ‘feel’ resulting in pushing the levers too far towards the stow position, causing forward thrust to unexpectedly occur momentarily;

3. Deliberately close maximum reverse thrust at the 60 knots by placing the reverse thrust levers into the stow position.

In the above three scenarios, the reverse thrust levers have not been allowed to pause at the detent 1 position. Pausing at detent 1 is important as %N1 requires several seconds to reduce to idle thrust after maximum reverse thrust has been used, and it is during this ‘wind down’ period, as the reverse sleeves fully close, that %N1 will transition through 55-60%N1, which is forward thrust.

By not allowing the reversers to pause momentarily at detent 1, to enable thrust to disparate below 55-60%N1, may cause the aircraft to momentarily accelerate. This can be rather disconcerting, especially on a short field landing or landing in marginal conditions.

So What Do I do (normal procedure)

At touchdown I engage reverse thrust – either detent 2 or maximum reverse thrust (or part thereof).
Approaching 60 knots I slowly and smoothly retard the reverse thrust levers to detent 1.
I always allow a few seconds at detent 1 to enable %N1 to dissipate.
Approaching taxi speed I disarm the speedbrake and close the reversers.

At no time do, unless in an emergency, do I close the reversers suddenly; I always close the reversers smoothly and slowly. This enables %N1 to dissipate gradually.

Final Call

The procedure to deploy reverse thrust is straightforward and very easy to accomplish, and there is little argument that reverse thrust should be used on all, but the longest runways in optimal environmental conditions. However, there is confusion and often disagreement to when the reversers are deployed, whether maximum reverse thrust should be used, and for how long the reversers should be left in the open position before retraction and stowing.

It is unfortunate that the information written in the Flight Crew Training Manual (FCTM) and Flight Crew Operations Manual (FCOM) does not provide a more objective ‘black and white’ answer to this procedural dilemma.

Video

The below video shows the REV indicators on the Primary Engine Display (when reverse thrust is commanded) and the REVERSE annunciators on the ECC panel (AFT overhead). Video taken directly from ProSim-AR 737 avionics suite (virtual software). Video upload to U-Tube rather than VIMEO).

May 22, 2022

Circle-to-Land Approach Procedure

May 22, 2022/ Flaps 2 Approach

Although a circle-to-land is a VFR approach, it is recommended to use whatever automation and equipment is available. This includes the FMC to generate waypoints and radials to increase situational awareness (Petr Beran, Aerosvit Boeing 737-400 on final approach into Tivat Airport, CC BY-SA 4.0)

Landing can be a challenge to new virtual flyers, and this is especially so when there are so many different types of approaches that an airline pilot can use. Often the approach selected is based on environmental conditions, the type of equipment used in the aircraft, and the type of equipment and technology available at the airport.

This article will explain the technique used in circle-to-land (CTL) approaches. I have purposely tried to simply the details to cater to all levels of experience. However, aviation often is not a simple subject; issues can be complex and overlap.

What is a Circle-to-Land

A circle-to-land approach is similar to entering a VFR traffic pattern, but you are following a published approach prior to entering the pattern directly. A CTL is an hybrid between a standard non-precision visual approach and a precision approach; you use information gleaned from the circle-to-land information block on the chart in a 100% visual environment.

The initial approach used can be either a precision or non-precision approach. RNAV (GPS), NDB, VOR and ILS approach types are allowed, however, only CAT 1 approaches can be used (CAT 2 & CAT 3 approaches cannot be used). It is important to realise that if an ILS is used, you do not fly the ILS. Rather, you fly the Localizer and use Vertical Speed (V/S) to descend at the appropriate rate of descent (following the ILS vertical guidance).

Although the approach is VFR, you still utilise whatever instruments necessary to increase spatial awareness and lower pilot workload. The autopilot, autothrottle and vertical speed are often used during the approach, however, this is not a hard and fast rule and flying the aircraft manually is allowed. Boeing recommend the use of the autopilot when intercepting the landing profile.

The approach is usually executed at a low altitude; typically 1000 feet AGL within a defined boundary around the airport (usually a 4.5 nautical mile ‘protection area’). This is in contrast to a standard visual traffic pattern whereby an altitude of ~1500 feet AGL is used.

Approach chart for Hobart, Tasmania (YMHB). Note the circle-to-land information block outlined in red. Also note the MDA and visibility for a Category C aircraft highlighted in yellow © Navigraph/Jeppesen

MDA and Speed Management

The minimum altitude that the CTL is to be flown is specified by the MDA, while the minimum required visibility and other pertinent points are displayed in the circle-to-land section of the approach chart (see chart diagram). The general rule is that if something is not prohibited, then it is allowed. If there is no note on the chart prohibiting a CTL then circling to land is implicit.

The MDA is the lowest altitude that you can descend to when conducting a CTL. This said, there is absolutely no reason why you must descend to this altitude. Providing horizontal visibility is within range, often a higher altitude (similar to a pattern altitude) will make the approach easier. If using a higher altitude than the MDA, ensure you do not exceed the boundary as defined by the 'protected area'.

Speed management and a stabilised approach is paramount, as the aircraft is relatively low to the ground and is in landing configuration. The aircraft’s speed should not be below Vref+15 (approximately 160 kias) as the aircraft will need to be banked in a standard 25 degree turn when it has reaches the MDA. The final approach speed and descent occurs during the turn to short final and on final.

NOTE: I purposely have not discussed ICAO and US TERPS. If you want to read about the differences between the two protocols, navigate to Skybrary.

Circle-to-Land Procedure

Consult the approach chart to determine the Minimum Descent Altitude (MDA). Whatever this figure is, round the number up to a even number by adding 100. For example, if the MDA is 1430 feet round the number to the nearest 100 feet, which is 1500 feet. Dial this altitude into the altitude window on the MCP (if desired, a higher altitude to the published MDA can be used).
Fly the Localiser and use V/S to alter your rate of descent. Speed management is important. Although not required, it is a good idea to adjust your heading selector on the MCP to read 45 degrees either left or right of the localiser course. This saves you doing it when the aircraft reaches the MDA.
The landing gear and flaps(flaps 15) are to be extended no later than the MDA. However, if necessary this can be done prior to the MDA to aid in establishing a stabilised approach (for example, between 10 and 7 nautical miles from the runway). The speed brake should be armed.
Fly the localiser to the MDA until ALT HOLD (ALT ACQ will be displayed on the FMA) and level off at the MDA. Set the Missed Approach Altitude (MAA) in the altitude window on the MCP. If you are not visual by this stage, a Go Around must be executed. Note that is if VNAV is being used select ALT HOLD on the MCP (this will disable VNAV).
Press Heading Select (HDG SEL) on the MCP. The aircraft will turn 45 degrees L/R at a 25 degree bank (assuming you preset the HDG SEL as mentioned earlier). Once the wings are level (more or less by a few degrees) continue to fly this course for 20 seconds. Use the timer to record the elapsed time.
After 20 seconds has elapsed (some procedures suggest 30 seconds), adjust your heading (HDG SEL) to fly downwind (the original localiser course). Fly this heading until the aircraft is abeam of the runway threshold (the triangle that represents the aircraft on the ND should align with the end of the runway). Either look out of the window to gauge your position and/or use the Navigation Display to check the aircraft’s position in relation to the runway.
Start the clock when the aircraft is abeam of the runway and fly outbound for 3 seconds per 100 feet AGL. For example, if the MDA is 1500 feet, you divide 1500 by 100 and times by 3 to determine the time (t) of the outbound leg – which is 45 seconds (t=1500/100*3).
When 45 seconds has elapsed, call for landing flaps, adjust the speed, and set the HDG SEL on the MCP to the runway heading. Begin a descent using V/S at 300 fpm and complete the landing checklist.

It is recommended to use the position trend vector on the Navigation Display, in conjunction with outside references (runway PAPI, etc), to judge the turn. The aircraft’s bank should not exceed 25 degrees during the turn.

The prevailing wind and distance from the runway will determine if the turn is continuous or to base and then final.

If using the autopilot, remember to adjust the bank angle selector accordingly, otherwise the aircraft’s bank may exceed stipulated parameters. Intercept the normal visual glide path (final) and disconnect the autopilot and autothrottle. Verify that the missed approach altitude is set on the MCP and recycle the Flight Director switches (if required).

After disconnecting the autothrottle, an initial 'good' thrust setting is around 55%N1; from this point you can increase or decrease thrust to maintain Vref+5. Also, as you turn to final, glance at the runway PAPI lights and adjust vertical speed accordingly. As a rough guide:

PAPI Lights

4 RED - do nothing (maintain V/S).

3 RED - increase V/S to 500 fpm.

2 RED - increase V/S 800-850 fpm

1 RED - increase V/S 1000 fpm

9. If the MDA is breached or visual references are lost, a Go Around must be executed. Depending upon the aircraft’s position, climb to the Missed Approach Altitude (MAA) remaining in the ‘protected area’ (fly in circles)

If a Go Around is executed prior to the final approach, always turn the aircraft in the direction of the runway, as this will ensure the aircraft remains in the ‘protected area’.

Winds

Any tail or crosswind must be taken into consideration. Failure to do so will place the aircraft in the wrong position relative to the approach.

To correct for wind, you take half the tail component and subtract it from the outbound time. For example, if the tail component is 5 knots and the outbound time is 24 seconds, you would subtract 5 from 24 giving you an adjusted time of 19 seconds.

Another way to determine this is to press the progress page (page 2) on the CDU (PROG)

and halve the tailwind component displayed.

The Navigation Display showing several aids that have been used to facilitate a circle-to-land on runway 30 at Hobart, Tasmania (YMHB). A circle ring at 4 mile, a radial (030), and a point/bearing/distance waypoint (RW301). The heading bug has been preset to a turn of 45 degrees

Aids to Increase Spatial Awareness

Although this is a visual only approach, there is no reason not to use whatever tools are at hand to increase spatial awareness and make the approach a little easier.

Use the CDU to:

Make a waypoint (Place/Bearing/Distance waypoint) at whatever distance desired that is adjacent to the runway. This waypoint will act a point in space that the turn to base is made.
Note that this waypoint/fix is only for added reference and is not a point from which to create a route.
Create a radial 90 degrees from the end of the runway. This will display a straight line from the runway that will be a visual reminder when the aircraft is abeam of the runway.
Create distance rings. The rings are displayed on the Navigation Display. At the very least, a ring should be used to delineate the 'protected area' around the airport. Further rings can be used to help show the MDA and other flight specific events.
Use the Vertical Bearing Indicator (VBI). The VBI provides a defined vertical speed that can be used as a reference to the correct 3 degree glide path.

