Throttle Quadrant Rebuild - Flaps Lever Uses String Potentiometer

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Flaps lever set to Flaps 30.  The throttle quadrant is from a Boeing 737-500 airframe. The flaps lever arc is the curved piece of aluminium that has has cut-out notches that reflect the various flap positions.  It was beneath this arc that micro-buttons had been installed

There are several ways to enable the flaps lever to register a particular flaps détente when the flaps lever is moved to that position on the flaps arc.

In the earlier conversion, the way I had chosen worked reasonably well.  However, with constant use several inherent problems began to develop.

In this article, we'll examine the new system.  But before going further, I'll briefly explain the method that was previously used.

Overview of Previously Used System

In the earlier conversion, nine (9) micro-buttons were used to register the positions of the flaps lever when it was moved (Flaps UP to Flaps 40). 

The micro-buttons were attached to a half moon shaped piece of fabricated aluminium.  This was mounted beneath the flaps lever arc and attached to the quadrant.  Each micro-button was then connected to an input on a PoKeys 55 interface card.  Each input corresponded to an output.

Calibration was straightforward as each micro-button corresponded to a specific flaps position.

Problems

The system operated reasonably well, however, there were some problems which proved the system to be unreliable.  Namely:

(i)    The vertical and lateral movement of the chain located in the OEM throttle quadrant interferred with the micro-buttons when the trim was engaged; and,

(ii)  The unreliability of the PoKeys 55 interface card to maintain an accurate connection with the micro-buttons.

Movement of OEM Chain

The chain, which is similar in appearance to a heavy duty bicycle chain, connects between two of the main cogs in the throttle quadrant.  When the aircraft is trimmed and the trim wheels rotate, the chain revolves around the cogs.  When the chain rotates there is considerable vertical and some lateral movement of the chain, and it was this movement that caused three micro-buttons to be damaged; the chain rubbed across the bottom section of the micro-buttons, and with time the affected buttons became unresponsive.

First Officer side of a disassembled throttle quadrant  (prior to cleaning and conversion).  The large notched cog is easily seen and it's around this cog that the OEM chain rotates (the chain has been removed)

It took some time to notice this problem, as the chain only rotates when the trim buttons are used, and the micro-buttons affected were primarily those that corresponded to Flaps 5, 10 and 15.  The chain would only rub the three micro-buttons in question when the flap lever was being set to Flaps 5, 10 or 15 and only when the trim was simultaneously engaged.

The cog and chain resides immediately beneath the flaps arc (removed, but is attached to where you can see the four screws in the picture). 

Although there appears to be quite a bit of head- space between the cog and the position where the flaps arc is fitted, the space available is minimal.  Micro-buttons are small, but the structure that the button sits is larger, and it was this structure that was damaged by the movement of the chain (click to enlarge).

An obvious solution to this problem would be to move the chain slightly off center by creating an offset, or to fabricate a protective sleeve to protect the micro-buttons from the movement of the chain.     However, the design became complicated and a simpler solution was sought.

Replacement System

Important criteria when designing a new system is: accuracy, ease of installation, calibration, and maintenance.  Another important criteria is to use the KIS system.  KIS is an acronym used in the Australian military meaning Keep It Simple.

The upgraded system has improved reliability and has made several features used in the earlier system redundant.  These features, such as the QAMP (Quick Access Mounting Plate) in which linear potentiometers were installed, have been removed.

String Potentiometer Replaces Micro-buttons

Single-string potentiometer enables accurate calibration of flaps UP to flaps 40.  The potentiometer is mounted on a customised bracket screwed to the First Officer side of the throttle quadrant superstructure.  The terminal block in the image is part of the stab trim wheel system

A Bourne single-string potentiometer replaced the micro-buttons and previously used linear potentiometers.  The string potentiometer is mounted to a custom-designed bracket on the First Officer side of the throttle quadrant.  The bracket has been fabricated from heavy duty plastic.

A string potentiometer was selected ahead of a linear potentiometer because the former is not limited in throw; all the flap détentes can be registered from flaps UP through to flaps 40.  This is not usually possible with a linear potentiometer because the throw of the potentiometer is not large enough to cater to the full movement of the flaps lever along the arc.

A 'string' is also very sensitive to movement, and any movement of the string (in or out) can be accurately registered.

Another advantage, is that it's not overly important where the potentiometer is mounted, as the string can move across a wide arc, whereas a linear potentiometer requires a straight direction of pull-travel.

Finally, the string potentiometer is a closed unit.  This factor is important as calibration issues often result from dust and grime settling on the potentiometer.  A closed unit for the most part is maintenance free.

The end of the potentiometer string is attached to the lower section of the flaps lever.  As the flaps lever moves along the arc, the string moves in and out of the potentiometer. 

The ProSim737 software has the capability to calibrate the various flap détentes.  Therefore, calibration using FSUIPC is not required.  However, if ProSim737 is not used, then FSUIPC will be needed to calibrate the flap détente positions.

Advantages

Apart from the ease of calibration, increased accuracy, and repeatability that using a string potentiometer brings, two other advantages in using the new system is not having to use a Pokeys 55 card or micro-buttons.

Unreliability of PoKeys 55 Interface Card

The PoKeys card, for whatever reason, wasn't reliable in the previous system.  There were the odd USB disconnects and the card was unable to maintain (with accuracy and repeatability) the position set by the micro-buttons.

I initially replaced the PoKeys card, believing the card to be damaged, however, the replacement card behaved in a similar manner.  Reading the Internet I learned that several other people, who also use ProSim737 as their avionics suite, have had similar problems.

Micro-buttons can and do fail, and replacing one or more micro-buttons beneath the flaps arc is a time-consuming process.  This is because the upper section of the throttle quadrant must be completely dismantled and the trim wheels removed to enable access to the flaps arc.

Registering the Movement of the Flaps Lever in Windows

The movement of the flaps lever, prior to calibration must be registered by the Windows Operating System.  This was done using a Leo Bodnar 086-A Joystick interface card.  This card is mounted in the Throttle Interface Module (TIM).    The joystick card, in addition to the flaps lever, also registers several other button and lever movements on the throttle quadrant.  

Final Call

The rebuild has enabled a more reliable and robust system to be installed that has rectified the shortfalls experienced in the earlier system.  The new system works flawlessly.

  • This article displays links to the majot journal posts concerning the 737 throttle: OEM Throttle Quadrant. 

Acronyms and Glossary

  • OEM - Original Aircraft Manufacture (real aircraft part).

Using OEM Panels in the MIP

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OEM Captain-side DU panel.  Note the thick engraving and specialist DZUS fasteners

The introduction of the Boeing 737 Max has meant that many carriers are updating their fleets and retiring earlier production 737 NG airframes.  This has flow on benefits for flight simulator enthusiasts, because more and more OEM NG parts are becoming available due to NG airframes being stripped down and recycled.  

Although some items, such as high-end avionics are priced outside the realm of the average individual, many other parts have become reasonably priced and are often a similar price to the equivalent reproduction part.

This article primarily relates to the panels used in the Main Instrument Panel (MIP), and lower kick stand.  The term panel means the aluminum plate that is secured to the framework of the MIP, and lightplate refers to the engraved plate that is secured to the panel.

Do You Notice The Difference

This is a common question.  The resounding answer is yes – the difference between OEM and reproduction parts can be noticed, especially if you compare the identical parts side by side.  This said, some high-end companies manufacturer panels that are almost indiscernible from the OEM panel.  These panels are bespoke, expensive, and usually are only made to a custom order.  Therefore, it really depends on which manufacturer/company you are comparing the OEM panel against.

Close up detail of OEM lightplate and general purpose knobs

By far the biggest difference between an OEM and reproduction panel, other than appearance, is the tactile feel of a knob, the overall robustness of the panel, and the firmness felt when rotating a commercial-grade switch; the later feels very accurate in its movement. 

There is litle compromise with backlighting as an OEM panel has a consistent colour temperature and intensity without hot and cold spots.  

Using a real panel helps to provide immersion and, as your're using a real aircraft part there is no second-guessing whether the panel is an accurate copy; using an OEM panel is literally 'as real as it gets'.  Furthermore, it’s  environmentally friendly to use second hand parts.  New parts (reproduction or otherwise) are made from  finite resources. 

Limitation

Not every OEM part can work in a home simulator.  For example, the OEM potentiometer responsible for the dimming function in the lower kickstand DU panels cannot be used.  This is because Boeing use a rheostat instead of a potentiometer.  Without going into detail, a rheostat is designed to take into account 115 volts AC commonly used in aircraft.  If using these panels. you will need to change the rheostat to a high-end commercial potentiometer.  

Table 1 outlines 'some' of the main differences between the OEM panels and their reproduction equivalents.

Table 1:  Main differences between OEM and reproduction panels (MIP only).

The information presented in the above table, should not be taken in a way that reflects poorly on the manufacturer of reproduction panels.  There are a few high-end companies whose panels are indiscernible from the real item; it’s the purchaser’s knowledge and the manufacturer’s skill that will define whether a reproduction panel replicates the real item.  ‘Caveat Emptor’should always be at the forefront of any purchase decision.

Potential Problems Using OEM Panels in the MIP

Potential problems often surface when attempting to mate OEM parts to the framework of the MIP.  This is because reproduction MIPs rarely echo the identical dimensions of their OEM counterpart. 

OEM Stand-by instrument panel. Although difficult to see from a picture, the overall robustness of this panel surpasses all but the very best reproductions

It's not possible to document every potential problem, as all reproduction MIPs are slightly different to each other.  However, some issues encountered may be the misalignment of screw holes between the MIP framework and the OEM panel, the inability to use the panel's DZUS fasteners, the panel being too large or too small for the MIP in question, or the open framework structure at the rear of the panel (which incorporates the wiring lume and Canon plugs) interfering with the infrastructure of the reproduction MIP, or the mounting of the computer screens.

In general, OEM panels cannot be mounted to a reproduction MIP without major work being done to the framework of the MIP.   The solution is to use a MIP that has been designed 1:1 with the OEM MIP, or fabricate a MIP in-house to the correct dimensions.

Specifics to the FDS MIP

The MIP used in the simulator is manufactured by Flight Deck Solutions (FDS), and although the MIP is made to a very high quality, the dimensions of the MIP are not 1:1. 

The most problematic issue is that the MIP length is slightly too narrow to enable the OEM panels to be fit correctly to the front of the framework.  For example, the OEM chronograph panel is 1 cm wider than the FDS chronograph panel.  Furthermore, most of the OEM panels (such as the standby instrument, chronograph and landing gear panel) measure 130 mm in height as opposed to the FDS panels that measure 125 mm in height.  This causes problems when trying to line up the bottom of each panel with the bottom of the display bezels. 

The standby instrument panel does fit, however, there is a few centimeters of space between the panel and the adjacent display bezel frame.  In the real aircraft, the display bezel and the edge of the standby instrument panel almost abut one another.  The autobrake panel does fit as do the lower kickstand panels.

FDS use screws to attach their panels to the upper MIP framework, however, OEM panels use DZUS fasteners.  The screw holes on the FDS MIP do not align with the position of the DZUS fasteners in the OEM panel.  The lower MIP panel (kickstand) in the real aircraft also incorporates a DZUS rail to which the panels are attached.  The FDS kickstand does not use a DZUS rail, and screws or reproduction DZUS fasteners are needed to secure the OEM kickstand panels.

The above said, FDS does not state that their MIP is I:1, and when asked will will inform you that OEM panels will not fit their products without considerable fabrication.

DZUS fastener that secures DU panel to the MIP framework

Specialist DZUS Fasteners

The OEM panels used in the upper MIP incorporate into the panel a specialist DZUS fastener.  This fastener is used to tightly secure the panel to the framework of the MIP; screws are not used.  Screws are only used to secure the lightplate to the panel. 

The DZUS fastener is shaped differently to the fasteners used to secure the panels located in the lower kickstand, overhead and center pedestal, and these parts are not interchangeable. 

Reproductions rarely replicate these DZUS fasteners.  However, like many things it's often the small things that make a difference (at least aesthetically).

Rear of OEM Captain-side DU panel.   Note heavy duty rotary switches (Cole & Jaycor brand), neat and sturdy wiring lume, and easy connect Canon plug.  The use of the correct bracket in the panel enables the AFDS unit to fit snugly to the panel.  Note the depth of the external frame which can cause placement issues

Advantages Using OEM Wiring Lume and Canon Plugs

A major plus using any OEM panel is that the part usually includes an expertly-made wiring lume that terminates at Canon plug.    If possible, the original wiring lume should be kept intact and additional wiring should be done from the Canon plug.  It’s very difficult to duplicate the same level of workmanship that Boeing has done in relation to the wiring.  Furthermore, the wire that has been used is high-end aviation grade wire.

