Barometer and Radio Altitude Settings (EFIS) - How To Use Them
/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.
diagram SHOWING RELATIONSHIP BETWEEN TRANSITION altitude and level (© icon) (click to enlarge)
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.
STD press button on the EFIS
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).
RA
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.
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.