How to make a distance ring, radial, waypoint, and use the Vertical Bearing Indicator (VBI)

Although this has been mentioned elsewhere on this website, a review is in order. In the following examples I will use the approach chart YMHB Runway 30 (see chart diagram below). This is a VOR approach, however, it could equally another approach type. LSK1L means Line Select 1 Left.

NOTE: There are differences between avionics suites. ProSim737 use the acronym RW to define a runway. PMDG use RWY.

Before continuing, the following functionality overlaps with each other. Therefore, it is easy to become discombobulated. When you are in the simulator you will find it makes sense.

Distance Rings

Distance rings are created from the FIX page in the CDU.

Open the FIX page and type into the scratchpad a known waypoint or navaid (For example YMHB or RW30).
Up-select the identifier to the FIX page (LSK1L). A dashed-green coloured circle will be displayed around the waypoint in the Navigation Display.
To enlarge the ring to a desired distance around the waypoint, type into the scratchpad the distance (for example /2). Up-select this to LSK2L. This will display the ring around the waypoint at a distance of 2 miles.

Creating a Radial to a Specified Waypoint

To create a radial a set distance from a known point (waypoint/navaid). For example RW30.

Open the FIX page and type into the scratchpad the desired waypoint/navaid, bearing vector and distance.
Type into the scratchpad the bearing and distance of the radial wanted (for example 030/2).
Up-select this to the appropriate line in the FIX page (LSK2L). For example, entering RW30030/2 will create a green dashed line along the 030 bearing to intersect with a circle surrounding RW30 at a distance of 2 miles.
If you want the point (where the line insects the circle) to become a waypoint, read the next section.

Creating a Specified Waypoint (Place/Bearing/Distance Waypoint)

There are a few ways to do this. I have discussed one way (which works with ProSim737).

Type into the scratchpad RW30. This will create a green coloured circle around RW30 on the Navigation Display (ND).
Type in the scratchpad the bearing and distance (030/2).
Up-select this information to the FIX page (LSK2L). This will place a green-coloured radial at 030 degrees from RW30 that intersects the circle at 2 miles on the ND.
Next, select the 030/2 entry from the FIX page (press LSK2L). This will copy the information to the scratchpad. Note the custom-generated name - RW30030/2.
Open the LEGS page and up-select the copied information to the route. Press EXECUTE.
RW30030/2 will now have an amended name - RW301. Note that RW301 will form part of the active route.
Copy RW301 to the scratchpad.
Open a new FIX page (there are 6 FIX pages that can be used).
Up-select RW301 to the FIX page (LKL1L). This will create a circle around RW301 on the ND.
To remove the waypoint (RW301) from the route, open the LEGS page and delete the entry.
Press EXECUTE.

RW30 will be displayed on the Navigation Display

There is a less convolted way to do this, however, the method is not supported by ProSim737.

VBI

To input a variable into the VBI, an appropriate approach must be selected from the ARRIVALS page. This approach information can be deleted from the route after the information for the VBI has been generated.

Select the DEP/ARR page in the CDU.
Select ARR and then select RW30. RW30 is shown on the last page.
Choose a desired distance to generate a runway extended line (RWY EXTLSK3R).
Open the LEGS page and close any discontinuity; or,
Delete all entries except RW30 (unless wanting them). Ensure RW30 is the active leg (LSK1L). The entry will be coloured magenta.
Press EXECUTE.

Open the VBI by pressing DES on the CDU. RW30 should be displayed in the VBI.

Important Points:

A quicker way to do this is to select RW30 to the scratchpad and then up-select to the upper most entry (LSK1L). This will delete all entries except this one (assuming you do not want other entries).
When loading an approach, often a RX-XX will be displayed. The RX-XX waypoint is not part of the database but is a generated waypoint based on the approach type selected (it will have a different altitude). Do not use the RX-XX entry (delete it).

**Diagram 1:** representing a circle-to-land approach © Boeing FCOM

Go Around

To perform a Go Around using a published missed approach you need to enter the missed approach details into the FMC (the missed approach is displayed in the LEGS page immediately AFTER RW30).

Select DEP/ARR in the CDU and select an approach for Runway 30. This will display in the LEGS page an appropriate approach, runway and a missed approach.
Open the LEGS page and delete all entries prior to runway 30 (RW30) and clean up any discontinuity. Check the LEGS page to ensure the runway and missed approach are correct.

Additional information: Creating Waypoints on the Fly .

Important Points:

A circle-to-land approach can only be conducted when the pilot flying is able to see the airport and runway. If at anytime visual reference is lost, a Go Around must be executed.
The aircraft must not descend below the Minimum Descent Altitude (MDA) stipulated on the approach chart. Although the aircraft must not descent below the MDA, a higher MDA can be used if desired.
The initial approach can be flown using one of several chart types. If using an ILS approach it is recommended to not engage the ILS mode (if you do, ensure you do not accdently descend past the MDA - change out to V/S prior to reaching the MDA). If using an RNAV approach make sure that VNAV is disengaged at the MDA.
Speed management is critical as you are flying at low altitude in landing configuration. A stable approach is paramount.
Do not construct a route in the CDU to overlay onto the circle-to-land route. The procedure is designed to be flown using HDG SEL.
The circle-to-land is VFR. Do not end up 'tail-up' with your head in the CDU. Look outside!

To learn why an overlay is not recommended, watch this video by Mentour Pilot.

Recommended Actions:

To aid in spatial awareness the following actions are suggested:

If the Captain is flying the aircraft, try and turn right as this will place the airport on the left side of the aircraft enabling the pilot flying better visual reference. Vice versa if the First Officer is the pilot flying (unless the direction is stipulated otherwise in the approach chart).
Use the CDU to create distance rings and a waypoint/radial. Use the VBI.

Flight Simulation - avionics suite

Unfortunately, not all flight simulation avionics suites are identical to each other. This is readily apparent when using the CDU to program the FMC. Users report subtle difference between ProSim737, PMDG and the real aircraft. If any of the above commands do not function correctly, you will need to try and find a workaround; often this is quite easy, but does require a little lateral thought. Hopefully, one day all major suites will be identical.

Variability

Many things in aviation can be done multiple ways. The rules concerning the circle-to-land procedure are for the most part solid. It would be foolish to descend below the MDA, navigate outside the 'protected area' or to continue landing when viability has obscured the runway.

Wind, however, is one aspect that can alter the time used to fly the various legs; 30 seconds may be more prudent than 20 seconds, while an initial 40 degree turn may be more effective than a 45 degree turn.

Likewise, the boundary of the 'protected area' and the pilot's ability and confidence will determine the distance from the runway they fly. One pilot will be confident flying a tight pattern with a continuous descending turn from downwind to final while another may want to extend the distance to enable more time to carry out the landing. Variably is allowed provided you keep within the parameters discussed earlier.

Airline Operator Policy

In the real world, an operator will often publish their own approved limitations, including those for circling approaches. They are usually based on several factors, including the speed category of the aircraft and also a minimum height to fly at while carrying out any sort of visual approach (this is sometimes referred to as the Approach Ban).

The objective of the exercise is to fly the published procedure safely by remaining clear of cloud, in sight of the surface and keeping as close as possible to the landing runway. This is best achieved by the pilots flying at a familiar height which is typical of a normal visual circuit.

Video and Discussion Paper

A good video on U-Tube showing a circle-to land can be watched at: https://youtu.be/zDsaXqUVa2A.
An interesting discussion paper concerning Circling to Approach.

Useful Points:

Using the ILS during the initial approach is not recommended as the aircraft can easily descend below the MDA (unless you are vigilant). Use the localiser and V/S.
If the ILS glideslope is used, enter into the altitude window the MDA + 500 feet. Then, when the altitude horn sounds (750 feet ASL) change the descent mode to V/S with an appropriate descent rate. This will ensure that the aircraft does not descent below the MDA.
As you descend to the MDA dial the offset heading into the heading window (rather than wait until you reach the MDA). Then, when you reach the MDA and ALT HOLD is displayed on the MDA select the HDG selector.
When turning to the offset course, always use a 45 degree turn left or right for roughly 20 seconds (factor in wind).
Change the degree of bank selector to 20 degrees (if using the MCP to navigate the aircraft).
To aid in spacial awareness, set-up a suitable approach in the FMC so that navigational cues can be followed when turning to final (for example, an IAN Approach will display diamond markers on the PFD. Using the Vertical Bearing Indicator (VBI) in CDU will display a rate of descent to the runway threshold).
When flying downwind, it can be advantageous to fly a little longer than the time calculated. This enables more time to turn to final and stabilise the aircraft prior to reaching 1000 feet ASL.
Select gear down when adjacent to the runway (if not before). Then, after flying the stipulated downwind time select landing flaps, set speed, and set a 300 feet descent rate using V/S. Then begin the turn to final.
At 300 feet AGL the aircraft wings should be level and the aircraft aligned to the runway.

Final Call

The circle-to-land approach is not difficult, however, depending upon your flight simulator set-up, it can be challenging because you cannot look out of a physical window and see the airport. By far the most important variables are speed management and a stabilised approach regime.

Review and Updates

Released 27 May 2022.

Updated 01 June 2022.

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Discussion Paper

May 10, 2021

Integrated Approach Navigation (IAN) - Procedures

May 10, 2021/ Flaps 2 Approach

I have rewritten the previous article relating to IAN (published in 2015). The new article is more in-line with current practices, has been streamlined. and I hope is easier to read and understand. The original article had been linked to several outside websites, and to maintain the links and url, I have only replaced the content.

Click here to read the article: Integrated Approach Navigation (IAN) - Review and Procedures

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February 03, 2021

Flow Sequence To Enter Information To Flight Management Computer (FMC)

February 03, 2021/ Flaps 2 Approach

Specific information must be entered into the Control Display Unit (CDU) if the Flight Management Computer (FMC) and Flight Management System (FMS) is to function correctly. To ensure that all the appropriate data is entered, a flow sequence is usually used by a flight crew to enter data into the CDU.

Each aircraft is normally equipped with two Control Display Units; one on the Captain-side and one the First Officer-side. Each CDU can be used either in tandem or independently of each other.

In this article, I will discuss the preferred flow sequence that should be used to enter information into the CDU pre-flight. It should be noted that, like many aspects of aviation there are usually several ways to achieve a similar if not identical result. Often airline policy will dictate the sequence that the CDU is configured, and by which pilot. Therefore, the below information should be treated as a guideline rather than an inflexible set of rules.

The information used comes in part from the aircraft’s flight plan and load sheet.

The content of this article has been reviewed by a Boeing 737 Captain for accuracy.

FMC Software

The Flight Management System (FMS) is controlled by software and the software version used is often dependent on the age of the aircraft; not all software is identical. The information in this article refers to Software Version U10.8A. U10.8A is the version used by ProSim737 (other simulation avionics suites may differ). An earlier article discusses software variants.