OEM landing gear panel. Like any OEM part, the neatness in relation to the wiring is immaculate.  A Canon plug enables the panel to be connected to a lume which then connects with whatever interface card is in use

The Canon plug deserves further mention, as the use of a Canon plug (or any connector for that matter) enables you to easily remove the panel for service work should this be required.  If at all possible, the original Canon plug (and wiring) should be used because it’s neat and tidy and ensures a good connection.  However, if the correct Canon plug cannot be procured then a reproduction plug should be fabricated.  There is nothing worse than having to disconnect wires from an interface card to remove a part.

Configuring an OEM Panel

Configuring an OEM panel to use in flight simulator depends on which panel you are referring to. 

Panels with knobs, toggles and switches are relatively straightforward to interface with a respective interface card (Phidget card, PoKeys card, FDS SYS card or similar).  Determining the pinouts on the Canon plug that control backlighting requires the use of a multimeter, and then connection to a 5 volt power supply.  If the panel includes annunciators (korrys), then these will need to be connected to a 28 volt power supply (using the correct pinouts).

Technology is rarely static, and there are other ways to interface and configure OEM panels.  The ARINC 429 protocol is becomminginceasingly common to use along with specialist interface cards, and these will be discussed in separate articles.

Rear of DU panel showing korry connections and AFDS bracket

The Future

The FDSMIP can, with some work, be modified to mount the OEM panels.  However, an easier option is to find another MIP that has been designed to mount the panels, or fabricate a MIP in-house to OEM dimensions.

Final Call

Aesthetically, nothing beats the use of an OEM panel, and the panels used in the upper MIP and lower kickstand offer little comparison to their reproduction equivalents, with possible exception to bespoke reproductions. By far the biggest challenge is determining the pin-outs for the Canon plug, but once known, configuration using a Phidget or other traditional card is relatively straightforward. 

As straightforward as it may seem, potential problems surface when attempting to mate OEM panels to an existing reproduction MIP.  To resolve these issues, often a replacement MIP is needed that has been made to the identical dimensions of the OEM counterpart.

Additional Information

The following articles may provide further information in relation to using OEM parts.

Acronyms

  • ARINC 429 - Aircraft communication protocol

  • DU - Display Unit

  • Lume - A harness that holds several wires in a neat way

  • OEM - Original Equipment Manufacturer

  • MIP - Main Instrument Panel

Sounds Reworked - Flight Sim Set Volume (FSSV) - Review

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737 Titan Airways (RHL Images from England, Titan Airways B737, G-POWC (3842154945), CC BY-SA 2.0)

Immersion is a perception of being physically present in a non-physical world.  The perception is created by surrounding the user of the simulator in images, sound or other stimuli that provide an engrossing total environment.  When something does not replicate its real world counterpart, the illusion and immersion effect is degraded.

Engine Sound Output

The sound output generated by a jet aircraft as heard from the flight deck is markedly different when the aircraft is at altitude.  This is because of differences in air density, temperature, the speed of the aircraft, drag, and thrust settings.  The noise emitted from the engines will always be highest at takeoff when full thrust is applied.  At this time, the noise generated from wind blowing over the airframe will be at its lowest.  At some stage, these variables will change and wind noise will dominate over engine noise.

As an aircraft gathers speed and increases altitude, engine sound levels lower and wind levels, caused by drag, increase.  Furthermore, certain sounds are barely audible from the flight deck on the ground let alone in the air; sounds such the movement of flaps and the extension of flight spoilers (speedbrake).

Being a virtual flyer, the sound levels heard and the ratio between wind and engine sound at altitude is subjective, however, a visit to a flight deck on a real jet liner will enlighten you to the fact that that Flight Simulator’s constant-level sound output is far from realistic.

Add On Programs

Two programs which strive to counter this shortcoming (using different variables) are Accu-Feel by A2A Simulations and FS Set Volume (FSSV).  This article will discuss the attributes of FSSV (Sounds Reworked).

Flight Sim Set Volume (FSSV)

FSSV is a very basic program that reads customized variables to alter the volume of sound generated from Flight Simulator.  The program is standalone and can be copied into any folder on your computer, however, does require FSUIPC to connect with Flight Simulator.  Wide FS enables FSSV to be installed on a client computer and run across a network.  

The following variables can be customised:

(i)     Maximum volume

(ii)    Minimum volume

(iii)   Upper mach threshold

(iv)   Lower mach threshold

(v)    Engine volume ratio

Each of the variables will alter to varying degrees the Mach, engine %N1, rounded engine speed and volume percentage.  

For the program to have effect it must be opened either prior to or after the flight simulator session is opened. 

FSSV pop-up screen showing customised variables (default) that can be set and current reads-outs for the simulator session

It’s an easy fix to automate the opening of the program to coincide with Flight Simulator opening by including the program .exe in a batch file. 

A pop-up window, which opens automatically when the program is started, will display the variables selected and the outputs of each variables.  If the window is kept open, the variables can be observed ‘on the fly’ as the simulation session progresses.  Once you are pleased with the effects of the various settings, a save menu allows the settings to be saved to an .ini file.  The pop-up window can then be set to be minimized when you start a flight simulator session.  

How FSSV Works

The program reads the sound output from the computers primary sound device and alters the various sound outputs based upon customized variables.  The program then lowers the master volume at the appropriate time to match the variables selected.  FSSV will only alter the sound output on the computer that the program is installed.  Therefore, if FSSV is installed to the same computer as Flight Simulator (server computer) then the sound for that computer will only be affected.

Possible Issue (depends on set-up)

An issue may develop if FSSV is installed on a client computer and run across a network via Wide FS, then the program will not only affect the sound output from the server computer, but it also will affect the sound output from the client computer.  

A workaround to rectify this is to split the sound that comes from the sever computer with a y-adapter and connect it to the line-in of another computer, or use a third computer (if one is spare).

In my opinion, it’s simpler to install and run the program via a batch file on the server computer that flight simulator is installed.  The program is small and any drop in performance or frame rates is insignificant.

Summary

The program, although basic, is very easy to configure and use - a little trial and error should enable the aircraft sounds to play with a higher degree of realism.  However, the level that you alter the variables to is subjective; it depends on your perception to the level of sound heard on a flight deck – each virtual flyer will his or her own perception to what is correct. 

The program functions with FSX and P3D flawlessly. 

Finally, If you are unhappy with the result, it’s only a matter of removing/deleting the folder you installed the program to, or close the program during your simulator session to return the sound levels to what they previously were. 

  • FS Set Volume can be downloaded at no charge here. 

Video

The below video is courtesy of the FSSV website.

 
 
 

RNAV Approaches

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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 navigation.  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

B737 NG Display Unit Bezels By Fly Engravity

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The bezels that have replaced the acrylic bezels made by FDS. The landing gear, clock annunciators (korrys) and brake pressure gauge are OEM parts converted for flight simulator use - First Officer side. Note OEM Korrys and clock

I recently upgraded the display unit bezels (frames) on the Main Instrument Panel (MIP).  

The previous bezels, manufactured by Flight Deck Solutions (FDS), lacked the detail I was wanting.  Increasingly, I found myself being fixated by glaringly incorrect hallmarks that did not conform to the original equipment manufacturer (OEM) – in particular, the use of incorrectly positioned attachment screws, the lack of a well-defined hinge mechanism, and the use of acrylic rather than aluminum.

Although it is not necessary to have replicated items that conform to a real part, it does add to the immersion level, especially if you are using predominately OEM parts.  The MIP in my case is a skeleton on which to hang the various real aircraft parts that have been converted for flight simulator use. 

This is not a review, but more a reason to why sometimes there is a need to change from one product to another.

The OEM display is a solid unit that incorporates the avionics, display and bezel in the one unit.  This unit has the protective plastic attached to the screen

OEM Display Units

The OEM display units used in the Boeing Next Generation airframes comprise a large rectangular box that houses the necessary avionics and glass screen for the display.   

The display unit is mounted by sliding the box into the MIP along two purpose-built sliding rails.  The unit is then locked into the MIP by closing the hinge lever and tightening the thumb screw on the lower right hand side of the bezel.  The hinge mechanism is unique to the OEM unit in that once the thumb screw is loosened; one side of the lower display adjacent to the hinge becomes a lever in which to pull the unit free of its locking points in the MIP.

The units are usually manufactured by Honeywell.

The display unit is one piece which incorporates the bezel as part of the assembly; therefore, it is not possible to obtain just the bezel – this is why a reproduction is necessary.

Reproduction Bezels

Reproduction bezels are manufactured by several companies – Open Cockpits, SimWorld, Fly Engravity and Flight Deck Solutions to name a few.  As with all replica parts, each company makes their products to differing levels of accuracy, detail and quality.

I looked at several companies and the closest to the  OEM item appeared to be the bezels manufactured by Fly Engravity and CP Flight (CP Flight are a reseller of Fly Gravity products).  

The main reasons for changing-out the FDS bezels were as follows:

  • FDS bezels have two Philips head screws in the upper left and right hand side of the bezel.  These are used to attach the bezel to the MIP.  The real bezel does not have these screws.

  • FDS bezels are made from acrylic.  The bezels in the real B737, although part of a larger unit, are made from aluminum.  Fly Engravity make their bezels from aluminum which are professionally painted with the correct Boeing grey.  

  • FDS have not replicated the hinge in the lower section of the bezel.  Rather, they have lightly engraved into the acrylic a facsimile of the hinge .   Fly Engravity fabricate a hinge mechanism, and although it does not function (there is absolutely no need for it to function) it replicates the appearance of the real hinge.

  • FDS use 1mm thick clear Perspex whereby the real aircraft uses smoke grey-tinted glass.  Fly Engravity bezels use 3 mm smoke grey-tinted Perspex.

  • The Perspex used by FDS is very thin and is attached to the inside of the bezel by double-side tape.  The thinness of the material means that when cleaning the display it is quite easy to push the material inwards which in turn breaks the sticky seal between the Perspex and the inside of the bezel.  Fly Engravity use thicker Perspex that is attached to the inside of the bezel by four screws.  It is very solid and will not come loose.

Table 1 provides a quick reference to the assailant points.

Detail showing the hinge mechanism in the Fly Engravity bezel.  Although the hinge is non-functional, the detail and depth of the cut in the aluminium frame provides the illusion of a functioning hinge mechanism

Attaching the Bezels to the FDS MIP

The FDS and Fly Engravity bezels are identical in size; therefore, there is not an issue with the alignment of the bezels with MIP – they fit perfectly.

Attaching the Fly Engravity bezels to the FDS MIP is not difficult.  The Fly Engravity bezels are secured to the MIP using the same holes in the MIP that were used to secure the FDS bezels. However, the screws used by Fly Engravity are a larger diameter; therefore, you will have to enlarge the holes in the MIP.  

Detail of the hinge thumb knob on the Fly Engravity bezel.  Although the internal screw is missing from the knob, the cross-hatched pattern on the knob compensates.  The knob is screwed directly into the aluminium frame and can be loosened or tightened as desired.  The circular device is a facsimile of the ambient light sensor (

For the most part the holes align correctly, although with my set-up I had to drill two new holes in the MIP.

The Fly Engravity bezels, unlike the FDS bezels, are secured from the rear of the bezel via the backside of the MIP.  The bezel and Perspex have precut and threaded holes for easy installation of the thumb screws.

Cross section of the Fly Engravity bezel showing the detail of the Perspex and attachment screw

Upgrade Benefits - Advantages and Disadvantages

It depends – if you are wishing to replicate the real B737 MIP as much as possible, then the benefits of upgrading to a Fly Engravity bezel are obvious.  However, the downside is that the aluminum bezels, in comparison to acrylic-made bezels are not inexpensive.

The smoke grey-tinted Perspex has definite advantages in that the computer monitor screens that simulate the PFD, ND and EICAS appear a lot sharper and easier to see.  But a disadvantage is that the computer monitors colour calibration alters a tad when using the tinted Perplex.  This is easily rectified by calibrating your monitors to the correct colour gamut.  I was concerned about glare and reflections, however, there is no more using the tinted Perspex than there is using the clear Perspex.

The Fly Engravity bezels have one minor inaccuracy in that the small screw located in the middle of the hinge thumb knob is not simulated.  This is a small oversight, which can be remedied by having a screw fitted to the knob.

Improvements

A possible improvement to the Fly Engravity bezels could be to use flat-headed screws, or to design a recessed head area into the rear of the Perspex (see above photograph which shows the height of the screw-head).  A recessed area would allow the screw head to sit flush enabling the monitor screen to be flush with the rear of the Perspex. 

The inability of the monitor screen to sit flush with the Perspex does not present a problem, but it is good engineering for items to fit correctly.

Final Call

Although the bezels made by FDS do not replicate the OEM item, they are still of good quality and are functional.  However, if you are seeking authenticity and prefer an aluminum bezel then those produced by Fly Engravity are superior.

Endorsement and Transparency

I have not been paid by Fly Engravity or any other reseller to write this post.  The review is not endorsed and I paid full price for the products discussed.

Glossary

  • EICAS – Engine Indicator Crew Alert system.

  • MIP – Main Instrument Panel.

  • ND – Navigation Display.

  • OEM – Original Equipment Manufacturer (aka real aircraft part).

  • Perspex - Poly(methyl methacrylate), also known as acrylic or acrylic glass as well as by the trade names Plexiglas, Acrylite, Lucite, and Perspex among several others.

  • PFD – Primary Flight Display. 

Throttle Quadrant Rebuild - New Wiring Design and Rewiring of Center Pedestal

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oem 737-500 center pedestal. the panels change as oem components are purchased and converted

Put bluntly, the wiring in the center pedestal was not to a satisfactory standard.  Several panels were daisy chained together, the wires were not colour coded, and the pedestal looked like a rat’s nest of wires.  Likewise, the wiring of the Master Caution System (MCS) required upgrading as several of the original wires showed signs of fraying.  

A word of thanks goes to a friend (you know who you are...) who helped wade through the labyrinth of wires!

This post shares several links to other pages in the website.

Wiring Redesign (pedestal and panels)

The set-out of the inside of the center pedestal was redesigned from the ground up, and several of the pedestal panels re-wired to ensure conformity to the new design standard, which was neater and more logical than its predecessor.  Additionally, the MCS was rewired using colour-coded wire and the wires labeled accordingly.

New Design (panels must be stand-alone)

The new design called for each panel (module) that was installed into the pedestal to be stand-alone.  Stand-alone means that if removal of a panel was necessary, it would be a simple process of unscrewing the DZUS fasteners, lifting the panel out and disconnecting a D-Sub plug and/or 5 volt backlighting wire.   Doing this with panels that were daisy chained together was impossible.

The following panels have been re-wired:

(i)      EVAC panel;

(ii)     Phone panel;

(iii)   ACP units (2);

(iv)    On/off lighting/flood panel; and,

v)      Radar panel.

737-800 EVAC panel, although not a panel that resides in the pedestal, it demonstrates the 'stand-alone' panel philosophy.  One D-Sub plug with labelled and colour-coded wire.  The mate of the D-sub resides inside the pedestal with the wires connected to the appropriate busbar

All the panels have been retrofitted with colour-coded and labeled D-Sub connections.  Removing a panel is a simple as unfastening a DZUS connector, disconnecting a D-Sub connector, and unscrewing the 5 volt backlighting wire from the 5 volt terminal block (if ued).  If a USB cable is needed for the panel, then this must also be disconnected.

A word concerning the ACP units, which were converted some time ago with an interface card located on a separate board outside of the unit.  As part of the rebuild, the two ACP units were completely re-wired to include the interface card within the unit.  Similar to the fire suppression panel, the ACP units are now stand-alone, and only have one USB cable which is used to connect to the computer.  The First Officer side ACP is daisy chained to the Captain-side unit.

Center Pedestal Flat Board

A flat board 1 cm in thickness and constructed from wood was cut to the same dimensions of the pedestal base.  The board was then attached to the inside bottom of the pedestal by screws.  The wood floor has been installed only to the rear two thirds of the pedestal, leaving the forward third open to allow easy access to the platform floor and area beneath the floor structure..

Attached to the flat board are the following items:

(i)       FDS 5 Volt IBL-DIST panel power card (backlighting for FDS panels);

(ii)      28 Volt busbar;

(iii)     5 Volt busbar (backlighting);

(iv)     12 Volt relay (controls backlighting on/off tp panel knob);

(v)      Terminal block (lights test only);

(vi)     Light Test busbar;

(vii)    OEM aircraft relay; and a,

(viii)    Powered USB hub (NAV, M-COM, ACP & Fire Suppression Panel connection).

The 5, 12 and 28 volt busbars (mounted on the flat board) receive power continuously from the power supplies, mounted in the Power Supply Rack (PSR) via the System Interface Module (SIM). Each panel then connects directly to the respective busbar depending upon its voltage requirement.  

In general, 5 volts is used for panel backlighting while 12 and 28 volts is used to power the fire suppression panel, EVAC, throttle unit, phone panel and other OEM components

The flat board has a fair amount of real-estate available; as such, expanding the system is not an issue if additional items need to be mounted to the board.

Lights Test busbar.  Similar in design to the 5 volt busbar, its use centralizes all wires and reduces  the number of connections to a power supply.  Despite the pedestal rewire, there is still a lot of loose wire that cannot be 'cleaned up'.  The grey coloured object is the flat board

Lighting Panel Knob (backlighting on/off)

All the panels in the center pedestal require 5 volt power to illuminate the backlighting.  The general purpose knob located on the pedestal OEM lights panel is used to turn the backlighting on and off.  

Instead of connecting each panel’s wire to the on/off lights panel knob – a process that would consume additional wire and look untidy, each wire has been connected to a 10 terminal 5 volt busbar.  The busbar in turn is connected to a 12 volt relay which is connected directly with the on/off knob.

When panel lights knob is turned from off to on, the relay closes the circuit and the busbar is energised; any panel connected to the busbar will automatically receive power.

The busbar and relay are mounted to the flat board.

This system has the advantage that it minimizes the number of wires that are connected to the lights panel knob.  It also enables one single high capacity wire to connect from the relay to the knob rather than several smaller gauge wires.  This minimises the heat produced from using several thinner wires.  It is also easier to solder one wire to the rear of the panel knob than it is to solder several wires.

Lights Test and DIM Functionality

The center pedestal also accommodates the necessary components (Lights Test busbar) to be able to engage the Lights Test and DIM functionality.  These functions are triggered by the Lights Test Toggle located on the Main Instrument Panel (MIP).  

All wires have been corrected colour coded to various outputs and wire ends use ferrules to connect to the card

Interface Cards

In the previous throttle quadrant, a number of interface cards were mounted within the center pedestal. 

To ensure conformity, all the interface cards have been removed from the pedestal and are now mounted within one of the interface modules located forward of the simulator. 

Furthermore, all the wiring is colour-coded and the wire ends that connect into the I/O cards use ferrules.

The First Officer-side MCS completely rewired.  The MCS has quite a bit of wiring, and making the wire neat and tidy, in addition to being relatively accessible, was a challenge

The use of ferrules improves the longevity of the wiring, makes wire removal easier, and looks neater.

Wiring and Lumens

Needless to say, the alterations have necessitated rewiring on a major scale.  Approximately 80% of the internal wiring has had to be replaced and/or re-routed to a position that is more conducive to the new design.

The majority of the wiring required by the throttle unit now resides in a lumen which navigates from the various interface modules (located forword of the simulator) to the Throttle Communication Module (TCM).  

From the TCM the lumen routes through the throttle firewall, along the Captain-side of the throttle unit before making its way to the flat board in the center pedestal.  

The exception to the above is the cabling required for a powered USB hub located within the center pedestal, the wires required for the Lights Test (from the Lights Test Toggle located in the MIP), and the various power wires navigating to the pedestal from the Power Supply Rack.  These wires have been bundled into a separate lumen, which resides beneath the floor structure.

Identifying the voltage of wires is an important aspect of any simulation build

Wire Management

Building a simulator using OEM parts, requires an inordinate amount of multi-voltage wiring of various gauges, and it can be challenge to maintain the wire in a neat and tidy manner. 

Running the wire through conduits and lumens does help, but in the end, due to the amount of wire, the number of connections, and the very limited space that is available, the wire is going to appear a little messy.  Probably more important, is that the wire conforms to an established design standard – meaning it is colour-coded and labelled accordingly.

A dilemma often facing builders is whether to use electrical tape to secure or bind wires.  Personally, I have a strong dislike for electrical tape - whilst it does have its short-term usages, it becomes sticky very easily, and becomes difficult to remove if left on wires for a considerable time .

My preferred method is to use simple cable ties, snake skin casing, or to protect the wires near terminals of OEM parts. to use electrical shrink tubing (which can be purchased in different colours for easy identification of wires and terminals).

Final Product

The design and rewiring of many parts in the simulator has been time consuming.  But, the result has been:

(i)     That all the wires are now colour-coded and labelled for easy identification;

(ii)     The wiring follows a defined system in which common-themed items have been centralised.  

(iii)    Panels that were daisy chained have been rewired with separate D-Sub plugs so they are now stand-alone;

(iv)    The  frayed wires from the MCS have been replaced with new wires; and,

(v)    The wires in general are neater and more manageable (the rat's nest is cleaner...).

Throttle Quadrant Rebuild - Four Speed Stab Trim and Stab Trim Indicator Tabs

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Captain-side stab trim wheel with manual trim handle extended.  The white line on the trim wheel is an aid to indicate that the trim wheels are rotating

This post will document several changes that have been made to enable the stab trim wheels to utilise four speeds.  I will also discuss several problems that were encountered and their solution.  Finally, I will provide some possible reasons for the erratic behavior of the stab trim indicator tabs.

In the previous throttle unit, the power to rotate the trim wheels was from a inexpensive 12 Volt pump motor, and the forward and aft rotation speed of the stab trim wheels was controlled by an I/O card.  The system worked well, but the single speed was far from realistic.

The upgrade to the throttle quadrant enables the stab trim wheels to rotate at four speeds which are identical to the speeds observed in a Boeing aircraft.  The speed is controlled by three adjustable speed controller cards, five relays and a Phidget 0/0/8 interface card – all of which are mounted within the Throttle Interface Module (TIM).  

To generate the torque required to rotate the trim wheels at varying speeds, the pump motor was replaced with an encoder capable 12 volt dual polarity brush motor.  The replacement motor is mounted on a customized bracket attached to the inside frame of the throttle unit.  This style of motor is often used in the robotics industry.

Boeing Rotation Speed

The speed at which the trim wheels rotate is identical to the Boeing specification for the NG series airframe.  Simply written, it is:

(i)     Manual trim  - speed without flaps (slow speed);

(ii)    Manual trim  - speed with flaps extended (very fast speed);

(iii)   Autopilot trim  - speed without flaps extended (very slow speed); and,

(iv)   Autopilot trim - speed with flaps extended (faster speed than iii but not as fast as ii).

To determine the correct number of revolutions, each trim wheel cycle was measured using an electronic tachometer.  Electronic tachometers are often used in the automobile industry to time an engine by measuring the number of revolutions made by the flywheel.

It is important to understand that it is not the rotation speed of the trim wheels which is important, but more the speed at which the aircraft is trimmed.  With flaps extended, the time taken to trim the aircraft is much quicker than the time taken if the flaps were retracted.

Electric stab trim switch on Captain-side yoke.  Whenever the trim is engaged the stab trim wheels will rotate with a corresponding movement in the stab trim indicator tabs

Is There a Noticeable Difference Between the Four Speeds

There is definitely a noticeable difference between the speed that the trim wheels rotate at their slowest speed and fastest speed; however, the difference is subtle when comparing the intermediate speeds.

Design and Perils of Stab Trim

If you speak to any real-world pilot that flies Boeing style aircraft, they all agree upon a dislike for the spinning of the trim wheels.  The wheels as they rotate are noisy, are a distraction, and in some instances can be quite dangerous, especially if your hand is resting on the wheel and the trim is engaged automatically by the autopilot.  This is not to mention the side handle used to manually rotate the trim wheels, which if left extended, can easily damage your knee, during an automatic trimming operation.

If you look at the Airbus which is the primary rival of Boeing, the trim wheels pale by comparison; they are quiet, rotate less often, and are in no way obtrusive.  So why is this case?

Boeing when they deigned the classic and NG series aircraft did not design the throttle unit anew.  Rather, they elected to build upon existing technology which had changed little since the introduction of the Boeing 707.  This saved the company considerable expense.

Airbus, on the other hand, designed their throttle system from the ground-up and incorporated smaller and less obtrusive trim wheels from the onset.

Interestingly, Boeing in their design of the Dreamliner have revamped the design of the stab trim wheels and the new design incorporates smaller, quieter and less obtrusive trim wheels than in the earlier Boeing airframes – no doubt the use of automated and computer controlled systems has removed the need for such a loud and visually orientated system.

Problems Encountered (Teething Issues)

Three problems were encountered when the trim wheels were converted to use four speeds.  They were:

(i)      Excessive vibration when the trim wheels rotate at the fastest speed;

(ii)     Inconsistency with two of the speeds caused when CMD A/B is engaged; and,

(iii)    Fluttering (spiking) of the stab trim indicator tabs when the electric stab trim switch was engaged in the down position.

Point (i) is discussed immediately below while points (ii) and (iii), which are interrelated, have been discussed together.

(i)    Excessive vibration

When the trim wheels rotate at their highest speed there is considerable vibration generated, which causes the throttle quadrant to shake slightly on its mounts.

Stab trim wheel cog and mechanism (before cleaned) from the First Officer side.  The picture shows some of the internal parts that move (and vibrate) when the trim wheels rotate at very high speeds.  The high and narrow shape of the throttle unit is easily noted

One of the reasons for the excessive vibration becomes obvious when you compare the mounting points for the throttle quadrant in a homemade simulator to those found in a real aircraft – the later has several solid attachment points between the throttle unit, the center pedestal, the main instrument panel (CDU Bay), and the rigid floor of the flight deck. 