Which Pilot Does What And When Is It Done

It is not uncommon for the pilot’s to share the task of setting up the CDU. Usually the pilot flying (PF) will enter parameters that are essential to flight, while the pilot not flying/monitoring (PM) will enter information pursuant to the route.

However, the hierarchy in a flight deck is that the Captain is the Pilot In Command (PIC), and it is assumed that the First Officer will complete most of the mundane, albeit important, navigation tasks leaving the Captain to deal with other matters.

CDU Verification and Cross Checking Procedure

The CDU is nothing more than a ‘glorified keypad’ and the maxim of ‘rubbish in rubbish out’ applies. Until execution (pressing the illuminated execute button on the keypad), none of the information entered into the CDU will be reflected in the FMC and FMS. Therefore, it is important that prior to execution, each pilot review and confirms the other’s inputs. Cross checking and verification minimises the chance that incorrect information has been entered.

At a minimum, a flight crew should compare the filed flight plan with the airways and waypoints entered on the ROUTE pages. The flight plan total distance and estimated fuel remaining at the destination should also be reviewed on the progress page of the CDU. If a discrepancy is noted, the LEGS page must be updated to ensure it is identical to the airways and waypoints in the filed flight plan. A cross check using the Navigation Display in PLAN mode and the CDU in STEP function (LEGS page) will aid is verification of the flight plan and in determining if there are any discontinuities that need to closed.

Taxi and Flight

Before taxi, the Captain or First Officer may make CDU entries. However, when possible, CDU entries should be made prior to taxi or when stopped. If CDU entries must be made during taxi, the pilot monitoring makes the entries and the pilot flying concentrates on steering the aircraft.

In flight, the pilot monitoring usually makes the CDU entries, however, the pilot flying may make simple CDU entries, but only when the workload allows. Essentially, the pilot flying concentrates on flying the aircraft and if they wish to enter data to the CDU, then the responsibly of flying the aircraft should be transferred to the First Officer.

The pilot flying is responsible for setting up the approach page in the CDU. To do this, the pilot flying will transfer command of the aircraft to the pilot not flying, and then make any amendments to the approach in the CDU. Upon completion, the command of the aircraft will be transferred and the pilot not flying will check the information.

Which Page in the CDU is Opened During Takeoff

The pilot flying usually will have the takeoff reference page displayed to enable the crew to have immediate access to V-speeds. This is to counter against the rare event that the V-speeds are inadvertently removed from the airspeed display on the Primary Flight Display (PFD) due to a display failure. Alternatively, the pilot flying may also elect (following the takeoff briefing in the Before Takeoff Procedure) to display the CLB page for takeoff.

The pilot monitoring normally displays the LEGS page during takeoff and departure to allow timely route modification if necessary.

CDU Sequence Flow

There are numerous ways to flow from one CDU function to another. The two commonly used methods are to use the Alpha Keys or the Line Select Keys (i.e. LSKL6). For example, LSKL6 refers to line select key left 6 or the sixth lower button on the left hand side.

As stated, the pilot flying will enter any information relevant to the takeoff of the aircraft, while the pilot not flying will enter information pertaining to the route of the aircraft (i.e. route, legs).

Bold CAPITALletters indicate that the command is an ALPHA menu key.

PILOT NOT FLYING (PM)

INIT REF / INDEX (LSKL6).
POS (LSL2) – Enter airport identifier into Ref Airport.
RTE or ROUTE (LSKR6) - Enter airport identifier (origin and destination), flight number (Flt No) and runway.
DEP ARR – Enter departure information (DEP LSKL1) - SID and runway.
LEGS– Enter airways, waypoints and navaids as required to a build a navigation route.
DEP ARR – Enter arrival information (ARR LSKL2) - STAR, approach, transition and runway.
On the EFIS, select PLAN and using the STEP function (LEGS Page) or PREV-NEXTPage, cycle through the waypoints checking the route on the Navigation Display. Check the route and close any route discontinuities. Return EFIS to MAP.
ACTIVATE (LSKL6) / EXECor RTE / ACTIVATE (LSKR6) / EXEC.

PILOT FLYING (PF)

INIT REF – Enter Zero Fuel Weight (ZFW), Fuel Reserves, Cost Index, Cruise Altitude (Crz Alt), Cruise Wind (Crz Wind), ISA Deviation (ISA Dev), Outside Air Temperature (T/C OAT) and Transition Altitude (Trans Alt).
N1 LIMIT (or LSKR6) – Enter Derates as desired.
LEGS / RTE DATA (LSKR6) – Enter wind (this determines fuel quantity on progress page). Note #1.
INIT REF / displays TAKEOFF REF page – Enter Flaps setting for departure and Centre of Gravity (Flaps and Trim). Go to page 2/2 and input data to various fields as and if required.
EXEC– Press illuminated execute key (this triggers the V-Speeds to be displayed on the TAKEOFF REF page).
To select V-Speeds, press Line Select keys beside each V-Speed to activate (LSR1, 2 & 3). Note #2.

Notes:

NOTE #1: Wind direction and speed (point 3) can be addedprior to or after the EXEC button has been selected. The flow sequence will alter dependent upon when this information is added. If the winds are not added, the flow will alter and TAKEOFF (LSKR6) will be selected instead of INIT REF.
NOTE #2: If V-Speeds on the Takeoff page are not displayed, it is because either the EXEC key has been pressed prior to the Takeoff Page being opened and data entered. If this occurs, cycle the QRH (LSKR6) on and off. The V-Speeds will then be displayed. Another reason that the V-Speeds may not be displayed is failure to input other essential pre-flight information.

There is often confusion to what the QRH designation means. When QRH is not selected (turned off) the V-Speeds will be automatically promulgated. If QRH is selected (turned on) the V-Speeds will be shown in green beside the appropriate line. This enables the flight crew to change the V-Speeds prior to executing them (note that ProSim-AR enables this to be altered in the IOS/Settings).

Additional Information

I usually do not link to outside resources, however, this U-Tube video from Mentour Pilot demonstrates the procedure quite well. Scroll to 0:31 seconds to begin video.

For those interested in reading more about how the CDU, FMC and FMS and how they interrelate concerning information input, Randy Walter from Smiths Industries has put together a very good article called Flight Management Systems.

Final Call

The CDU is an essential item that must be configured correctly if the aircraft’s internal navigation database is to be used. Likewise, LNAV or VNAV will only operate if the information has been entered into the CDU correctly.

The sequence you enter the information into the CDU is important, and although some latitude to the flow is accepted, a correct sequence flow will ensure all essential variables are inputted. Finally, cross verification of data, or any change to the data, ensures correct and accurate information is being entered.

Acronyms and Glossary

ALPHA Menu Key - Refers to the menu function keys.
CDU - Control Display Unit (the keypad).
FMC - Flight Management Computer.
FMS - Flight Management System.
LSK - Line Select Key. Used to enter lower level pages.
QRH - Quick Reference Handbook.

Download Flow Sequence

March 03, 2018

Wind Correction (WIND CORR) Function - CDU

March 03, 2018/ Flaps 2 Approach

OEM 737 CDU showing WIND CORR display in Approach Ref page

Wind Correction (WIND CORR)

The approach page in the CDU has a field named WIND CORR (Wind Correction Field or WCF).

WIND CORR can be used by a flight crew to alter the Vref + speed (speed additive) that is used by the autothrottle during the final approach. This is to take into account wind gusts and headwind that is greater than 5 knots.

Changing the Wind Correction to match increased headwind and gusts increases the safety margin that the autothrottle operates, and ensures that the autothrottle command a speed is not at Vref.

WIND CORR Explained

The algorithm of the autothottle includes a component that includes a speed additive. The speed additive is 1.23 times greater than the stall speed of the aircraft (at whatever flap setting). When the autothrottle is engaged, the speed additive is automatically added to Vref. This provides a safety buffer to ensure that the autothrottle does not command a speed equal to or lower than Vref. This added speed is usually 'bled off' during the flare ensuring landing is at Vref.

Although the autothrottle algorithm is a sophisticated piece of software, there is a time lag between when the sensors register a change in airspeed to when the physcial engines increase or decrease their spool (power). By having a speed additive (based on headwind and gust component) the speed of the aircraft (as commanded by the autothrottle) should not fall below Vref.

A Vref+ speed higher than +5 can be inputted when gusty or headwind conditions are above what are considered normal. By increasing the additive speed (+xx), the speed commanded by the autothrottle will not degrade to a speed lower than that inputted.

The default display is +5 knots. Changing this figure will alter how the algorithm calculates the command speed for the autothrottle; any change will be reflected in the LEGS page, however not in the APPROACH REF page.

The data entered into the Wind Correction field will only be used by the Flight Management System (FMS) when the aircraft is following an RNAV approach, or when using VNAV to fly an approach that has been manually constructed in the CDU. This is because these approach modes use the data from the FMS to fly the approach (as opposed to an ILS or other mode that doesn't use the FMS data).

If hand flying the aircraft, or executing another approach type, Wind Correction is advisory (you will need to add the speed additive (Vref+ xx knots) by mental mathematics).

Important Points:

Wind Correction is automatically added to Vref when flying an RNAV approach, or when using VNAV to fly an approach that has been manually constructed in the CDU.
Wind Correction is advisory for all other approach types or when manually flying an approach; +xx knots must be added to Vref by mental mathematics.

How To Use WIND CORR

The WIND CORR feature is straightforward to use.

Virtual CDU (ProSim737) showing the difference in landing speed with a Vref between a +5 and +13 Knot (Wind Correction) change. Vref altered from 152 knots to 160 knots

Navigate to the approach page in the CDU (press INIT REF key to open the Approach Reference page). Then double press the key adjacent to the required flaps for approach (for example, flaps 30). Double selecting the key causes the flap/speed setting to be automatically populated to the FLAP/SPD line.

Type the desired additive into the scratch pad of the CDU and up-select to the WIND CORR line. The revised speed will change the original Vref speed and take the headwind component into account. If you navigate to the LEGS page in the CDU, you will observe the change.

If the headwind is greater than 5 knots, then WIND CORR can be used to increase the additive from the default +5 knots to anything up to but not exceeding 20 knots.

It’s important to understand that the figure generated in the CDU is the Vref speed. This is the speed that the aircraft should be at when crossing the runway threshold or at a altitude of approximately 50 feet.

To this speed you must add the appropriate wind correction - either by mental mathematics or by using WIND CORR (if flying an FMS generated approach).

Boeing state that the +XX knots should be bled off during the flare procedure ensuring that touchdown speed is at Vref, however this rarely occurs in real life.