In a simulator, replicating these attachment points can be difficult.   Also, the throttle is a relatively high yet narrow structure and any vibration will be exacerbated higher in the structure.

Another reason for the cause of the vibrations is the material used to produce the center pedestal.  In the classic airframe the material used was aluminum; however, in the NG carbon fiber is used, which is far more flexible than aluminum.  Any vibration caused by the rotation of the trim wheels has a tendency to become amplified as it travels to the less rigid center pedestal and then to the floor of the flight deck.

Solution

Solving the vibration issue is uncomplicated – provide stronger, additional, and more secure mounting points for the throttle quadrant and the attached center pedestal, or slow the rotation of the trim wheels to a more acceptable speed.  Another option is to replace the platform’s floor with a heavier grade of steel or aluminum.  This would enable the throttle quadrant and center pedestal to be attached to the floor structure more securely.  However, this would add significant weight to the structure.  In my opinion, a heavy steel floor is excessive.

By far the simplest solution, is to reduce the fastest speed at which the trim wheels rotate.  The rotation speed can be altered, by the turn of the screwdriver, on one of three speed controller cards mounted within the Throttle Interface Module (TIM).

For those individuals using a full flight deck including a shell, the excessive vibration is probably not going to be an issue as the shell provides additional holding points in which to secure the throttle quadrant, MIP and floor structure.

(ii)    Inconsistency with two of the speeds caused when CMA A/B is engaged

When the autopilot (CMD A/B) was selected and engaged on the MCP, the rotation of the trim wheels would rotate at an unacceptable very high speed (similar to run-away trim).  

The mechanics of this issue was that when the autopilot was engaged, the electronics was not activating the relay that is responsible for engaging the speed controller card.

(iii)       Fluttering of the stab trim indicators

When the electric stab trim switch was depressed to the down position, it was observed that the stab trim indicator tabs would often flutter.  Although the fluttering was mechanical and had no bearing on the trim accuracy, or speed at which the aircraft was trimmed, it was visually distracting.

A possible cause for the run-away trim was electromagnetic interference (RF) generated by the high torque of the trim motor.  The higher than normal values of RF were being  ‘picked up’ by the relay card, which were causing the relay to not activate when the autopilot was engaged.  Similarly, the fluttering of the stab trim indicator tabs, was thought to have been caused by RF interfering with the servo motor.

There were several possibilities for RF leakage.

(i)     The high torque of the motor was generating and releasing too much RF;

(ii)    The wire lumen that accommodates the cabling for the throttle is mounted proximal to the servo motor.  If the lumen was leaking RF, then this may have interfered with the operation of the servo motor;

(iii)    The servo motor was not digital and did not have an RF shield attached;

(iv)   The straight-through cable from the Throttle Communication Module (TCM) to the Throttle Interface Module (TIM) did not have RF interference nodules attached to the cable.

Solution

To counter the unwanted RF energy several modifications were made:

(i)     Three non-polarized ceramic capacitors were placed across the connections of the trim wheel motor;

(ii)    The analogue servo motor was replaced with a higher-end digital servo with an RF shield;

(iii)   The straight-through cable between the TIM and TCM was replaced with a cable that included high quality RF nodes; and,

(iv)   The wires from the servo motor were re-routed and shielded to ensure they were not lying alongside the wire lumen.

Manual Trimming

Manual trimming (turning the trim wheels by hand) is not implemented in the throttle quadrant, but a future upgrade may incorporate this feature.

Stab trim cut out switches with spring-loaded cover open on main and closed on autopilot

Cut-out Stab Trim Button (throttle mounted)

In the earlier conversion, the stab trim cut-out toggle was not functional and the toggle had been programmed to switch off the circuit that powers the rotation of the trim wheels.  Having the ability to disconnect the rotation of the trim wheels is paramount when flying at night, as the noisy trim wheels kept family members awake.

The new conversion does not incorporate this feature as the trim cut-out toggle is fully functional.  Rather, a push-to-engage, green-coloured LED button has been installed to the forward side of the Throttle Interface Module (TIM).  The button is connected to a relay, which will either open or close the 12 volt circuit responsible for directing power to the trim motor.

Stab trim indicator tabs (Captain side).  The throttle is from  B737-500.  The indicator tabs on the NG airframe are slightly different - they are more slender and pointed

Stab Trim Indicator Tabs

The method used to convert the stab trim indicators has not been altered, with the exception of replacing the analogue servo with a RF protected digital servo (to stop RF interference).  

LEFT:  Stab trim indicator tabs (Captain side).  The throttle is from  B737-500.  The indicator tabs on the NG airframe are slightly different - they are more slender and pointed (click to enlarge).

To review, a servo motor and a Phidget advanced servo card have been used to enable the stab trim tab indicators to move in synchronization to the revolution and position of the stab trim wheels.  The servo card is mounted within the Throttle Interface Module (TIM) and the servo motor is mounted on the Captain-side of the throttle unit adjacent to the trim wheel.  There is nothing exceptional about the conversion of the stab trim indicator tabs and the conversion is, more or less, a stock standard.

Is Variable Rotation Speed Important to Simulate

As discussed earlier, it is not the actual rotation of the trim wheels that is important, but more the speed at which the aircraft is trimmed.   In other words, the speed at which the trim wheels rotate dictates the time that is taken for the aircraft to be trimmed.  

If the trim wheels are rotating slowly, the movement of the stab trim indicator tabs will be slow, and it will take longer for the aircraft to be trimmed.  Conversely, if the rotation is faster the stab trim indicator tabs will move faster and the aircraft will be trimmed much more quickly.

Stab Trim Wheel Braking

The amperage of the motor is controlled by a motor controller card; a lower amperage ensures the trim wheel rotates slowly while a high amperage causes the trim wheel to rotate faster.  A brake has not been used to stop the rotation of the trim wheel and the wheel rotation stops by inertia or by pushing the electric trim switch (forward or reverse). 

A future upgrade may look at using a dynaclutch system or magnetic braking.  Another method to install braking is to use software rather than a mechanical system.  A motor controller card with a H-Bridge circuit (not available at the time of conversion) could also possibly be used as a brake to stop the trim wheel rotation when the electric trim switch is relesed.

Final Call - is Four-speed Trim Worthwhile

Most throttle conversions implement only one speed for the forward and aft rotation of the trim wheels with the conversion being relatively straightforward.

Converting the throttle unit to use four speeds has not been without problems, with the main issue being the excessive vibration caused by the faster rotation speed.  Nevertheless, it is only in rare instances, such as when the stab trim is engaged for longer than a few seconds at a time, and at the fastest rotation speed, that the vibration becomes an issue.  If the rotation for the fastest speed is reduced, any vibration issues are alleviated – the downside to this being the fastest speed does not replicate the correct Boeing rotation speed.

For enthusiasts wishing to replicate real aircraft systems, there is little excuse for not implementing four-speed trim, however, for the majority of flight deck builders I believe that two-speed trim, is more than adequate.

Video

Below is a short video, which demonstrates the smooth movement of the stab trim indicator tabs from the fully forward to fully aft position.  The video is only intended to present the functionality of the unit and is not to represent in-flight settings.

 

737 Throttle Quadrant trim tab indicator movement

 

Below is short video that demonstrates two of the four rotation speeds used.  In the example, manual trim is has been engaged, beginning with flaps UP, flaps extended, and then flaps UP again.  The rotation speed of the trim wheels with flaps extended (in this case to flaps 1) is faster than the rotation speed with flaps UP.  The video does not reflect in-flight operations and is only to present the functionality of the unit in question.

 

737 Throttle Quadrant variable speed of trim wheels

 

Glossary

  • Electromagnetic Interference (RF) – RF is a disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation  emitted from an external source (see Wikipedia definition).

  • MCP – Mode Control Panel.

  • MIP – Main Instrument Panel.

  • Stab Trim Indicator Tabs – The two metal pointed indicators located on the throttle unit immediately adjacent to the %CG light plate.  If not using a workable throttle unit, then these tabs maybe located in the lower EICAS as a custom user option.

  • Servo Motor – Refers to the motor that powers the stab trim indicator tabs.

  • Trim Motor – Refers to the motor that powers the stab trim wheels.

Throttle Quadrant Rebuild - Clutch, Motors, and Potentiometers

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Captain-side of throttle quadrant showing an overview of the new design.  The clutch assembly, motors, and  string potentiometer can be seen, in addition to a portion of the revised parking brake mechanism

An earlier article, Throttle Quadrant Rebuild – Evolution Has Led to Major Changes has outlined the main changes that have been made to the throttle quadrant during the rebuild process. 

This article will add detail and explain the decision making process behind the changes and the advantages they provide.  As such, a very brief overview of the earlier system will be made followed by an examination of the replacement system.

Limitation

It is not my intent to become bogged down in infinite detail; this would only serve to make the posts very long, complicated and difficult to understand, as the conversion of a throttle unit is not simplistic.

This said, the provided information should be enough to enable you to assimilate ideas that can be used in your project.  I hope you understand the reasoning for this decision.

The process of documenting the throttle quadrant rebuild will be recorded in a number of articles.  In his article I will discuss the clutch assembly, motors, and potentiometers. 

Why Rebuild The Throttle Quadrant

Put bluntly, the earlier conversion had several problems; there were shortfalls that needed improvement, and when work commenced to rectify these problems, it became apparent that it would be easier to begin again rather than retrofit. Moreover, the alterations spurred the design and development of two additional interface modules that control how the throttle quadrant was to be connected with the simulator.

TIM houses the interface cards responsible for the throttle operation while the TCM provides a communication gateway between TIM and the throttle.

Motor and Clutch Assembly - Poor Design (in previous conversion)

The previous throttle conversion used an inexpensive 12 volt motor to power the thrust lever handles forward and aft.  Prior to being used in the simulator, the motors were used to power electric automobile windows.  To move the thrust lever handles, an automobile fan belt was used to connect to a home-made clutch assembly.

This system was sourly lacking in that the fan belt continually slipped.  Likewise, the nut on the clutch assembly, designed to loosen or tighten the control on the fan belt, was either too tight or too loose - a happy medium was not possible.   Furthermore, the operation of the throttle caused the clutch nut to continually become loose requiring frequent adjustment.

The 12 volt motors, although suitable, were not designed to entertain the precision needed to synchronize the movement of the thrust levers; they were designed to push a window either up or down at a predefined speed on an automobile.

The torque produced from these motors was too great, and the physical backlash when the drive shaft moved was unacceptable.  The backlash transferred to the thrust levers causing the levers to jerk (jump) when the automation took control (google motor backlash).

This system was removed from the throttle.  Its replacement incorporated two commercial motors professionally attached to a clutch system using slipper clutches.

Close up image of the aluminium bar and ninety degree flange attachment.  The long-threaded screw connects with the tail of the respective thrust lever handle. An identical attachment at the end of the screw connects the screw to the large cog wheel that the thrust lever handles are attached

Clutch Assembly, Connection Bars and Slipper Clutches - New Design

Mounted to the floor of the throttle quadrant are two clutch assemblies (mounted beside each other) – one clutch assembly controls the Captain-side thrust lever handle while the other controls the First officer-side. 

Each assembly connects to the drive shaft of a respective motor and includes in its design a slipper clutch.  Each clutch assembly then connects to the respective thrust lever handle.  A wiring lumen connects the clutch assembly with each motor and a dedicated 12 volt power supply (mounted forward of the throttle quadrant).  See above image.

Connection Bars

diagram 1: crossection and a cut-away of a slipper clutch

To connect each clutch assembly to the respective thrust lever handle, two pieces of aluminium bar were engineered to fit over and attach to the shaft of each clutch assembly. 

Each metal bar connects to one of two long-threaded screws, which in turn connect directly with the tail of each thrust lever handle mounted to the main cog wheel in the throttle quadrant. 

Slipper Clutches

close up of slipper clutch showing precision ball bearings

A slipper clutch is a small mechanical device made from tempered steel, brass and aluminum.  The clutch consists of tensioned springs sandwiched between brass plates and interfaced with stainless-steel bearings.  The bearings enable ease of movement and ensure a long trouble-free life.

The adjustable springs are used to maintain constant pressure on the friction plates assuring constant torque is always applied to the clutch.  This controls any intermittent, continuous or overload slip.

A major advantage, other than their small size, is the ease at which the slipper clutches can be sandwiched into a clutch assembly.

Anatomy and Key Advantages of a Slipper Clutch

A number of manufacturers produce slipper clutches that are specific to a particular industry application, and while it's possible that a particular clutch will suit the purpose required, it's probably a better idea to have a slipper clutch engineered that is specific to your application. 

The benefit of having a clutch engineered is that you do not have to redesign the drive mechanism used with the clutch motors.

Key advantages in using slipper clutches are:

  • Variable torque;

  • Long life (on average 30 million cycles with torque applied);

  • Consistent, smooth and reliable operation with no lubrication required;

  • Bi-directional rotation; and,

  • Compact size.