Recall from above, that any change using the Wind Correction field will have no bearing on calculations, unless the aircraft is being flown in RNAV / VNAV, or the approach has been manually constructed in the CDU.

For a full review on how to calculate wind speed, refer to this article: Crosswind landing Techniques - Calculations. A prompt sheet is displayed for quick reference.

Important Variables - Aircraft Weight and Fuel Burn

To obtain the most accurate Vref for landing, the weight of the aircraft must be known minus the fuel that has been consumed during the flight.

Fortunately, the Flight Management System updates this information in real-time and provides access to the information in the CDU. It's important that if an approach is lengthy (time consuming) and/or involves holds, the Vref data displayed on the CDU will not be up-to-date (assuming you calculated this at time of descent); the FLAPS/Vref display will show a different speed to that displayed in the FLAP/SPD display. To update this data, double press the key adjacent to the flaps/speed required and the information will update to the new speed.

How To Manually Calculate Fuel Burn

If wishing to manually calculate the final approach speed well before the approach commences, then it's necessary to manually calculate the fuel burn of the aircraft. Open the PROGRESS PAGE on the CDU and take note of the arrival fuel. Subtract this value from how much fuel you have in the tanks - this is the fuel burn (assuming all variables are constant).

Interestingly, the difference that fuel burn and aircraft weight can play in the final Vref speed can be substantial (assuming all variables, except fuel, are equal). To demonstrate:

Aircraft weight at 74.5 tonnes with fuel tanks 100% full – flaps/Vref 30/158.
Aircraft weight at 60.0 tonnes with fuel tanks 25% full – flaps/Vref 30/142.

Important Points:

During the approach, V speeds are important to maintain. A commanded speed that is below optimal can be dangerous, especially if the crew needs to conduct a go-around, or if winds suddenly increase or decrease. An increase or decrease in wind may cause pitch coupling.
If executing an RNAV Approach or using VNAV, it's important to update the WIND CORR field to the correct headwind speed based on wind conditions. This is because an RNAV approach and VNAV use the data from the Flight Management System (to which Wind Correction is added).
If an approach is lengthy (for example, during a STAR or when requested to hold), the Vref speed will need to be updated to take into account the fuel that has been used by the aircraft during the holding time.
Changing the WIND CORR speed in the CDU, does not alter the Vref speed displayed on the Primary Flight Display (PFD). Nor is the APPROACH REF page on the CDU updated. The change is only reflected in the LEGS page.
Boeing state that the speed additive should be 'bled off' during the flare so that the actual landing speed is Vref.

Autoland

Autolands are rarely done in the Boeing 737, however, if executing an autoland, the WIND CORR field is left as +5 knots (default). The autoland and autothrottle logic will command the correct approach and landing speed.

Simulated in Avionics Suite

WIND CORR may or may not be functional in the avionics software you use. Wind Correction is functional in the ProSim737 avionics suite.

Additonal Information

A very good video that discusses this in detail can be viewed at FlightDeck2Sim.

Acronyms

CDU – Control Display Unit
FMC – Flight Management Computer
FMS – Flight Management System (comprising the FMC and CDU)
Vref - The final approach speed is based on the reference landing speed
Vapp – Vapp is your approach speed, and is adjusted for any wind component you might have. You drop from Vapp to Vref usually by just going idle at a certain point in the flare

Updated 21 March 2022 (increased clarity)

April 26, 2017

Altitude and Speed Intervention Explained

April 26, 2017/ Flaps 2 Approach

The flight deck can be an extreme work environment, especially during the high-task descent and approach phase of the flight.

Altitude and Speed Intervention were designed to allow pilots to easily and quickly change either the altitude or speed of their aircraft without re-programming the FMC, disengaging VNAV, or spending excessive time 'heads down'.

The intervention buttons are strategically located on the MCP. When the buttons are selected, the aircraft's altitude or speed can be altered quickly on ‘the fly’.

In this article, I will examine the use of Altitude and Speed Intervention and demonstrate the use of these modes. In a follow-on article, I will discuss alternate methods that can be used to change altitude whilst maintaining Vertical Navigation. The reason for separating the two articles, is to avoid confusion that can develop between the different modes.

In this article I use the words Cruise Altitude (CRZ ALT) and Flight Level (FL) interchangeably. Also to avoid confusion the Control Display Unit (CDU) is the keypad used to interface with the Flight Mode Computer (FMC) that forms part of the Flight Management System (FMS).

I recommend reading the appropriate section in the Flight Crew Operations Manual (FCOM), Flight Crew Training Manual (FCTM) and the Cockpit Companion for a more thorough understanding.

Furthermore, whether intervention modes function in the simulator will depend upon which avionics suite and FMC software version is used. This article will deal only with ProSim-AR (ProSim737 avionics suite) which at the time of writing uses U10.8 A.

Important Points:

Altitude and Speed Intervention are company options that may or may not be ordered at the time of airframe purchase.
Altitude and Speed Intervention will only operate when a route has been programmed in the CDU, and is active. VNAV must be selected for either intervention mode to function.
Altitude and Speed Intervention is more often used when a temporary change in altitude and/or speed is required with a return to the original altitude/speed imminent.

MCP, VNAV & FMA Nomenclature and Displays

Prior to examining Altitude and Speed Intervention, it may be fruitful to quickly discuss common words that are used when describing the operation of VNAV and the MCP.

(i) CONDITION means that a mode will become active only when a condition(s) occurs;

(ii) ARM means that a mode is armed pending engagement;

(iii) ACTIVE means the mode is engaged/selected;

(iv) SELECT means to select or engage the mode (turn on); and,

(v) DESELECT means to deselect or disengage (turn off) the mode.

**Table 1:** FMA displays observed when Altitude and Speed Intervention is engaged

An often misunderstood facet of the MCP is that the annunciators illuminate to indicate a particular mode is active. This is not entirely correct, as the presence of an illuminated annunciator (light) does not always indicate whether a mode is active or not.

For example, the VNAV annunciator on the MCP will remain illuminated when VNAV is either active or armed. Furthermore, active modes that are not able to be deselected, do not display an illuminated annunciator.

To determine whether a mode is active or not, the Flight Mode Annunciator (FMA) should be consulted. The FMA is located above the Primary Flight Display (PFD) and displays various alerts and status messages.

Refer to Table 1 (download button at bottom of article) for a synopsis regarding the various displays that the FMA will generate when intervention is used.

Important Points:

A mode change highlight symbol (green rectangle) is displayed around the command name, in the Flight Mode Annunciator (FMA), whenever a mode has been armed and is about to become active. The green rectangle will remain displayed for a period of 10 seconds.
It’s prudent to cross reference between the FMA, MCP and CDU to determine what mode is armed or active at a given time.
Altitude and Speed Intervention, when active, will take precedence over VNAV, although VNAV will remain armed.

Scenario

The aircraft is flying at FL150 (15,000 feet) at 275 kias. The FMS has an active route (Company Route) that includes altitude and speed constraints (in the LEGS page of the CDU).

In level flight, with autopilot, LNAV and VNAV selected, the following will be observed:

(i) LNAV and VNAV will be active;

(ii) The FMA will display MCP SPD / LNAV / VNAV PTH or VNAV ALT;

(iii) The annunciators on the MCP - LNAV, VNAV & CMD A/B will be illuminated;

(iv) The speed window located on the MCP will be blank (no speed displayed); and,

(v) LNAV/VNAV will be displayed in white text on the PFD.

LNAV will be controlling the lateral navigation of the aircraft while VNAV will be controlling the speed and vertical altitude of the aircraft.

ATC request a decrease in speed from 275 kias to 240 kias.

Speed Intervention (SPD INTV)

Select (press) the SPD INTV button on the MCP. The MCP speed window becomes active and displays the current speed of 275 kias. Dial into the speed window on the MCP the new speed requirement of 240 kias.

Notice the speed indicator display above the speed tape on the PFD has changed from 275 kias to the new speed of 240 kias. Also note that the VNAV annunciator light on the MCP remains illuminated - in this case VNAV is active. The speed of the aircraft will be reduced to 240 kias.

If you cross check with the Cruise Altitude in the CDU (CRZ ALT key/TGT SPD), the CDU will still indicate the original cruise speed of 275 kias. This is because the speed is an intervention speed and, as such, will not have been updated in the FMC.

If you wish to stay at this speed (240 kias), you will need to manually change the cruise speed to 240 kias in the CDU. However, in this case the reduction in speed is momentary, and ATC advise you to return to your original speed.

Returning to Original Speed

Press the SPD INTV button (or unselect and reselect VNAV on the MCP). Doing this, will return the speed to the original speed (275 kias). It will also change the speed indication on the PFD from 240 kias back to 275 kias. The MCP speed window will become blank (no speed displayed) to indicate the VNAV is the controlling mode.

Important Point:

When SPD INTV is active, the FMA will display MCP SPD. When SPD INTV is not active (deselected) the FMA will revert to FMC SPD.

Altitude Intervention (ALT INTV)

Altitude Intervention is slightly more sophisticated in comparison to Speed Intervention. This is because, amongst other factors, the relationship changes dependent on whether the aircraft is ascending or descending, and whether there are active restrictions (constraints) programmed for waypoints (U10.8.A).

In level flight, with autopilot, LNAV and VNAV engaged, the following will be observed:

(i) LNAV and VNAV will be active;

(ii) The FMA will display FMC SPD / LNAV / VNAV PTH;

(iii) The annunciators on the MCP - LNAV, VNAV & CMD A/B will be illuminated;

(iv) The speed window located on the MCP will be blank (no speed displayed); and,

(v) LNAV/VNAV will be displayed in white text on the PFD.

ATC request a descent from FL150 to FL120.

DESCENT Using ALT INTV (descent from FL150 to FL120)

Dial into the altitude window on the MCP the new altitude (FL120).

CDU cruise page showing 12000 in scratch pad. Selecting line select 1 left (LS1L) will update the CDU to the new Flight Level

Notice the altitude display above the altitude tape on the PFD has changed from FL150 to the new altitude of FL120. Also note that the VNAV annunciator light on the MCP remains illuminated - in this case VNAV is armed. ALT INTV takes precedence over VNAV.

Select (press) ALT INTV button on the MCP and the FMA will annunciate FMC SPD / LNAV / VNAV PTH. The aircraft will descend at 1000 fpm (default descent speed) until FL120 is reached.

If you cross-check the Cruise Altitude in the CDU (INIT PERF/PERF/CRZ ALT or CRZ key/CRZ ALT), it will display the original Cruise Altitude of FL150. The FMC has NOT automatically updated the Flight Level to the lower altitude – this is normal and not a fault.

If you want to remain at FL120, you will need to manually update the Cruise Altitude in the CDU (INIT PERF/PERF/CRZ ALT), or (CRZ key/CRZ ALT) and press the EXEC key.

Important Points:

When the CDU page is open on CRZ (CRZ key), it will display in the scratch pad any change to the altitude in the MCP. This provides a ‘shortcut’ to insert the new flight level should it be desired to make it permanent. All that is needed is to press the CRZ/CRZ ALT (in the CRZ page) and the FMC cruise altitude will be updated. The altitude in the LEGS page will also be updated.
By default, Altitude Intervention will always maintain a vertical descent at 1000 fpm.

Returning to Original Flight Level

To return to the original Flight Level (FL150), dial into the MCP the previous Flight Level (FL150) and press ALT INTV. The aircraft will ascend to FL150.

Important Points:

The FMC will NOT automatically update the Flight Level to the lower altitude. If desired, this will need to be done manually.
When returning to the original Flight Level, VNAV will not engage unless the original Flight Level (FL150) is dialled into the altitude window of the MCP. For VNAV to be active, the Cruise Altitude in the CDU and the altitude set in the MCP must be identical.
ALT INTV takes precedence over VNAV. The VNAV annunciator on the MCP will remain illuminated and VNAV will be in armed mode (when ALT INTV is selected).
To determine if VNAV is the active mode (or not) the FMA display must be consulted – not the annunciator light on the MCP.
U10.8A bring some important changes from earlier U releases. If there are no altitude restrictions, pressing ALT INTV will automatically update the altitude in the CDU to the lower selected altitude. However, if an altitude restriction is present the lower altitude will not be updated.

ASCENT Using ALT INTV (ascent from FL120 to FL150)

The ALT INTV button operates a little differently when you ascend. For a start, it automatically replaces (updates) the Flight Level (CRZ ALT) in the CDU. It will also update the altitude in the LEGS page in the CDU.

The FMA will annunciate N1 / LNAV / VNAV SPD during the climb phase of the flight, changing to FMC SPD / LNAV / VNAV PTH when the new flight level is reached. When climbing using ALT INTV, the thrust mode uses N1.

Important Points:

When a Flight Level of a higher altitude is dialled into the altitude window and ALT INTV selected, the new Flight Level will be updated in the CDU.
U10.8A bring some important changes from earlier U releases. If the selected MCP altitude is BELOW any altitude restriction, then that restriction will be DELETED. Also, altitude restrictions will be DELETED if they are between the current altitude and the selected MCP altitude (when ALT INTV is pressed).
If ascent and descent do not function correctly. In the first instance consult the FMS software for the U version in service.

Considerations When Using ALT INTV

When using ALT INTV, several variables that relate to the altitude constraint (s) will change, depending upon whether you are in VNAV climb, cruise or descent. Rather than rephrase what already has been written, I have scanned the appropriate page (below) from the Cockpit Companion written by Bill Bulfer.

Using ALT INTV and SPD INTV During a VNAV Approach Phase

ALT INTV is a very handy tool, if during an VNAV approach, the flight crew fail to change the altitude in the MCP to the next lowest altitude constraint.

To demonstrate, the aircraft is flying a published STAR that will join an VNAV approach. VNAV and LNAV are active and the flight plan has several altitude and speed constraints. To meet these constraints, the crew must update the MCP altitude to the next lowest altitude (displayed in the LEGS page of the CDU) prior to the aircraft crossing the constraint.

If the crew fail to update the MCP to the next lowest altitude constraint, then the aircraft will transition from descending flight (VNAV PTH) to level flight (VNAV ALT). In this situation a crew could engage LVL CHG or V/S, however, doing so would deselect VNAV.

A simpler solution is to change the altitude in the MCP window to the next lowest altitude constraint (or MDA) and press ALT INTV. This will command VNAV to descend the aircraft, at a variable descent rate, to meet the required constraint. By using ALT INTV, the aircraft will remain in VNAV.

Additionally, SPD INTV is a straightforward way to control the speed of the aircraft during the approach while maintaining VNAV. Company policy at some airlines insist that Speed Intervention be used approximately 2 nautical miles from of the Final Approach Fix (FAF).

Reliability of ALT INTV in Descent Mode - ProSim-AR

ProSim-AR (Version 1.49) exhibits difficulty in holding a lower altitude level when ALT INTV is used.

The Boeing system is designed that once the V-Path is intercepted, the Flight Director (FD) cross hairs maintain the new altitude by pitch. In ProSim-AR this pitch is often difficult to hold and a resultant pitching of the aircraft (up and down) occurs as the system attempts to hold the lower altitude. When using LVL CHG or V/S this does not occur. Note that this behaviour does not occur when using INTV ALT to ascend.

It is not certain if this behaviour is common only to my system or is more widespread; but a way to solve the issue is to either:

(i) Use an alternate descent mode; or,

(ii) Manually change the altitude values in the CDU (INDEX/PERF/CRZ ALT), or (CRZ key/CRZ ALT) and press EXEC.

Procedure (ii) manually changes the Cruise Altitude (CRZ ALT) to the lower altitude in the CDU. This causes the command logic to switch from the logic that commands Altitude Intervention to the logic that commands altitude in thr FMC. The aircraft will not pitch and will be stable.

The developers at ProSim-AR are continually tweaking these variables. In future software releases (post version 221.b12) this issue may well be rectified.

Final Call

There are many of reasons an aircraft will need to alter altitude and/or speed; be it to divert around a localized weather event, or to abide by an Air Traffic Control directive. Whatever the reason, often the changes are short-lived and a return to the original altitude/speed constraint imminent.

In these situations Altitude and Speed Intervention enable the aircraft to easily and quickly transition between Flight Level changes whilst VNAV is active. Furthermore, the use if this functionality can minimise the time spent in the ‘heads down’ position during the high-task descent and approach phase of a flight.

In this article, I have explained the Altitude and Speed Intervention functionality of the Boeing 737. I also have documented "work-arounds" should VNAV not function as anticipated.

Acronyms and Glossary

Annunciator - A push button to engage a particular mode – often has a light that illuminates
ALT INTV - Altitude Intervention
CDU – Control Display Unit (display screen and keyboard to input data into the FMC)
Flight Level – Altitude that the aircraft will fly at (set in FMC)
FMA – Flight Mode Annunciator
FMC – Flight Management Computer (part of the Flight Management System)
FMS – Flight Management System
LNAV – Lateral Navigation
MCP – Mode Control Panel
PFD – Primary Flight Display
SPD INTV - Speed Intervention
VNAV – Vertical Navigation

Download Table 1

July 19, 2016

VNAV 'Gotchas' - Avoiding Unwanted Level-Offs

July 19, 2016/ Flaps 2 Approach

One aspect of using VNAV during published instrument departures, arrivals, and approaches is that it can cause unnecessary level-offs.

These level-offs can cause engines to spool needlessly, increase fuel cost and stagger a Continuous Descent Final Approach (CDFA) such as when executing an RNAV approach.

It is not only domestic airliners that must meet altitude constraints; military aircraft also must meet the same requirements when landing at a non-military airport (click to enlarge). Image is copyright xairforces.net. For those interested in flying the Wedgetail, there is a model available for ProSim-AR users on their forum page.

To avoid this, and ensure that minimum altitude constraints are met, two techniques can be used.

METHOD 1: Constraints Are Not Closely Spaced.

This technique is normally used when waypoints with altitude constraints are not closely spaced (in other words, there is a moderate distance between altitude constraints).

During climbs, the maximum or hard altitude constraints should be set in the Mode Control Panel (MCP).

Minimum crossing altitudes need not be set in the MCP as the FMC message function will alert the crew if these constraints cannot be satisfied.

During descent, the MCP altitude is set to the next constraint or clearance altitude, whichever will be reached first.

Immediately prior to reaching the constraint, when compliance with the constraint is assured, and when cleared to the next constraint, the MCP altitude is reset to the next constraint/altitude level.

METHOD 2: Constraints Are Closely Spaced.

Where constraints are closely spaced to the extent that crew workload is adversely affected, and unwanted level-offs maybe a concern, the following is approved:

For departures, set the highest of the closely-spaced constraints.

For arrivals, initially set the lowest of the closely-spaced altitude constraints or the Final Approach Fix (FAF) altitude, whichever is higher.

IMPORTANT: When using either technique, the FMS generated path should be checked against each altitude constraint displayed in the CDU to ensure that the path complies with all constraints. Furthermore, the selection of a pitch mode other than VNAV PTH or VNAV SPD should be avoided, as this will result in the potential violation of altitude constraints.

To enlarge more on VNAV is beyond the scope of this post. A future post will address this topic in more detail.

Crew Controls Automation - Not Vice Versa

However, the system is only as good as the knowledge of the person pushing the buttons. It is very important that a flight crew control the automation rather than the automation control the flight crew.

If VNAV begins to do something that is unplanned or unexpected, do not spend precious time ‘thinking about the reasons why’ – disconnect VNAV and use a more traditional method or hand floy the aircraft. Then, determine why VNAV did what it did. The most common comment heard in today's modern cockpits is ‘What is it doing now…’

Final Call

VNAV is an easy concept to understand, but it can be confusing due to innumerable variables associated with vertical navigation. VNAV is probably one of the more complicated systems that virtual and real pilots alike have to understand. When using VNAV it is paramount to maintain vigilance on what it is doing at any one time, especially during descent and final approach. Furthermore, it is good airmanship to always have a redundancy plan in place – a ‘what if’ should VNAV fail to do what was anticipated.

The below article also discuss VNAV:

Speed Intervention (SPD INTV) and VNAV Use

An interesting article concerning VNAV:

Cognitive Engineering Analysis of the Use of VNAV (.pdf)

Acronyms and Glossary

CDU - Control Display Unit (aka FMC)
FAF – Final Approach Fix
FMC - Flight Management Computer
FMS - Flight Management System. Supply of data to the FMC and CDU
Gotcha - An annoying or unfavorable feature of a product or item that has not been fully disclosed or is not obvious.
LNAV – Lateral Navigation
MCP – Mode Control Panel
NPA - Non Precision Approach
VNAV – Vertical Navigation
VNAV PTH – Vertical Navigation Path
VNAV SPD – Vertical Navigation Speed

July 09, 2016

RNAV Approaches

July 09, 2016/ Flaps 2 Approach

RNAV 07 L - one of several RNAV approach charts for Los Angeles International Airport (LAX). The most important aspect of an RNAV approach is that it is a Non-Precision Approach (NPA). Note the word GPS is written in the title of the approach plate

My previous post provided of overview on RNAV and RNP navigatio n. This article will explain what a RNAV approach is, provide incite to the operational requirements, and discuss the approach. I will also briefly discuss Approach Procedures and Vertical Guidance (APV) and RNP/ANP values.