The clutch assembly as seen from the First Officer side of the throttle quadrant.  Note the slipper clutch that is sandwiched between the assembly and the connection rods.  Each thrust lever handle has a dedicated motor, slipper clutch and connection rod.  The motor that powers the F/O side can be seen in the foreground

Clutch Motors

The two 12 Volt commercial-grade motors that provide the torque to drive the clutch assembly and movement of the thrust lever handles, have been specifically designed to be used with drives that incorporate slipper clutches.

In the real world, these motors are used in the railway and marine industry to drive high speed components.  As such, their design and build quality is excellent.

Each motor is manufactured from stainless steel parts and has a gearhead actuator that enables the motor to be operated in either forward or reverse.  Although the torque generated by the motor (18Nm stall torque) exceeds that required to move the thrust lever handles forward and aft, the high quality design of the motor removes all the backlash evident when using other commercial-grade motors.  The end result is an extraordinary smooth, and consistent operation when the thrust lever handles move.

A further benefit using this type of motor is its size.  Each motor can easily be mounted to the floor of the throttle quadrant; one motor on the Captain-side and the second motor on the First Officer-side.  This enables a more streamlined build without using the traditional approach of mounting the motors on the forward firewall of the throttle quadrant.

captain-side 12 Volt motor, wiring lumen and dual string potentiometer that control thrust levers

String Potentiometers - Thrust Levers 1/2

Two Bourne dual-string potentiometers have been mounted in the aft section of the throttle unit.  The two potentiometers are used to accurately calibrate the position of each thrust lever handle to a defined %N1 value.  The potentiometers are also used to calibrate differential reverse thrust.

The benefit of using Bourne potentiometers is that they are designed and constructed to military specification, are very durable, and are sealed.  The last point is important as sealed potentiometers will not, unlike a standard potentiometer, ingest dust and dirt.  This translates to zero maintenance.

Traditionally, string potentiometers have been mounted either forward or rear of the throttle quadrant; the downside being that considerable room is needed for the operational of the strings.  

In this build, the potentiometers were mounted on the floor of the throttle housing (adjacent to the motors) and the dual strings connected vertically, rather than horizontally.  This allowed maximum usage of the minimal space available inside the throttle unit.

Automation, Calibration and Movement

The automation of the throttle remains as it was.  However, the use of motors that generate no backlash, and the improved calibration gained from using string potentiometers, has enabled a synchronised movement of both thrust lever handles which is more consistent than previously experienced.

Reverse Thrust 1/2

Micro-buttons were used in the previous conversion to enable enable reverse thrust - reverse thrust was either on or off, and it was not possible to calibrate differential reverse thrust. 

Dual Bourne string potentiometer that enables accurate calibration of thrust lever handles and enables differential thrust when reversers are engaged

In the new design, the buttons have been replaced by two string potentiometers (mentioned earlier).  This enables each reverse thrust lever to be accurately calibrated to provide differential reverse thrust.  Additionally, because a string potentiometer has been used, the full range of movement that the reverse thrust is capable of can be used.

The video below demonstrates differential reverse thrust using theProSim737 avionics suite. The first segment displays equal reverse thrust while the second part of the video displays differential thrust.

 
 

Calibration

To correctly position the thrust lever handles in relation to %N1, calibration is done within the ProSim737 avionics software  In calibration/levers, the position of each thrust lever handle is accurately ‘registered’ by moving the tab and selecting minimum and maximum.  Unfortunately, this registration is rather arbitrary in that to obtain a correct setting, to ensure that both thrust lever handles are in the same position with identical %N1 outputs, the tab control must be tweaked left or right (followed by flight testing).

When tweaked correctly, the two thrust lever handles should, when the aircraft is hand-flown (manual flight), read an identical %N1 setting with both thrust levers positioned beside each other.  In automated flight the %N1 is controlled by the interface card settings (Polulu JRK cards or Alpha Quadrant cards).

Have The Changes Been Worthwhile

Comparing the new system with the old is 'chalk and cheese'.  

One of the main reasons for the improvement has been the benefits had from using high-end commercial-grade components.  In the previous conversion, I had used inexpensive potentiometers, unbalanced motors, and hobby-grade material.  Whilst this worked, the finesse needed was not there.

One of the main shortcomings in the previous conversion, was the backlash of the motors on the thrust lever handles.  When the handles were positioned in the aft position and automation was engaged, the handles would jump forward out of sync.  Furthermore, calibration with any degree of accuracy was very difficult, if not impossible. 

The replacement motors have completely removed this backlash, while the use of string potentiometers have enabled the position of each thrust lever handle to be finely calibrated, in so far, as each lever will creep slowly forward or aft in almost perfect harmony with the other.

An additional improvement not anticipated was with the installation of the two slipper clutches.  Previously, when hand-flying there was a binding feeling felt as the thrust lever handles were moved forward or aft.  Traditionally, this binding has been difficult to remove with older-style clutch systems, and in its worst case, has felt as if the thrust lever handles were attached to the ratchet of a bicycle chain.

The use of high-end slipper clutches has removed much of these feeling, and the result is a more or less smooth feeling as the thrust lever handles transition across the throttle arc.

Future Articles

Future articles will address the alterations made to the speedbrake, parking brake lever, and internal wiring, interfacing and calibration.  The rotation of the stab trim wheels and movement of the stab trim indicator tabs will be discussed.

This article is one of several that pertain to the conversion of the OEM throttle quadrant. A summary page with links can be viewed here: OEM Throttle Quadrant. 

Update

on 2018-04-11 01:08 by FLAPS 2 APPROACH

This article was not able to be published at an earlier time because of issues with confidentiality and potential patents.  The article has been re-written (March 2018). 

Autobrake System - Review and Procedures

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air berlin 737-700 -  autobrake set, flaps 30, spoilers deployed, reverse thrust engaged (Marcela, GFDL 1.2 www.gnu.org/licenses/old-licenses/fdl-1.2.html, via Wikimedia Commons)

The autobrake, the components which are located on center panel of the Main Instrument Panel (MIP), is designed as a deceleration aid to slow an aircraft on landing.  The system uses pressure, generated from the hydraulic system B, to provide deceleration for pre-selected deceleration rates and for rejected takeoff (RTO). An earlier post discussed Rejected Takeoff procedures.  This article will discuss the autobrake system.

General

The autobrake selector knob (rotary switch) has four settings: RTO (rejected takeoff), 1, 2, 3 and MAX (maximum).  Settings 1, 2 and 3 and RTO can be armed by turning the selector; but, MAX can only be set by simultaneously pulling the selector knob outwards and turning to the right; this is a safety feature to eliminate the chance that the selector is set to MAX accidentally.  

When the selector knob is turned, the system will do an automatic self-test.  If the test is not successful and a problem is encountered, the auto brake disarm light will illuminate amber.

The autobrake can be disengaged by turning it to OFF, by activating the toe brakes, or by advancing the throttles; which deactivation method used depends upon the circumstances and pilot discretion.  Furthermore, the deceleration level can be changed prior to, or after touchdown by moving the autobrake selector knob to any setting other than OFF.  During the landing, the pressure applied to the brakes will alter depending upon other controls employed to assist in deceleration, such as thrust reversers and spoilers.

The numerals 1, 2, 3 and MAX provide an indication to the severity of braking that will be applied when the aircraft lands (assuming the autobrake is set).

In general, setting 1 and 2 are the norm with 3 being used for wet runways or very short runways.  MAX is very rarely used and when activated the braking potential is similar to that of a rejected take off; passenger comfort is jeopardized and it is common for passenger items sitting on the cabin floor to move forward during a MAX braking operation.  If a runway is very long and environmental conditions good, then a pilot may decide to not use autobrakes favouring manual braking.

Often, but not always, the airline will have a policy to what level of braking can or cannot be used; this is to either minimize aircraft wear and tear and/or to facilitate passenger comfort. 

The pressure in PSI applied to the autobrake and the applicable deceleration is as follows:

  • Autobrake setting 1 - 1250 PSI equates to 4 ft per second squared.

  • Autobrake setting 2 - 1500 PSI equates to 5 ft per second squared.

  • Autobrake setting 3 - 2000 PSI equates to 7.2 ft per second squared.

  • Autobrake setting MAX and RTO - 3000 PSI equates to 14 ft per second (above 80 knots) and 12 ft per second squared (below 80 knots).

Conditions

To autobrake will engage upon landing, when the following conditions are met:

  • The appropriate setting on the auto brake selector knob (1, 2, 3 or MAX) is set;

  • The throttle thrust levers are in the idle position immediately prior to touchdown; and,  

  • The main wheels spin-up.

If the autobrake has not been selected before landing, it can still be engaged after touchdown, providing the aircraft has not decelerated below 60 knots. Setting the autobrake usually forms part of the approach cehcklist.

To disengage the autobrake system, any one of the following conditions must be met:

  1. The autobrake selector knob is turned to OFF (autobrake disarm annunciator will not illuminate);

  2. The speed brake lever is moved to the down detent position;

  3. The thrust levers are advanced from idle to forward thrust (except during the first 3 seconds of landing); or,

  4. Either pilot applies manual braking.

The last three points (2, 3 and 4) will cause the autobrake disarm annunciator to illuminate for 2 seconds before extinguishing.

Important Facet

It is important to grasp that the 737 NG does not use the maximum braking power for a particular setting (maximum pressure), but rather the maximum programmed deceleration rate (predetermined deceleration rate).  Maximum pressure can only be achieved by fully depressing the brake pedals or during an RTO operation.  Therefore, each setting (other than full manual braking and RTO) will produce a predetermined deceleration rate, independent of aircraft weight, runway length, type, slope and environmental conditions.

Autobrake Disarm Annunciator

The autobrake disarm annunciator is coloured amber and illuminates momentarily when the following conditions are met:

  • Self-test when RTO is selected on the ground;

  • A malfunction of the system (annunciator remains illuminated - takeoff prohibited);

  • Disarming the system by manual braking;

  • Disarming the system by moving the speed brake lever from the UP position to the DOWN detente position; and,

  • If a landing is made with the selector knob set to RTO (not cycled through off after takeoff).  (If this occurs, the autobrakes are not armed and will not engage.  The autobrake annunciator remains illuminated amber).

The annunciator will extinguish in the following conditions:

  • Autobrake logic is satisfied and autobrakes are in armed mode; and,

  • Thrust levers are advanced after the aircraft has landed, or during an RTO operation.  (There is a 3 second delay before the annunciator extinguishes after the aircraft has landed).

Preferences for Use of Autobrakes and Anti-skid

When conditions are less than ideal (shorter and wet runways, crosswinds), many flight crews prefer to use the autobrake rather than use manual braking, and devote their attention to the use of rudder for directional control.   As one B737 pilot stated - ‘The machine does the braking and I maintain directional control’.

Anti-skid automatically activates during all autobraking operations and is designed to give maximum efficiency to the brakes, preventing brakes from stopping the rotation of the wheel, thereby ensuring maximum braking efficiency.  Anti-skid operates in a similar fashion to the braking on a modern automobile.

Anti-skid is not simulated in FSX/FS10 or in ProSim737 (at the time of writing).

To read about converting an OEM Autobrake.

Rejected Takeoff (RTO) - Review and Procedures

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The Rejected Takeoff is part of the Auto Brake Selector Panel located on the Main Instrument Panel (MIP).  RTO can be selected by turning the selector knob to the left from OFF by one click. The knob is from a classic 737-500 knob

A takeoff may be rejected for a variety of reasons, including engine failure, activation of the takeoff warning horn, ATC direction, blown tyres, or system warnings.  For whatever reason, Boeing estimates that 1 takeoff in every 2000 will be rejected (Boeing Corporation).

This is an OEM (Original Equipment Manufacture) autobrake assembly that has been converted for use in the simulator.  Note that the selector knob is not NG compliant but is from a 500 series airframe.  In time this knob will be replaced.  (click image to enlarge)

Performed incorrectly, an RTO can be a dangerous procedure; therefore, protocols have been are established that need to be followed.  

This is the first of two consecutive posts that will discuss components of the autobrake system.  In this post RTO procedures will be explained.  In the second post the auto brake will be examined.

Rejected Takeoff (RTO)

The Auto Brake and Rejected Takeoff (RTO) are part of Auto Brake System, the components which are located on center panel of the Main Instrument Panel (MIP).  An RTO is when the pilot in command makes the decision to reject the takeoff of the aircraft.  

The Boeing Flight Crew Training Manual (FCTM) states:

  • ‘A flight crew should be able to accelerate the aircraft, have an engine failure, abort the takeoff, and stop the aircraft on the remaining runway'; or,

  • 'accelerate the aircraft, have an engine failure, and be able to continue the takeoff utilizing one engine’.  

Two important variables of pre-flight planning need to be established for an RTO to be executed safely - V speeds and runway length.

V Speeds and Runway Length

There are three V speeds that are critical to a safe takeoff and climb out: V1, Vr and V2.  