The operational criteria for RNAV approaches is complicated and not easy to explain. There are a number of RNAV approaches (often different for differing areas of the globe) and each is defined by the accuracy of the equipment used in the execution of the approach. As such, this article is not all encompassing and I encourage you to read other technical articles available on this website and elsewhere.

RNAV Approaches - Background Information

The Global Positioning System (GPS) is the brand name owned by the US military. Initially all RNAV approaches were GPS orientated, however, in recent years this has changed to include Global Navigation Satellite System (GNSS) applications. GNSS applications are not owned (or controlled) by the US military. As such, an RNAV approach chart uses the words GPS and GNSS interchangeably.

What is an RNAV Approach

The definition for an RNAV approach is 'an instrument approach procedure that relies on the aircraft's area navigation equipment for navigational purposes'. In other words, a RNAV approach is any non ILS instrument-style approach that does not require the use of terrestrial navigation aids such as VOR, NDB, DME, etc.

Rather than obtain navigational information directly from land-based navigational applications, the aids for the approach are obtained from a published route contained within the aircraft's Flight Management System (FMS) and accessible to the crew through the Control Display Unit (CDU). Broadly speaking, the approach uses signals, that are beamed from navigational satellites orbiting the Earth, and compares this data with the information from the FMC navigation database.

All Boeing Flight Management Systems (FMS) are RNAV compliant and have the ability to execute an RNAV approach.

Important Point:

An RNAV approach is classified as a Non-Precision Approach (NPA).

Non-Precision Approaches (NPA)

Before writing further, a very brief overview of Non-Precision Approaches is warranted.

There are three ways to execute a Non-Precision Approach.

(i) IAN (integrated Approach Navigation). IAN is a airline customer option and makes a NPA similar to an ILS approach. A separate article has been written that addresses IAN.

(ii) Vertical Speed (V/S). V/S is not normally used when flying a RNAV approach that uses positional information from the aircraft's database. However, V/S can be used for other Non-Precision Approaches and to transition to a RNAV approach.

(iii) VNAV (Vertical Navigation). VNAV is the preferred method to execute an NPA (provided the approach is part of the FMS database).

(iv) LNAV (Lateral Navigation). LNAV is mandatory for all approaches that are GPS/GNSS/RNP based.

RNAV Approach Types

The following are RNAV approaches:

(i) RNAV (GPS) approach;

(ii) RVAV (RNP) approach;

(iii) RVAV (RNP) AR approach; and,

(iv) RNAV (GNSS) approach.

The RNAV (GNSS) approach can further be sub-divided into an additional three possible types of approach, each identified by a different minima. These approaches are:

(i) RNAV (GNSS) LNAV;

(ii) APV Baro VNAV approach;

(iii) APV SBAS approach.

It's easy to become confused by the various types of RNAV approaches, however, the actual flying of a RNAV approach does not differ greatly between each approach type. The main difference lies in the level of accuracy required for the approach to be flown.

Approach Procedures with Vertical Guidance (APV)

APV refers to any approach which has been designed to provide vertical guidance to a Decision Height (DH). An APV approach is characterised by a constant descent flight path, a stable airspeed, and a stable rate of descent. This type of approach rely upon Performance Based Navigation (PBN).

The difference between the two APV approaches (ii and iii mentioned above) is that an APV Baro VNAV approach uses barometric altitude information and data from the FMS database to compute vertical guidance. in contrast the APV SBAS approach uses satellite based augmentation systems, such as WAAS in the US and Canada and EGNOS in Europe, to determine lateral and vertical guidance.

I will now discuss the RNAV (GNSS & RNP) approach.

Flying The RNAV (GNSS) Approach

The RNAV (GNSS) approach is designed to be flown with the autopilot engaged. The recommended roll mode is LNAV or HDG SEL. The preferred method for pitch is VNAV. If LNAV and VNAV are engaged, the aircraft will fly the lateral and vertical path as determined by the FMS database; the route is displayed in the LEGS page of the CDU.

The aircraft uses the FMS database to determine its lateral and vertical path. As such, it is very important that the RAW data published in the navigational database is not altered by the flight crew. Furthermore, the data presented in the CDU should be cross-checked with the data on the approach chart to ensure it is identical.

As discussed previously, an RNAV (GNSS) approach is classified as a Non-Precision Approach. Therefore, minima is at the Minimum Descent Altitude (MDA). It is good airmanship to add +50 feet to the MDA to reduce the chance of descending through the MDA. If a RNAV (RNP) or APV approach is being flown, the minima changes from a MDA to a Decision Height (DH). Whatever the requirement, the minima will be annotated on the approach chart.

LIDO chart (Lufthansa Systems) depicting the RNAV (RNP) 01 approach into BNE-YBBN (Brisbane Australia). Note that this chart has a Decision Altitude (DA) rather than a Minimum Descent Altitude (MDA). Chart courtesy of NaviGraph

RNAV (RNP) Approaches

RNP stands for Required Navigation Performance which means that specific navigational requirements must be met prior to and during the execution of the approach.

There are two types of RNAV (RNP) approaches:

(i) RNAV (RNP) approach; and,

(ii) RNAV (RNP) AR approach.

Both approaches are similar to a RNAV (GNSS) approach, however, a RNAV (RNP) approach, through the use of various sensors and equipment, achieves far greater accuracy through the use of Performance Based Navigation (PBN ), and can therefore be flown to a DA rather than a MDA.

RNP/ANP - How It Works

An RNAV (RNP) approach compares the position that the aircraft should be in with the actual position of the aircraft. If this value exceeds the prescribed distance (RNP exceeds ANP), the approach must be aborted. The use of RNP/ANP enables greater accuracy in determining the position of the aircraft.

RNP/ANP Alerts

If an anomaly occurs between RNP and ANP one of two RNP alerts will be generated:

(i) VERIFY POSITION - displayed in the scratchpad of the CDU; or,

(ii) UNABLE REQD NAV PERF-RNP - displayed on the Navigation Display (ND) (if EFIS is set to MAP).

It should be noted that different versions of CDU software will generate different alerts. This is because newer software takes into account advances in PBN. To determine which software version is in use, press IDENT from the CDU main page (LSK1L) and check OP PROGRAM. ProSim-AR uses U10-8a.

The variables for RNP/ANP can be viewed in the CDU in the POS REF page (page 3), the LEGS page when a route is active, and also on the Navigation Display (ND).

A second type of RNP approach is the RNAV (RNP) AR approach. This approach enables you to have curved flight paths into airports surrounded by terrain and other obstacles. Hence why special aircraft and aircrew authorization (AR) is required for these approaches. Other than AR and additional flight crew training, the approach is identical to the RNAV (RNP) approach.

Advantages of RNAV and RNAV (RNP) Approaches

The benefit of using an RNAV approach over a traditional step-down approach is that the aircraft can maintain a constant angle (Continuous Descent Final Approach (CDFA)) until reaching minima. This has positive benefits to fuel savings, engine life, passenger comfort, situational awareness, and also lowers flight crew stress (no step-downs to be followed). Additionally, it also minimises Flight Into Terrain (CFIT) events.

A further advantage is that the minimas for an RNAV approach are more flexible than those published for a standard Non-Precision Approach not using RNAV. RNAV approach charts have differing descent minima depending upon the type of RNAV approach.

For example, if flying a RNAV (RNP) approach the MDA is replaced by a DH. This enables a lower altitude to be flown prior to a mandatory go-around if the runway threshold is not in sight. The reason that a RNAV (RNP) approach has a DH rather than a MDA (and its resulting lower altitude constraint) is the far greater accuracy achieved through the use of Performance Based Navigation (PBN).

Approach To Land Using RNAV

The following addresses the basics of what is required to execute an RNAV approach.

Prior to beginning the approach, the crew must brief for the approach and complete ant required preparation. This includes, but is not limited to, the following items:

(i) Equipment must be operational prior to starting the approach;

(ii) Selection of the approach procedure, normally without modifications from the aircraft's navigation database (CDU);

(iii) For airplanes without Navigation Performance Scales (NPS), the map display should be set to the 10 NM or less range. This is to monitor path tracking during the final approach Segment and provide greater navigational awareness;

(iv) For airplanes with NPS, the map display range may be set to whatever distance is desired;

(v) TERR display must be selected on either the Captain or First Officer side of the ND;

(vi) For airplanes without Navigation Performance Scales (NPS), the RNP progress page on the CDU should be displayed. For airplanes equipped with NPS, selection of the CDU page is at the crew's discretion;

(vii) The navigation radios must be set according to the type of approach; and,

(viii) There must be no alerts generated (UNABLE REQD NAV PERF and/or VERIFY POSITION).

In addition to the above, airline Standard Operational Procedures (SOPs) may require additional caveats. For example, the setting of range rings on the ND to provide enhanced situational awareness at specific points (range rings can be set on the FIX page in the CDU).

Important Points:

Select the approach procedure from the arrivals page of the CDU and cross-check this data with that published on the approach chart, especially the altitude constraints and the Glide Path (GP).
If the Initial Approach Fix (IAF) in the CDU has an ‘at or above’ altitude restriction, this may be changed to an ‘at’ altitude restriction that uses the same altitude. Speed modifications (using speed intervention) are allowed as long as the maximum published speed is not exceeded. No other lateral or vertical modifications should be made at or after the IAF.

Beginning the Approach

Select LNAV no later than the IAF. If on radar vectors, select LNAV when established on an intercept heading to the final approach course. VNAV PTH must be engaged and annotated in the Flight Mode Annunciator (FMA) for all segments that contain a Glide Path (GP) angle, as shown on the LEGS page, and must be selected no later than the Final Approach Fix (FAF) or published glide path intercept point.

Speed Intervention (INTV), if desired, can be used prior to the GP. Good airmanship directs that the next lower altitude constraint is dialled into the MCP altitude window as the aircraft passes through the previous constraint. When 300 feet below the Missed Approach Altitude (MAA) re-set the altitude window in the MCP to the MAA.

Final Approach using RNAV

When initiating descent on the final approach path (the GP), select landing flaps, slow to final approach speed, and do the landing checklist. Speed limits published on the approach chart must be complied with to enable adequate bank angle margins.

At minima, or as directed by the airline's SOP, the autopilot followed by the autothrottle is disconnected and a visual 'hands on' approach made to the runway threshold.

Once established on final approach, a RNAV approach is flown like any other approach.

Final Call

The Boeing aircraft is capable of several types of Non-Precision Approaches, however, outside the use of ILS and possibly IAN, the RNAV approach enables an accurate glide path to be followed to minima. While it's true that the differing types of RNAV approaches can be confusing due to their close relationship, the approach is straightforward to fly.

This short article is but a primer to understanding an RNAV approach. Further information can be found in the FCTM, FCOM and airlines SOP.

In my next article we will look some of the possible 'gotchas' that can occur when using VNAV.

References

Flight Crew Training Manual (FCTM), Flight Crew Operations Manual (FCOM) and airline SOP.

Acronyms and Glossary

Annunciator – Often called a korry, it is a light that illuminates when a specific condition is met
ANP - Actual Navigation Position
APV - Approach Procedure with Vertical Guidance
CFIT - Continuous Flight Into Terrain
DME – Distance Measuring Equipment
FAF - Final Approach Fix
FCOM - Flight Crew Operations Manual (Boeing)
FCTM - Flight Crew Training Manual (Boeing)
FMA - Flight Mode Annunciator
FMC – Flight Management Computer
FMS – Flight Management System
Gotcha- An unfavorable feature of a product or item that has not been fully disclosed or is not obvious.
GPS – Global Positioning System
GNSS - Global Navigation Satellite System
IAF - Initial Approach Fix
Korry - See annunciator
LNAV – Lateral Navigation
LPV - Localizer Performance with Vertical Guidance
MAA - Missed Approach Altitude
MCP – Mode Control Panel
ND – Navigation Display
NPA - Non Precision Approach
PBN - Performance Based Navigation
RNAV – Area Navigation
RNP - Required Navigation Performance
SOP - Airline Standard Operational Procedure. A manual that provides additional information to the FCTM and FCOM
SBAS - Satellite based augmentation systems. In the U.S. called WAAS and Europe called EGNOS.
VNAV – Vertical Navigation
VNAV PTH – Vertical Navigation Path
VNAV SPD – Vertical Navigation Speed
VOR – VHF Omni Directional Radio Range

Updated 11 November 2021

June 23, 2016

RNAV, RNP, LNAV and VNAV Operations - Overview

June 23, 2016/ Flaps 2 Approach

Collins Mode Control Panel (MCP) showing lnav and vnav buttons

New flyers to the Boeing 737NG often become confused understanding the various terminology used with modern on-board navigational systems.

Although the concepts are easy to understand, the inter-relationship between systems can become blurred when the various types of approaches and departures are incorporated into the navigational system.

This post will not provide an in-depth review of these systems; such a review would be lengthy, confusing and counterproductive to a new virtual flyer. Rather, this post will be a ‘grass-roots’ introduction to the concept of RNAV, RNP, LNAV and VNAV. I will also touch on the concept of Performance Based Navigation (PBN).

In the Beginning there was RNAV

RNAV is is an acronym for Area Navigation (aRea NAVigation).

Prior to complex computers, pilots were required to use established on-the-ground navigational aids and would fly directly over the navaid. Such a navaid may be a VOR, NDB or similar device. Flying over the various navaids was to ensure that the flight was on the correct route. Often this entailed a zigzag course as navaids could not be perfectly aligned with each other in a straight line - airport to airport.

When computers entered the aviation world it became possible for the computer to 'create' an imaginary navigation aid based on a direction and distance from a ground-based navaid. Therefore, a straight line could be virtually drawn from your origin to destination and several waypoints could be generated along this line. The waypoints were calculated by the computer based on ground VORs and positioned in such a way to ensure more or less straight-line navigation.

In essence, RNAV can be loosely defined as any 'straight line' navigation method similar to GPS that allows the aircraft to fly on any desired path within the coverage of referenced NAVAIDS.

Required Navigation Performance (RNP) and Performance Based Navigation (PBN)

Simply explained, Required Navigation Performance (RNP) is a term that encompasses the practical application of advanced RNAV concepts using Global Navigation Satellite Systems (GNSS).

However, there is a slight difference between RNP and RNAV although the principles of both systems are very similar.

RNAV airspace generally mandates a certain level of equipment and assumes you have a 95% chance of keeping to a stated level of navigation accuracy. On the other hand, RNP is performance based and requires a level of on-board performance monitoring and alerting. This concept is called Performance Based Navigation (PBN).

RNAV and RNP both state a 0.95 probability of staying within 1 nm of course. But RNP (through PBN) will let you know when the probability of you staying within 2 nm of that position goes below 0.99999. In essence, RNP and PBN enable an aircraft to fly through airspace with a higher degree of positional accuracy for a consistently greater period of time.

To achieve this level of accuracy a selection of navigation sensors and equipment is used to meet the performance requirements. A further enhancement of this concept is the use of RNP/ANP (Required Navigation Performance and Actual Navigation Performance. Advanced RNAV concepts use this comparative analysis to determine the level or error between the required navigation (the expected path of the aircraft) and the actual navigation (what path the aircraft is flying.) This information is then displayed to the flight crew.

LNAV and VNAV

LNAV and VNAV are parts of the Flight Guidance System, and are acronyms for Lateral Navigation and Vertical Navigation'. Both these functions form part of the automation package that the B737NG is fitted with.

LNAV is the route you fly over the ground. The plane may be using VORs, GPS, DME, or any combination of the above. It's all transparent to the pilot, as the route specified in the clearance and flight plan is loaded into the Flight Management System (FMS), of which the Flight Management Computer (FMC) is the interface.

The route shows up as a magenta line on the Navigation Display (ND), and as long as the LNAV mode on the Mode Control Panel (MCP) is engaged and the autopilot activated, the aircraft will follow that line across the ground. LNAV however, does not tell the plane what altitude to fly, VNAV does this.

VNAV is where the specified altitudes at particular waypoints are entered into the FMS, and the computer determines the best way to accomplish what you want. The inputs from VNAV are followed whenever the autopilot is engaged (assuming VNAV is also engaged).

The flight crew can, if necessary alter the VNAV constraints by changing the descent speed and the altitude that the aircraft will cross a particular waypoint, and the computer will re-calculate where to bring the throttles to idle thrust and begin the descent, to allow the aircraft to cross the waypoint, usually in the most economical manner.

VNAV will also function in climb and take into account airspeed restrictions at various altitudes and will fly the aircraft at the desired power setting and angle (angle of attack) to achieve the speed (and efficiency) desired.

There is not a fast rule to whether a flight crew will fly with LNAV and VNAV engaged or not; however, with LNAV and VNAV engaged and the autopilot not engaged, LNAV and VNAV will send their signals to the Flight Director (F/D) allowing the crew to follow the F/D cue display and hand fly the aircraft the way the autopilot would if it were engaged.

Reliance on MCP Annunciators

Flight Mode Annunciator (FMA) showing LNAV and VNAV Path Mode engaged. The Flight Director provides a visual cue to the attitude of the aircraft while the speed is controlled by the the FMC. CMD indicates that the autopilot is engaged (ProSim737 avionics suite)

LNAV and VNAV have dedicated annunciators located on the Mode Control Panel (MCP). These annunciators illuminate to indicate whether a particular mode is engaged.

However, reliance on the MCP annunciators to inform you of a mode’s status is not recommended. Rather, the Flight Mode Annunciator (FMA) which forms part of the upper area of the Primary Flight Display (PFD) should be used to determine which modes are engaged. Using the FMA will eliminate any confusion to whether VNAV (or any other function) is engaged or not.

This post explains the Flight Mode Annunciators (FMA) in more detail.

Final Call

RNAV is a method of area navigation that was derived from the use of VOR, NDBs and other navaids. RNP through it use of GNSS systems has enabled Area Navigation to evolve to include LNAV and VNAV which are sub-systems of the Flight Guidance System - LNAV is the course across the ground, and VNAV is the flight path vertically.

Historically, navigation has been achieved successfully by other methods, however, the computer can almost always do things better, smoother and a little easier – this translates to less workload on a flight crew.

In my next post, we will discuss RNAV approaches and how they relate to what has been discussed above.

References

The information for this article came from an online reference for real-world pilots.

Acronyms and Glossary

Annunciator – Often called a korry, it is a light that illuminates when a specific condition is met
DME – Distance Measuring Equipment
FMA - Flight Mode Annunciator
FMC – Flight Management Computer
FMS – Flight Management System
GPS – Global Positioning System
GNSS - Global Navigation Satellite System
LNAV – Lateral Navigation
MCP – Mode Control Panel
ND – Navigation Display
NPA - Non Precision Approach
PBN - Performance-based Navigation
RNAV – Area Navigation
RNP - Required Navigation Performance
VNAV – Vertical Navigation
VNAV PTH – Vertical Navigation Path
VNAV SPD – Vertical Navigation Speed
VOR – VHF Omni Directional Radio Range

November 06, 2015

Control Wheel Steering (CWS) Explained

November 06, 2015/ Flaps 2 Approach

Collins 737 Mode Control Panel (MCP) showing location of CWS buttons on Collins MCP. The CMD and CWS buttons are located on the First Officer side of the MCP. Each of the four press to engage buttons has a green annunciator which illuminates when the mode is engage

CWS is an acronym for Control Wheel Steering. Broadly speaking, it is a sub-set of the autopilot system which can used on either System A or B. When engaged, CWS maneuvers the aircraft in response to control pressures applied to the control wheel or column.

The control pressure is similar to that required for manual flight. When control pressure is released, the autopilot holds the existing attitude until CWS is disengaged, or the autopilot is engaged.

The Flight Crew Training Manual (FCTM) states:

‘Control Wheel Steering (CWS) may be used to reduce pilot workload. Follow the manually flown procedure but instead of disengaging the autopilot, engage CWS.’

CWS is a similar system to the ‘Fly By Wire’ system utilised by Airbus.

Advantages of CWS

The control pressures on the flight controls are in the order of 37 pounds push/pull value +- 3 pounds and continually applying this pressure for a protracted period of time can be tiring. As such, an obvious advantage of using CWS is that you do not have to continually apply positive pressure to the flight controls to maintain a set pitch or roll attitude.

CWS enables you to fly the aircraft using the flight controls rather than turning the heading knob on the Mode Control Panel (MCP) or configuring other modes such as Level Change, Vertical Speed, VNAV, etc. Being able to ‘feel’ the control surfaces through the yoke and column has obvious benefits that flying using the MCP cannot convey.

CWS is also advantageous when flying in turbulent conditions (additional information below) as it results in smoother transitions than when the autopilot is used. Furthermore, CWS also allows for greater control of the aircraft when performing touch and goes and circuits at lower altitudes.

CWS engaged during climb following flaps retraction. The FMA displays CWS R & CWS P, the vertical speed is 2650 and pitch mode is V/S after changing from TOGA thrust following climb out

Practical Example

CWS is often used during the climb to altitude with the autopilot being engaged at 10,000 feet.