V1 is the speed used to make the decision to ‘abort or fly’.  Vr is the rotation speed, or the speed used to begin the rotation of the aircraft by smoothly pitching the aircraft to takeoff attitude.  V2 is the speed used for the initial climb-out, and is commonly called the takeoff safety speed.  The takeoff safety speed ensures a safe envelope for single engine operations.

It stands to reason, that the runway must be long enough to cater towards the V speeds calculated from the weight of the aircraft and outside temperature.

Rejected Takeoff - Conditions and Procedure

In general, the protocol used to execute an RTO, is to:

  • Abort the takeoff for ‘cautions’ below 80 knots; and,

  • Between 80 knots and V1 speed, abort only for ‘bells’ (fire warning) and flight control problems.

If a problem occurs below V1 speed, the aircraft should be able to be stopped before reaching the end of the runway.  After exceeding V1 speed, the aircraft cannot be safely stopped and the only option is to takeoff, and after reaching a safe minimum altitude and speed, troubleshoot the problem.

Before takeoff, a flight crew will position the auto brake selector knob to RTO.  This action will trigger the illumination of the auto brake disarm annunciator, which will illuminate amber for 2 seconds; this is a self-test to indicate that the system is working.  After 2 seconds the annunciator will extinguish.

To arm the RTO prior to takeoff, the following conditions must be met:

  • The auto brake and anti-skid systems must be operational;

  • The aircraft must be on the ground;

  • The auto brake selector must be set to RTO;

  • The forward thrust levers must be in the idle position; and

  • The wheel speed must be less than 60 knots.

Once armed, the RTO system only becomes operative after the aircraft reaches 80 knots ground speed (some manuals state 90 knots).  If an ‘abort’ is indicated below 80 knots, the aircraft will need to be stopped using manual braking power.  

The auto brake will remain in the armed mode if the RTO abort was executed prior to 80 knots (the auto brake disarm annunciator does not illuminate).

To engage the RTO the following conditions must be met:

  • The auto brake must be set to RTO;

  • The thrust levers must be retarded to idle position;

  • The aircraft must have reached 80 knots; and,

  • The autothrottle must be disconnected.

When an RTO is executed and the auto brake system engages, the system will apply 3000 PSI to the brakes to enable the aircraft to stop.  Additionally, if the aircraft has reached a wheel speed in excess of 60 knots, and one or two of the reverse thrust levers are engaged, the spoiler panels will extend automatically to the UP position (deploy), and the speed brake lever on the throttle quadrant will move to the UP position.

The auto brake will disengage, if during the RTO either pilot:

  • Activates the toe brakes;

  • Turns the selector knob of the auto brake from RTO to off.   

If the reversers have been engaged and the speed brake lever is in the UP position, then the lever will abruptly move to the DOWN detente position.  When this occurs, the speed brake annunciator will illuminate amber for 2 seconds before extinguishing.  Braking will then need to be accomplished manually.

RTO Procedure

  1. Pilot flying calls ‘STOP’, ‘ABANDON’ or ‘ABORT’

  2. Pilot flying closes thrust levers and disengages autothrottle.

  3. Pilot flying verifies automatic RTO braking is occurring, or initiates manual braking if deceleration is not great enough, or autobrake disarm light is illuminated.

  4. Pilot flying raises speedbrake lever.

  5. Pilot flying applies maximum reverse thrust or thrust consistent with runway and environmental conditions.

  6. Once stopped, pilot flying engages parking brake and completes RTO checklist.

Important Point:

  • Point 4 is important as although the spoilers deploy automatically when the reverse thrust is engaged, the speedbrake lever must be extended manually by the pilot flying (prior to application of reverse thrust).  This is to minimise any delay in spoiler extension, as extension is necessary for efficient wheel braking.

What Circumstances Trigger An RTO

Prior to 80 knots, the takeoff should be rejected for any of the following:

  • Activation of the master caution system;

  • Unusual noise and vibration;

  • Slow acceleration;

  • Takeoff configuration warning;

  • Tyre failure;

  • Fire warning;

  • Engine failure;

  • Bird strikes;

  • Windshear warning;

  • Window failure; and/or,

  • If the aircraft is unsafe or unable to fly.

After 80 knots and prior to V1, the takeoff should be rejected for any of the following:

  • Fire warning;

  • Engine failure;

  • Windshear warning; and/or,

  • If the aircraft is unsafe or unable to fly.

After V1 has been reached, takeoff is mandatory.

Important Points:

Important points to remember when performing a Rejected Takeoff are:

  1. Engage the RTO selector knob before takeoff;

  2. Retard throttles to idle;

  3. Disengage the autothrottle (A/T);

  4. Engage one or both reverse thrust levers;

  5. Monitor RTO system performance, being prepared to apply manual braking if the auto brake disarm light annunciates;

  6. Manually raise speed brake lever if not already in the UP position BEFORE engaging reverse thrust; and,

  7. Remember that RTO functionality engages only after the aircraft has reached 80 knots ground speed, and remains armed if the RTO has been executed below 80 knots.

Procedural Variations

A successful RTO is dependent upon the pilot flying making timely decisions and using proper procedures.  Whether an RTO is executed fully or partly is at the discretion of the pilot flying (reverse thrust engaged to deploy spoilers).

It should be noted that If the takeoff is rejected before the THR HLD annunciation, the autothrottles should be disengaged as the thrust levers are moved to idle. If the autothrottle is not disengaged, the thrust levers will advance to the selected takeoff thrust position when released. After THR HLD is annunciated, the thrust levers, when retarded, remain in idle.

For procedural consistency, disengage the autothrottles for all rejected takeoffs.

Figure 1 provides a visual reference indicating the distance taken for an aircraft to stop after various variations of the Rejected Takeoff are executed (copyright, Boeing Flight Crew Training Manual FCTM).

figure 1: distance taken for an aircraft to stop after various variations of the Rejected Takeoff are executed (copyright, Boeing Flight Crew Training Manual FCTM)

This post has explained the basics of a Rejected Takeoff.  Further information can be found in the Flight Crew Training Manual (FCTM) or Quick Reference Handbook (QRH).

In the next post the autobrake system will be discussed.

Direct-To-Routing, ABEAM PTS and INTC CRS - Review and Procedures

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In an earlier post, a number of methods were discussed in which to create waypoints ‘on the fly’ using the Control Display Unit (CDU).  Following on a similar theme, this post will demonstrate use of the Direct-To Routing, ABEAM PTS and Course Intercept (INTC CRS) functionality.

CDU use an appear very convoluted to new users, and by far the easiest way to understand the various functionalities is by ‘trial and error and experimentation’. 

The software (Sim Avionics and ProSim737) that generates the math and formulas behind the CDU is very robust and entering incorrect data will not damage the CDU hardware or corrupt the software.  The worst that can happen is having to restart the CDU software. 

Line Style and Colour

The style and colour of the line displayed on the Navigation Display (ND) is important as it provides a visual reference to the status of a route or alteration of a route.

Dashed white-coloured lines are projected courses whilst solid magenta-coloured lines are saved and executed routes.  Similar colour schemes apply to the waypoints in the LEGS page.  A magenta-coloured identifier indicates that this is the next waypoint that the aircraft will be flying to (it is the active waypoint).

Direct-To Routing

A Direct-To Routing is easily accomplished, by selection of a waypoint from the route in the LEGS page, or by typing into the scratchpad (SP) a NAVAID identifier and up-selecting this to LSK 1L.  Once up-selected, the Direct-To route will be represented on the Navigation Display (ND) by a dashed white-coloured line.  Pressing the EXEC button on the CDU will accept the route modification and precipitate several changes:

  • The route line displayed on the ND, previously a white-coloured dashed line will become solid magenta in colour;

  • The previous displayed route will disappear from the ND;

  • All waypoints on the LEGS page between the aircraft's current position and the Direct-To waypoint in LSK 1L will be deleted; and,

  • The Direct-To waypoint in LSK 1L will alter from white to magenta.

Once executed the FMS will direct the aircraft to fly directly towards the Direct-To waypoint.

ABEAM PTS

Following on from the Direct-To function is the ABEAM PTS function located at LSK 5R. 

ABEAM points (ABEAM PTS) are one or more fixes that are generated between two waypoints from within a programmed route.  The ABEAM PTS functionality is found in the LEGS page of the CDU at LSL 5R and is only visible when a Direct-To Routing is being modified, within a programmed route (the LEGS page defaults to MOD RTE LEGS).  Furthermore, the ABEAM PTS dialogue will only be displayed if the the up-selected fix/waypoint is forward of the aircraft's position; it will not be displayed if the points are located behind the the aircraft.

If the ABEAM PTS key is depressed, a number of additional in-between fixes will be automatically generated by the Flight Management System (FMS), and strategically positioned between the aircraft’s current position and the waypoint up-selected to LSK 1L.  The generated fixes and a white-coloured dashed line showing the modified course will be displayed on the Navigation Display (ND).  

To execute the route modification, the illuminated EXEC button is pressed.  Following execution, the white-coloured line on the ND will change to a solid magenta-coloured line, and the original displayed route will be deleted.  Furthermore, the LEGS page will be updated to reflect the new route.

Nomenclature of Generated Fixes

The naming sequence for the generated fixes is the first three letters of the original waypoint name followed by two numbers (for example, TTR will become TTR 01 and CLARK will become CLA01).  If the fixes are regenerated, for instance if a mistake was made, the sequence number will change indicating the next number (for example, TTR01, TTR02, etc).  

Technique

  1. Up-select a waypoint from the route in the LEGS page to LSK 1L, or type into the scratchpad a NAVAID identifier.  This is a Direct-To Routing; when executed the waypoints between the up-selected waypoint and LSL 1L are deleted.

  2. Press ABEAM PTS in LSK 5R to generate a series of fixes along a defined course from the aircraft’s current location to the up-selected waypoint.  The fixes can be seen on the ND.

  3. Pressing the EXEC button will accept and execute the ABEAM PTS route.

Example and Figures

The below figures are screen captures using ProSim737 avionics suite.  The programming of the CDU has been done with the aircraft on the ground.  Click any image to enlarge.

FIGURE 1:  The LEGS page shows a route HB-TTR-CLARK-BABEL-DPO-WON.  The route is defined by a solid magenta-coloured line

FIGURE 2:  The Route is altered to fly from HB to BABEL.  Note that in the LEGS page, the title has changed from ACT to MOD RTE 1 LEGS.  The ND displays the generated ABEAM PTS and projected course (white-coloured dashed line), beginning from the aircraft’s current position and traveling through HB01, TTR01, CLA01 to BABEL.   The EXEC light is also illuminated

FIGURE 3:  When the EXEC light is pressed, the ABEAM PTS and altered route (Figure 2) will be accepted.  The former route will be deleted and the white-coloured dashed line will be replaced by a solid magenta-coloured line.  The magenta colour indicates that the route has been executed.  The LEGS page will also be updated and display the new route, with the waypoint HB01 highlighted in magenta

The Intercept Course (INTC CRS)

To understand the INTC CRS, it is important to have a grasp to what a radial and bearing is and how they differ from each other.  For all practical purposes, all you need to know is that a bearing is TO and a radial is FROM.  For example, if the bearing TO the beacon is 090, you are on the 270 radial FROM it. 

The Intercept Course (INTC CRS) function is located beneath the ABEAM PTS option in the LEGS page of the CDU at LSK 6R.  Like the ABEAM PTS function, the INTC CRS function is only visible when a when a Direct-To Routing, is being modified within a programmed route (the LEGS page defaults to MOD RTE LEGS).

The function is used when there is a requirement to fly a specific course (radial) to the fix/waypoint.  By default, the INTC CRC displays the current course to the fix/waypoint.  Altering this figure, will instruct the FMS to calculate a new course, to intercept the desired radial towards the fix/waypoint (1)  The radial will be displayed on the ND as a white-coloured dashed line, while the course to intercept the radial (from the aircraft’s current position) will be displayed as a magenta-coloured dashed line.

Visual Cues

An important point to note is that,  if the course (CRS) is altered, is that the displayed (ND) white-coloured line will pass directly through the fix/waypoint, but the line-style will be displayed differently dependent upon what side of the fix/waypoint the radial is, in relation to the position of the aircraft.  The line depicted by sequential long and short dashes (dash-dot-dash) shows the radial TOWARDS the fix/waypoint while the line showing dots, displays the radial AWAY from the fix/waypoint. 

It is important to understand, that for the purposes of the FMS, it will always intercept a course TO a fix/waypoint; therefore, the disparity in how the line-style is represented provides a visual cue to ensure a flight crew does not enter an incorrect CRS direction.

Intercept Heading

However, the flight crew may wish not fly directly to the fix/waypoint, but fly a heading to intercept the radial.  In this case, the flight crew should select the particular heading they wish to fly in the MCP heading selector window, and providing LNAV is armed, the aircraft will fly this heading until reaching the intercept course (radial), at which time the LNAV will engage and the FMS will direct the aircraft to track the inbound intercept course (radial) to the desired fix/waypoint.