In the example (left) the aircraft has CWS engaged during climb following flaps retraction. The FMA displays CWS R & CWS P, the vertical speed is 2650 and pitch mode is V/S after changing from TOGA thrust following climb out. Pitch and roll follows the FD bars and speed is 240 KIAS with altitude set to flight level 20900. If CWS remains engaged, the aircraft will continue at this attitude.

Airspeed is not protected when using CWS.

Following rotation, the Flight Director (FD) bars will be followed maintaining V2+15/20 until Acceleration Height (AH) is reached. At AH, the MCP speed will be increased to climb speed, or to a speed as required by Air Traffic Control. As airspeed increases the flaps will be retracted. When the flaps are retracted, the control column will be placed in a position that correlates to the Flight Director bars and CWS A or B will be engaged – the attitude of the aircraft will now be fixed.

The aircraft, in TOGA thrust, will maintain the established pitch as it ascends to the altitude set on the MCP. TOGA thrust is speed protected; therefore, as long as the FD bars are followed there will not be a speed incursion. If a roll mode is selected, the navigational data provided by this mode is also promulgated to the Flight Director. Once the desired altitude has been reached, LNAV / VNAV can be engaged.

Whether a flight crew used CWS is personal preference. Some flight crews use it regularly while others have never used it.

Turbulence (autopilot or CWS)

The Flight Crew Training Manual (FCTM) states:

‘That during times of turbulence the A/P system (CMD A/B) should be disengaged.’

When the aircraft is flying through turbulence, the autopilot is attempting to maintain an attitude (pitch) that is based upon a predefined barometric pocket of air that is present at the altitude you are flying at. In severe turbulence this pocket of air may not be stable and the autopilot will try to change altitude to match the changing barometric pressure. At its worse, the autopilot may unexpectedly disconnect.

CWS provides a stable buffer in which the aircraft will maintain its position when flying through turbulence. When CWS is engaged, it will maintain a preset attitude rather than the A/P attempting to match the attitude to changing barometric pressure.

Flight Crew Training Manuals differ in their content; each manual has been written with a particular airline. Many virtual flyers duplicate the procedures followed by Ryanair. This is because the documentation for Ryanair is relatively easy to find, and the policy of this airline is reasonably conservative. As such, I have transcribed from the Ryanair FCTM the segment on the use of CWS during turbulence.

The Ryanair FCTM states:

‘Flight through severe turbulence should be avoided, if possible. When flying at 30,000 feet or higher, it is not advisable to avoid a turbulent area by climbing over it unless it is obvious that it can be over flown. For turbulence of the same intensity, greater buffet margins are achieved by flying the recommended speeds at reduced altitudes. Selection of the autopilot Control Wheel Steering (CWS) is recommended for operation in severe turbulence’.

The recommended Ryanair procedures for flight in severe turbulence is:

Do not use Altitude Hold (ALT HLD) mode.
Target the airspeed to approximately 280 KIAS or 0.76 MACH, whichever is lower.
During severe turbulence there often will be large and often rapid variations in indicated airspeed. Do not chase the airspeed.
Engage the Yaw Damper.
If the autopilot is engaged, use CWS position, do not use ALT HLD mode.
Disengage the Autothrottle (stops the autothrottle from hunting a desired airspeed)
Maintain wings level and the desired pitch attitude. Use the attitude indicator as the primary instrument. In extreme down and updraft conditions extreme attitude changes may occur. Therefore, do not use sudden and excessive control inputs. After establishing the trim setting for penetration speed, do not change the stabilizer trim.

Autothrottle Use

When CSW is engaged, the autothrottle should not be engaged. The reason for this is because the autothrottle is coupled to the automation, and if there is a change in the aircraft's attitude there will be a corresponding change in engine thrust.

This said, I have spoken with several pilots who claim that they leave the autothrottle on when using CWS. In some respects it depends on the severity of the turbulence encountered.

Lazy Flying

Although not sanctioned by Boeing, some pilots use CWS as a 'lazy way' of flying, whereby they may establish the aircraft at a specific attitude and vertical speed with the autothrottle engaged. As CWS is a sub-set of the autopilot system, trim control will still be controlled by the system and the aircraft will maintain the desired attitude until CWS is cancelled.

A Virgin First Officer has stated that, after takeoff and flaps retraction, she will often use engage CWS to climb to a specific altitude, then she will engage LNAV, VNAV and the autopilot.

It's important to realise there are many ways, (although not sanctioned by Boeing or a specific airline policy) to fly the Boeing 737 aircraft.

Important Point:

There is no speed protection when CWS is engaged, except when the aircraft is in TOGA mode.

Technical Data (general)

The Flight Crew Training Manual states:

‘After autopilot engagement, the airplane may be manoeuvred using the control wheel steering (CWS) pitch mode, roll mode, or both using the control wheel and column. Manual inputs by the pilot using CWS are the same as those required for manual flight. Climbs and descents may be made using CWS pitch while the roll mode is in HDG SEL, LNAV or VOR/LOC. Autopilot system feel control is designed to simulate control input resistance similar to manual flight.'

The Mode Control Panel (MCP) has two CWS buttons located on the First Officer side of the MCP beneath the two CMD buttons (CMD A/B). Like the autopilot, CWS has a redundancy system (system A or system B). By default the CWS system is off (annunciator is not illuminated).

The CWS system has been designed so it can be used with or without the autopilot.

To engage the CWS system, either of the two CWS buttons must be pressed. When engaged, the CWS annunciator will illuminate green and the Flight Mode Annunciator (FMA) on the Primary Flight Display (PFD) will annunciate CWS P and/or CWS R.

CWS cannot be engaged when any of the following conditions are met:

Below 400 feet.
Below 150 feet RA with the landing gear in the down position.
After VOR capture with TAS 250 kt or less.
After LOC capture in the APP mode.

Important Points:

CWS can only be engaged when there is no pressure on the flight controls.
CWS can be engaged with the autopilot engaged or not engaged.

Operation - What CWS Does

As mentioned, the CWS system can be used with or without the autopilot being engaged.

CWS can be engaged two ways. Either by moving the control column when the autopilot is engaged, or by pressing the CWS button on the MCP.

To use CWS in its own right, the autopilot must be disengaged. This can be done manually by pressing the CMD button or by pressing CWS; the later will disconnect the autopilot (the CMD annunciation will extinguish and the CWS annunciation will illuminate). To access the CWS system partially, and still use the autopilot, the control column is moved (pitch/roll) while the autopilot is engaged.

Although the CWS concept is easy to understand, documenting exactly what it does is difficult and this can cause confusion. I wouldn't become too concerned with the 'technical jargon' below, as CWS is easy to master by using the function and remembering what it does:

The following information has been edited from documentation acquired from Smart Cockpit Airline Training.

1: CWS selected - PITCH and ROLL (autopilot not engaged)

Depressing the CWS button on the MCP engages the autopilot pitch and roll axes in the CWS mode. It also displays CWS-P and CWS-R on the FMA on both the Captain and First Officer Primary Flight Displays (PFD). (Note that CMD is not selected and the CMD annunciation is extinguished on the MCP).
With CWS engaged, the autopilot maneuvers the aircraft in response to control pressures applied to the control wheel or column. The control pressure is similar to that required for manual flight. When control pressure is released, the autopilot holds existing attitude and roll.

• If the column pressure is released with a bank angle 6 degrees or less, the autopilot rolls the wings level and holds existing heading. This feature is inhibited when any of the following conditions are met:

(i) Below 150 ft RA with the landing gear down;

(ii) After F/D VOR capture with TAS 250 kt or less; and,

(iii) After F/D LOC capture in the APP mode.

2: Moving control column - PITCH (autopilot engaged)

The FMA will display CWS-P.

The pitch axis engages in CWS while the roll axis is in CMD when:

(i) The autopilot pitch has been manually overridden with control column force and the force required for override is greater than normal CWS control column force. Note that manual pitch override is inhibited in the APP mode with both autopilots are engaged (autoland).

Important Points:

When approaching a selected altitude in CWS-P with the A/P in CMD, CWS-P changes to ALT ACQ and, when at the selected altitude, ALT HOLD engages.
If pitch is manually overridden while in ALT HOLD at the selected altitude, ALT HOLD changes to CWS-R If control force is released within 250 ft of the selected altitude, CWS-P changes to ALT ACQ and the autopilot returns to the selected altitude and ALT HOLD engages. If the elevator force is held until more than 250 ft from the selected altitude, pitch remains in CWS PITCH.

3: Moving control column - ROLL (autopilot engaged)

• The FMA will display CWS-R.

The roll axis engages in CWS while the pitch axis is in CMD when:

(i) The pitch has been manually overridden with control column force and the force required for override is greater than normal CWS control column force.

Important Point:

With CWS-R selected and the autopilot engaged, the aircraft will capture a selected radio course while the VOR/LOC or APP mode is armed. Upon intercepting the radial or localizer, the F/D and autopilot annunciation changes from CWS-R to VOR/LOC engaged and the autopilot tracks the selected course.

Using CWS (with the autopilot engaged) - Simplified

This segment has been added in response to some readers who stated they had difficulty in understanding some of the above content. I hope it explains, in easier terms, how the CWS system can be used when the autopilot is engaged.

Moving the flight controls (pitch/roll) during automated flight will cause the CWS system to engage. However, the autopilot (CMD) will remain selected and the CMD annunciator will remain illuminated on the MCP.

Flying the aircraft in this manner can be useful when hand flying an approach, but wishing to follow the automated inputs from the ILS and/or FMC.

During such a procedure the following will be noted:

Moving flight controls left or right (roll):

(i) The autopilot annunciation will remain illuminated;

(ii) The FMA on the PFD will alter from CMD to CWS-R;

(iii) The AFDS will illuminate A/P P/RST; and,

(iv) The heading annunciation on the MCP will extinguish, as will the LNAV annunciation if engaged.

The aircraft can now be flown using control wheel steering. To return to fully automated flight, press the heading button on the MCP. LNAV, if used, will also need selecting.

Moving flight controls up or down (pitch):

(i) The autopilot (CMD A/B) annunciation will remain illuminated;

(ii) The FMA on the PFD will alter from CMD to CWS-P;

(iii) The AFDS will illuminate A/P P/RST; and,

(iv) The heading annunciation on the MCP will extinguish, as will LNAV annunciation if engaged.

Important Point:

If the pitch is altered to cause the aircraft to ascend, the altitude window in the MCP must be changed to the new altitude prior to moving the flight controls (altitude capture is automatic). This is not required if the pitch is changed to cause the aircraft to descend.

Final Call

The use of CWS is very much underused and under-appreciated - whether used as a stand-alone system, or in conjunction with the autopilot.

Although surface control loading in a simulator rarely matches that of a real aircraft, the use of CWS in a simulator environment can still have positive benefits equating to better aircraft handling, especially when flying circuits and flying in turbulence.