Technique

  1. Up-select a waypoint from the route in the LEGS page to LSK 1L, or type into the scratchpad a NAVAID identifier and up-select.  This is a Direct-To Routing and will delete all waypoints that the aircraft would have flown to prior to the up-selected identifier.

  2. Type the course required into INTC CRS at LSK 6R.

  3. This will display on the ND a white-coloured long dashed line (course/radial).  Check the line-style and ensure that the course is TOWARDS the waypoint.  The line, closest to the aircraft should display sequential long and short dashes.

  4. Prior to pressing the EXEC button to confirm the route change, check that the intended course line crosses the current course line of the active route (solid magenta-coloured line).

  5. If wishing to fly a heading to intercept the radial, use the MCP heading window.  If LNAV is armed the FMS will direct the aircraft onto the radial.

Example and Figures

The below figures are screen captures using ProSim737 avionics suite.  The programming of the CDU has been done with the aircraft on the ground.  Click any image to enlarge.

FIGURE 1:  The LEGS page shows a route HB-TTR-CLARK-BABEL-DPO-WYY-WON.  The route is defined by a solid magenta-coloured line.   ATC request ‘QANTAS 29 fly 300 degrees until intercepting the 345 degree radial of BABEL; fly that radial to BABEL then remainder of route as filed

FIGURE 2:  From the LEGS page, locate in the route the waypoint BABEL (LSK 4L).  Recall that the INTC CRS will only function in Direct-To Routing mode. Up-select BABEL to LSK 1L.  Note that a dashed white-coloured line is displayed on the ND showing the new course from HB to BABEL.  The original course is still coloured magenta and the EXEC light is illuminated

FIGURE 3:  Type the radial required (345) into INTC CRS at LSK 6R.  This action will generate (fire across the page) a white-coloured dashed line displaying the 345 course to BABEL (the 165 radial).  Check the line-style and ensure the radial crosses the aircraft’ current course which is 300.  Recall that this line style indicates that the radial to TO BABEL

FIGURE 4:   Press EXEC to save and execute the new route.  The dashed line alters to a solid magenta-coloured line and joins with the remainder of the route at BABEL.  The magenta colour indicates this is now the assigned route.  Note that the magenta line continues across the ND away from the aircraft and BABEL.  This is another visual cue that the radial is traveling TO BABEL

If the aircraft continues to fly on a course of 300 Degrees, and LNAV is armed, the FMS will alter course at the intersection and track the 345 course to BABEL (165 radial).  The LEGS page is also updated to reflect that BABEL is the next waypoint to be flown to (BABEL is coloured magenta

Final Call

Direct-To Routings and ABEAM Points are usually used when a flight crew is required to deviate, modify or shorten a route.  Although the use of ABEAM PTS can be debated for short distances, the technology shines when longer routes are selected and several fixes are generated. The Intercept Course function, on the other hand, is used whenever published route procedures (STAR and SID transitions), or ATC require a specific course (radial) or heading to be followed to or from a navigation fix.

Caveat

The content of this post has been checked to ensure accuracy; however, as with anything that is convoluted minor mistakes can creep in (Murphy, aka Murphy's Law, reads this website).  If you note a mistake, please contact me so it can be rectified.

Acronyms and Glossary

  • ATC – Air Traffic Control

  • CDU – Control Display Unit

  • Direct-To Routing – Flying directly to a fix/waypoint that is up-selected to LSK 1L in the CDU.  All waypoints prior to the u-selected waypoint will be deleted

  • DISCO – refers to a discontinuity between two waypoints loaded in a route within the LEGS page of the CDU.  The DISCO needs to be closed before the route can be executed

  • DOWN-SELECT - Means to download from the CDU LEGS page to the scratchpad of the CDU)

  • FIX – A geographical position determined by visual reference to the surface, by reference to one or more NAVAIDs

  • FMC – Flight Management Computer

  • FMS – Flight Management System

  • Identifiers – Identifiers are in the navigation database and are VORs, NDB,s and published waypoints and fixes

  • LSK 5L – Line Select: LSK refers to line select.  The number 5 refers to the sequence number between 1 and 6.  L is left and R is right (as you look down on the CDU in plan view)

  • MCP – Mode Control Panel

  • NAVAIDS – Any marker that aids in navigation (VOR, NDB, Waypoint, Fix, etc.).  A NAVAID database consists of identifiers which refer to points published on routes, etc

  • ND – Navigation Display

  • RADIALS – A line that transects through a NAVAID representing the points of a compass.  For example, the 045 radial is always to the right of your location in a north easterly direction (Bearings and Radials Paper)

  • ROUTE – A route comprising a number of navigation identifiers (fixes/waypoints) that has been entered into the CDU and can be viewed in the LEGS page

  • SP - Scratchpad

  • UP-SELECT – Means to upload from the scratchpad of the CDU to the appropriate Line Select (LSK)

  • WAYPOINT – A predetermined geographical position used for route/instrument approach definition, progress reports, published routes, etc.  The position is defined relative to a station or in terms of latitude and longitude coordinates.

1:  The FMS will calculate the new course based on great circle course between the aircraft’s current location and the closest point of intercept to the desired course.  This course is displayed on the ND as a white dashed line.

B737-600 NG Fire Suppression Panel (Fire Handles) - Evolutionary Conversion Design

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737-600 Next Generation Fire Suppression Panel installed to center pedestal.  The lights test illuminates the annunciators

737-600 NG Fire Suppression Panel light plate showing installed Phidget and Phidgets relay card

Originally used in a United Airlines 737-600 Next Generation aircraft and purchased from a wrecking yard, the Fire Suppression Panel has been converted to use with ProSim737 avionic suite. The panel has full functionality replicating the logic in the real aircraft.

This is the third fire panel I have owned.  The first was from a Boeing 737-300  which was converted in a rudimentary way to operate with very limited functionality - it was backliut and the fire handles lit up when they were pulled. The second unit was from a 737-600; the conversion was an intermediate design with the relays and interface card located outside the unit within the now defunct Interface Master Module (IMM).  Both these panels were sold and replaced with the current 600 Next Generation panel. This panel is standalone, which means that the Phidget and relay card are mounted within the panel, and the connection is via the Canon plugs and one USB cable.

I am not going to document the functions and conditions of use for the fire panel as this has been documented very well in other literature.  For an excellent review, read the Fire Protection Systems Summary published by Smart Cockpit.

Nomenclature

Before going further, it should be noted that the Fire Suppression Panel is known by a number of names:  fire protection panel, fire control panel and fire handles are some of the more common names used to describe the unit.

Panel with outer casing removed showing installation of Phidget and and relays.  Ferrules are used for easier connection of wires to the Phidget card.  Green tape has been applied to the red lenses to protect them whilst work is in progress

Plug and Fly Conversion

What makes this panel different from the previously converted 737-600 panel is the method of conversion.  

Rear of panel showing integration of OEM Canon plugs to supply power to the unit (5 and 28 volts).  The USB cable (not shown) connects above the middle Canon plu

Rather than rewire the internals of the unit and connect to interface cards mounted outside of the unit, it was decided to remove the electronic boards from the panel and install the appropriate interface card and relays inside the unit.  To provide 5 and 28 volt power to illuminate the annunciators and backlighting, the unit uses the original Canon plugs to connect to the power supplies (via the correct pin-outs).  Connection of the unit to the computer is by a single USB cable.  The end product is, excusing the pun - plug and fly.

Miniaturization has advantages and the release of a smaller Phidget 0/16/16 interface card allowed this card to be installed inside the unit alongside three standard relay cards.  The relays are needed to activate the on/off function that enables the fire handles to be pulled and turned.

The benefit of having the interface card and relays installed inside the panel rather than outside cannot be underestimated.  As any serious cockpit builder will attend, a full simulator carries with it the liability of many wires running behind panels and walls to power the simulator and provide functionality. Minimising the number of wires can only make the simulator building process easier and more neater, and converting the fire handles in this manner has followed through with this philosophy.

Complete Functionality including Push To Test

The functionality of the unit is only as good as the flight avionics suite it is configured to operate with, and complete functionality has been enabled using the ProSim737 avionics suite. 

One of the positives when using an OEM Fire Suppression Panel is the ability to use the push to test function for each annunciator.  Depressing any of the annunciators will test the functionality and cause the 28 volt bulb to illuminate.  This is in addition to using the lights test toggle located on the Main Instrument Panel (MIP) which illuminates all annunciators simultaneously.

At the end of this post is a short video demonstrating several functions of the fire panel.

The conversion of this panel was not done by myself.  Rather, it was converted by a gentleman who is debating converting OEM  fire panels and selling these units commercially; as such, I will not document how the conversion was accomplished as this would provide an unfair disadvantage to the person concerned.

Differences - OEM verses Reproduction

There are several reproduction fire suppression panels currently available, and those manufactured by Flight Deck Solutions and CP Flight (Fly Engravity) are very good; however, pale in comparison to an OEM panel.  Certainly, purchasing a panel that works out of the box has its benefits; however the purchase cost of a reproduction panel is only marginally less that using a converted OEM panel.

By far the most important difference between an OEM panel and a reproduction unit is build quality.  An OEM panel is exceptionally robust, the annunciators illuminate to the correct light intensity with the correct colour balance, and the tension when pulling and turning the handles is correct with longevity assured.  I have read of a number of users of reproduction units that have broken the handles from overzealous use; this is almost impossible to do when using a real panel.  Furthermore, there are differences between reproduction annunciators and OEM annunciators, the most obvious difference being the individual push to test functionality of the OEM units.

737-300 Fire Suppression Panel. Note the different location of korrys

Classic verses Next Generation Panels

Fire Suppression Panels are not difficult to find; a search of e-bay usually reveals a few units for sale.  However, many of the units for sale are the older panels used in the 737 classic aircraft. 

Although the functionality between the older and newer units is almost identical, the similarity ends there.  The Next Generation panels have a different light plate and include additional annunciators configured in a different layout to the older classic units.

737-300 Fire Suppression Panel. this panel is slightly different to the above panel as it has extra korrys for moreadvanced fire logic

One of the reasons that Next Generation panels are relatively uncommon is that, unless unserviceable, the panels when removed from an aircraft are sold on and installed into another aircraft.

Video

The video demonstrates the following:

  • Backlighting off to on (barely seen due to daylight video-shooting conditions)

  • Push To Test from the MIP (lights test)

  • Push To Test for individual annunciators

  • Fault and overhead fire test

  • Switch tests; and,

  • A basic scenario with an engine 1 fire.

NOTE:  The video demonstrates one of two possible methods of deactivating the fire bell.  The usual method is for the flight crew to disable the bell warning by depressing the Fire Warning Cut-out annunciator located beside the six packs (part of the Master Caution System) on the Main Instrument Panel (MIP).  An alternative method is to depress the bell cut-out bar located on the Fire Suppression Panel. 

 

737-600 Fire Suppression Panel

 

B737 Autothrottle (A/T) - Normal and Non-Normal Operations

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Mode Control Panel (MCP) showing A/T on/off solenoid switch and speed window.  The MCP shown is the Pro model manufactured by CP Flight in Italy

The Autothrottle (A/T) is part of the Automatic Flight System (AFS) comprising the Autopilot Flight Director System (AFDS) and the autothrottle.  The A/T provides automatic thrust control through all phases of flight. 

The autothrottle functionality is designed to operate in unison with the Autopilot (A/P), Nevertheless, a flight crew will not always adhere to this use, some crews preferring to fly manually or partially select either the autopilot or autothrottle.

A search on aviation forums will uncover a plethora of comments concerning the use of the autothrottle which, combined with autopilot use and non-normal procedures, can be easily be misconstrued.  An interesting discussion can be read on PPRuNe.

This post will examine, in addition to normal A/T operation, some of the non-normal conditions, their advantages and possible drawbacks.  Single engine operation will not be addressed as this is a separate subject.

Additional Information:

Autothrottle (A/T) Use

The autothrottle is engaged whenever the A/T toggle is armed and the speed annunciator is illuminated on the Mode Control Panel (MCP).  Either of these two functions can be selected together or singularly. 

The autothrottle is usually engaged during the takeoff roll by pressing the TO/GA buttons located under the thrust lever handles.  This is done when %N1 stabilises for both engines at around 40%N1.  This will engage the autothrottle in the TO/GA command mode.  The reason the autothrottle is used during takeoff is to simplify thrust procedures during a busy segment of the flight.

FMA Captain-side PFD showing TO/GA annunciated during takeoff roll

Once engaged, the TO/GA command mode will control all thrust outputs to the engines until the mode is exited, either at the designated altitude set on the MCP, or by activating another automaton mode such as Level Change (LVL CHG).  When TO/GA is engaged, the Flight Mode Annunciator (FMA) will announce TO/GA providing a visual cue.

The use of the autothrottle is at the discretion of the pilot flying, however, airline company policy often dictates when the crew can engage and disengage the A/T. 

The Flight Crew Training Manual (FCTM) states:

‘A/T use is recommended during takeoff and climb in either automatic or manual flight, and during all other phases of flight’.

When to Engage / Disengage the Autothrottle

A question commonly asked is: ‘When is the autothrottle disengaged and in what circumstances’  Seemingly, like many aspects of flying the Boeing aircraft, there are several answers depending on who you speak to or what reference you read.

In the FCTM, Boeing recommends the autothrottle be used only when the autopilot is engaged (autopilot and autothrottle coupled).

In general, a flight crew should disengage the autothrottle system at the same time as the autopilot.  This enables complete manual input to the flight controls and follows the method recommended by Boeing.

My preference during an approach is to disconnect the autothottle and autopilot no later than 1500 feet AGL.  This corresponds to the altitude that the aircraft must be in landing configuration, gear down, flaps 30 and within vertical and lateral navigation constraints with landing checks completed.  Disconnecting the autothrottle and autopilot earlier in the approach provides additional time to transition from automated flight to manual flight, and establish a 'feel' for the aircraft before landing. 

It's not uncommon that  flight crew will manually fly the aircraft, especially 'old school' pilots who are very conversant with hand flying.   I know some crews that will fly from 10,000 feet to landing using the Flight Director (FD), ILS, VNAV and LNAV cues on the Primary Flight Display (PFD) for guidance and the information displayed on the Navigation Display (ND) for situational awareness.  Many pilots enjoy hand-flying the aircraft during the approach phase.

Important Point:

  • Whenever hand flying the aircraft with the autothottle not engaged, it's very important to monitor the airspeed.  This is especially so during the final approach, when thrust can easily decay to a speed very close to stall speed.

The Autothrottle is Designed to be used Coupled with the Autopilot

The autothrottle is a sophisticated automated system that will continually update thrust based on minor pitch and attitude changes, and operates exceptionally well when coupled with the autopilot.  But, when the autopilot is disengaged and the autothrottle retained, its reliability can be questionable.

Some crews believe that if a landing is carried out with the autopilot off and the autothrottle engaged, and a fall in airspeed occurs, such as during the flare, then the autothrottle will apply thrust causing the potential for a tail strike.  Likewise, if during the approach there are excessive wind gusts, pitch coupling (discussed below) may occur.

The advantages of using the autothrottle and autopilot together are:

(i)      Speed is stabilized;

(ii)     Speed floor protection is maintained;

(iii)    Task loading is reduced; and,

(iv)    Flight crews can concentrate on visual manoeuvring and not have to be overly concerned with wind additives

The disadvantages of using the autothrottlewithout the autopilot engaged are:

(i)     Additional crew workload and possible loss of situational awareness (due to workload);

(ii)    Potential excessive and unexpected throttle movement caused by pitch and attitude changes;

(iii)   Potential excessive airspeed when landing in windy conditions with gusts;

(iv)   The potential for pitch coupling to occur (discussed below); and,

(v)    A loss of thrust awareness (out of the loop).

Important Point:

  • The autopilot and autothrottle should not be used independent of one another.

737 Next Generation thrust levers

Boeing 737 Design

The design  of the 737 airframe is prone to pitch coupling because of its under wing mounted engines.  The engine position causes the thrust vector to pitch up with increasing thrust and pitch down with a reduction in thrust.

The autothrottle is designed to operate in conjunction with the autopilot, to produce a consistent aircraft pitch under normal flight conditions.  If the autopilot is disengaged but the autothrottle remains engaged, pitch coupling may develop.

Pitch Coupling

Pitch coupling is when the autothrottle system actively attempts to maintain thrust based on the pitch/attitude of the aircraft. It occurs when the autopilot is not engaged and manual inputs (pitch and roll) are used to control the aircraft. 

If the pitch inputs are excessive, the autothrottle will advance or retard thrust in an attempt to maintain the selected MCP speed.   This coupling of pitch to thrust can be potentially hazardous when manually flying an approach, and more so in windy conditions.

Scenario - pitch coupling

For example, imagine you are in level flight with autothrottle engaged and the autopilot not engaged, and a brief wind change causes a reduction in airspeed. The autothrottle will slightly advance the throttles to maintain commanded speed. This in turn will cause the aircraft to pitch slightly upwards, triggering the autothrottle to respond to the subsequent speed loss by increasing thrust, resulting in further upward pitch. The pilot will then correct this by pushing forward on the control column to decease pitch. As airspeed increases, the autothrottle will decrease thrust causing the aircraft to decrease more in pitch.

The outcome is that a coupling between pitch and thrust will occur causing a roll-a-coaster type ride as the aircraft increases and then decreases pitch, based on pilot input and autothrottle thrust control.

A/T ARM solenoid, N1 and speed button.  The N1 and speed button illuminate when either is in active mode.  In the image, the A/T is armed; however, the speed option is not selected (the annunciator is extinguished).  This enables thrust to be controlled manually

Autothrottle Non-Normal Operations (Arm Mode)

The primary function that the A/T ARM mode is to provide minimum speed protection.  A crew can ARM the throttle but not have it linked to a speed.  To configure the autothrottle in ARM mode, the  A/T toggle solenoid on the MCP is set to ARM, but the SPEED button is not selected (the annunciator is not illuminated).

Scenario - speed button not selected during approach

Some flight crews prefer during an approach, to arm the autothrottle, but not have the speed option engaged (speed annunciator extinguished). 

By doing this during a non-precision approach, it enables a Go-Around to be executed more expediently and with less workload  (the pilot flying only has to push the TO/GA buttons on the thrust lever and the autothrottle will engage).

If the approach proceeds smoothly and a Go-Around is not required, the crew will prior to landing, disengage the A/T solenoid switch on the MCP by either manually 'throwing' the toggle or pressing the A/T buttons located on the thrust levers.  Although favoured by some flight crews, this practice is not authorized by all airlines, with some company policies expressly forbidding the ARM A/T technique.

The recommendation by Boeing in the B737 Flight Crew Training Manual (FCTM) states:

‘The A/T ARM mode is not normally recommended because its function can be confusing. The primary feature the A/T ARM mode provides is minimum speed protection in the event the airplane slows to minimum maneuvering speed. Other features normally associated with the A/T, such as gust protection, are not provided’.  (When the A/T is armed and the speed button option not selected).

Autothrottle Speed Protection and Vref in Windy, Gusty and Turbulent Conditions

To provide sufficient wind and gust protection, when using the autothrottle during an approach in windy conditions, the command speed should set to the correct wind additive based on wind speed, direction and gusts (between Vref+5 and Vref +20).  

The use of an additive creates a safety envelope that takes into account potential changes in wind speed and minimises the chance of the autothrottle commanding a speed that falls below Vref.  Remember, that as wind speed varies the autothrottle will command a thrust based on the speed.

During turbulence, the autothrottle will maintain a thrust that is higher than necessary (an average) to maintain command speed (Vref).

Important Points:

  • When the autothrottle is not engaged, or the speed option on the MCP deselected, minimum speed protection is lost.

  • Always add a wind additive to Vref based on wind strength and gusts.  Doing so provides speed protection when the autothrottle is engaged.

Refer to Crosswind Landings Part 2 for additional information on Vref.

A/T disengage button on throttle thrust lever.  This is an OEM throttle from a B737-300 series.  The button is identical to that used in the NG with the exception that the handles are usually white and not grey in colour.  Depressing this button will disengage the autothrottle and disconnect the A/T solenoid switch on the MCP

Manual Override - Engaging the Clutch Assembly

Occasionally, for any number of reasons, the flight crew may need to override the autothrottle. 

The Boeing autothrottle system is fitted with a clutch assembly that enables the flight crew to either advance or retard the thrust levers whilst the autothrottle is engaged.  By moving the thrust levers, the clutch assembly is engaged and the autothrottle goes offline whilst the levers are moved.

The clutch is there to enable the autothrottle to be manually overridden, such as in an emergency or for immediate thrust control.

ProSim737 does not (as at 2018) support manual autothrottle override.

Simulation Nuances

The above information primarily discusses the systems that operate in the real aircraft.  Whether these systems are functional in a simulation, depends on the avionics suite used (Sim Avionics, Project Magenta, etc).

For example, the autothrottle may not maintain the speed selected in the MCP during particular circumstances (for example, turns in high winds). If this occurred in the real world, a crew would manually override the autothrottle.  However, if the avionics suite does not have this functionality, then the next best option is to either:

(i)      Disengage the autothrottle and manually alter thrust; or,

(ii)     Deselect the speed annunciator on the MCP.

Deselecting the speed annunciator will cause the throttle automation to be disengaged; however, the autothrottle will remain in the armed mode.  The second option is a good way to overcome this shortfall of not having manual override.  By deselecting the speed option, the thrust levers can be jiggled forward or aft to adjust the airspeed.  When the speed has been rectified by manual input, the autothrottle can be engaged again by depressing the speed  button.

It's important if the autothrottle is not engaged, or is in the ARM mode, that the crew maintains vigilance on the airspeed of the aircraft.  There have been several incidents in the real world whereby crews have failed to observe airspeed changes.

Manual Flying (no automation engaged)

The benefit of flying with the autothrottle and autopilot not engaged is the ease that the aircraft can be maneuvered.  The crew sets the appropriate %N1 that produces the correct amount of thrust to maintain whatever airspeed is desired; gone are the thrust surges as the autothrottle attempts to maintain airspeed.

Granted, it does take considerable time and patience to become competent at flying manually in a variety of conditions, but the overall enjoyment increases three-fold.

Company Policies

Airline policies often dictate how a flight crew will fly an aircraft, and while some policies are expedient, more often than not they are based on economics (cost savings) for the company in question.

Policies vary concerning autothrottle use.  For example, Ryanair has a policy to disconnect the autothrottle and autopilot simultaneously, as does Kenya Airways.  Air New Zealand and QANTAS have a similar policy, however, define an altitude that disconnection must occur at or before.   If an airline doesn't have a policy, then it's at the discretion of the flight crew who should follow Boeing's recommendation in the FCTM.

Confusion and Second Guessing - Vref with A/T Engaged or Disengaged

There is considerable confusion and second guessing when it comes to determining the Vref to select dependent on whether the autothrottle is engaged or disconnected at landing.  To simplify,

  • If the autothrottle is going to be disconnected before reaching the threshold, the command speed should be adjusted to take into account winds and gusts (as discussed above and refer to Crosswind Landings Part 2).  It's vital to monitor airspeed when the autothrottle is not engaged as during the approach the speed can decay close to stall speed.

  • If the autothrottle is to remain engaged during the landing (as in an autoland precision approach), the command speed should be set to Vref +5.  This provides speed protection by keeping the engine thrust at a level that is commensurate with the Vref command speed.  If wind and gust indicate a higher additive speed, then this should be added to Vref.

Refer to Wind Correction Function (WIND CORR) for information on how to use the Wind Correction function in the CDU.

Final Call

There is little argument that the use of the autothrottle is a major benefit to reduce task loading; however, as with other automated systems, the benefit can come at a cost, which has lead several airlines to introduce company policies prohibiting the use of autothrottle without the use of the autopilot; pitch coupling, excessive vertical speed, and incorrect thrust can lead to hard landings and possible nose wheel collapse, unwanted ground effect, or a crash into terrain.

Ultimately, the decision to use or not use the autothrottle and autopilot as a coupled system is at the discretion of the pilot in command, and depends upon the experience of the crew flying the aircraft, the environmental conditions, and airline company policy.  However,  the recommendation made by Boeing preclude autothrottle use without the autopilot being engaged.

Disclaimer

The content in this post has been proof read for accuracy; however, explaining procedures that are convoluted and often subjective, can be challenging.  Occasionally errors occur. If you observe an error, please contact me so it can be rectified.

Acronyms and Glossary

  • A/P – Autopilot (CMD A CMD B).

  • A/T – Autothrottle.

  • AFDS – Autopilot Flight Director System
.

  • Command Speed - In relation to the Autothrottle, Command Speed is Vref +5 knots.

  • FCTM – Flight Crew Training Manual (Boeing Corporation).

  • FMA – Flight Mode Annunciator.

  • Manual Flight – Full manual flying. A/T and A/P not engaged.

  • MCP – Mode Control Panel.

  • Minimal Speed Protection – Function of the A/T when engaged.  The A/T has a reversion mode which will activate according to the condition causing the reversion (placard limit). (For example, flaps, gear, etc).

  • Pitch Coupling – The coupling of A/T thrust to the pitch of the aircraft.  A/T thrust increases/decreases as aircraft pitch and attitude changes.  Pitch coupling occurs when the A/P is not engaged, but the A/T is enabled.

  • Selected/Designated Speed – The speed that is set in the speed window of the MCP.

  • Take Off/Go Around (TO/GA) – Takeoff Go-around command mode.  This mode is engaged during takeoff roll by depressing one of two buttons beneath the throttle levers.

  • Vref – Landing reference speed.

Updated and Amended 04 July 2019