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The purpose of FLAPS-2-APPROACH is two-fold:  To document the construction of a Boeing 737 flight simulator, and to act as a platform to share aviation-related articles pertaining to the Boeing 737; thereby, providing a source of inspiration and reference to like-minded individuals.

I am not a professional journalist.  Writing for a cross section of readers from differing cultures and languages with varying degrees of technical ability, can at times be challenging. I hope there are not too many spelling and grammatical mistakes.


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Journal Archive (Newest First)

Entries in A/T (2)


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

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. 

LEFT:  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 (click image to enlarge).

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.

For those interested in revising the AFDS system in detail, I recommend perusing the Boeing B737 Automatic Systems Review.

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.

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.

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

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 between 1500 feet.  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 autothrottle without 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 independency of one another.

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.

LEFT:  B737 NG thrust levers.

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.

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).

LEFT:  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.

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 manoeuvring 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.

Manual Override - Engaging the Clutch Assembly

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

LEFT:  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.

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 manoeuvres.  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 Fiunction (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.


The content in this post has been proof read for accuracy; however, explaining procedures that are convolved 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 Ammeded 04 July 2019


B737 Throttle Quadrant - Automated Thrust Lever Movement

In this final post dealing with the conversion of the throttle quadrant, we will discuss the automation and movement of the throttle thrust levers and look at some of the teething problems encountered during the throttle conversion.  We will also briefly discuss the use of potentiometers.  Part of this post will be repetitive as I briefly discussed automation in an earlier post.

LEFT:  The Autothrottle arming switch is a solenoid operated switch clearly identified on the main Instrument Panel (MCP).  The switch is linked to the IAS/MACH speed window (adjacent) and to two A/T disconnect buttons located either side of the throttle lever handles.

Avoiding Confusion - Automation

To avoid confusion, automation refers only to the movement of the two throttle thrust levers in relation to the %N1 output.  These N1 limits and targets are provided by the Flight Management Computer (FMC) and normally are used by the Autopilot Flight Director System (AFDS) and the Auto Throttle (A/T) to maintain airspeed and/or thrust setting.  

Automation and Movement - Interface Cards

Essentially, automation is the use of CMD A or CMD B (autopilot) to control the %N1 outputs from the Autothrottle (logic), and motorization is the moving of the throttle levers in unison with %N1 output.  To acheive this seemlessly, two interface and one controller card are used.

Alpha Quadrant Cards (2):  Each  motor controller card has the automation logic programmed directly to the card (via propiety software).  One card controls Auto Pilot CMD A while the other card controls Autopilot CMD B.

Phidget Advanced Servo Card (2):  This card acts as an interface and bridge between the Alpha Quadrant cards and the flight simulator platform used.

The card does not provide movement for the throttle thrust levers; this is controlled by a Phidget Motor Controller card.

Leo Bodnar BUO 836 A Joystick Controller Card:  This card will register in Windows the movement of levers, buttons and switches on the TQ.  Calibration of this card is done first in Windows, then in Flight Simulator (FSX/P3D), FSUPIC or the avionics suite used; for example, ProSim737.

The interface cards are mounted forward of the MIP within the Throttle Interface Module (TIM) and are connected to the throttle unit by custom VGA cables and to the computer by a single USB cable.

Main Controller Cards

The controller card I have used is not a Phidget card but a specialist card often used in robotics (Alpha Quadrant card).  The software to program the card has been independently developed by a software engineer and does not utilize Phidgets.

The technology used in the controller card is very similar to that utilized by NASA to control their robotic landers used in the space industry.  The technology is also used to control robots used in the car industry and in other mass production streams.  One of the benefits of the card is that it utilizes a software chip (firmware) that can be easily upgraded ore replaced.  

The Alpha Quadrant cards provide the logic from which the automation of the throttle unit operates.  The cards act a 'bridge' between the card and the avionics suite - "call it a language transfer if you will."

Being able to program each card allows replication of real aircraft logic and systems.  Whenever possible, these systems and their logic have been faithfully reproduced.

CMD A/B Autopilot - Two Independent Systems

Most throttle units only use one motor controller card which controls either CMD A or CMD B; whichever autopilot you select is controlled by the same card.  

In the real aircraft to provide for redundancy, each auto pilot system is separate.  This redundancy has been duplicated by using two Alpha Quadrant controller cards, rather than a single card.  Each controller card has been independently programmed and wired to operate on a separate system.  Therefore, although only one CMD is operational at any one time, a completely separate second system is available if CMD A or B is selected on the MCP.

Synchronized or Independent Lever Motorization

Synchronization refers to whether the two throttle thrust levers, based upon separate engine %N1 outputs, move in unison with each other (together) or move independently.

In the real aircraft, on earlier airframes (B707, B727 & some B737 classics), the levers were synchronized; however, the NG has a computer-operated fuel control system which can minutely adjust the %N1 of each engine.  This advanced fuel management can be observed in a real aircraft whereby each throttle lever creeps forward or aft independent of the other lever.

Programming flight simulator to read separate %N1 outputs for each engine and then extrapolating the data to allow two motors to move the throttle levers independently is possible; however, the outputs are often inaccurate (for varying reasons).  This inaccuracy can often be observed on reproduction throttle units that exhibit a gap between lever one and lever two when automating %N1 outputs.  

It was decided to maintain the older system and have both levers synchronized.  Although this is not replicating the NG system, it does make calibration easier.  If in the future incremental thrust lever movement is required, then it’s a matter of adding another 12 Volt motor to the front of the throttle bulkhead to power the second thrust lever.  

Be aware that although both thrust levers are synchronized, the throttle handles may still show a slight difference in position in relation to each other.  This is caused by the varying tension that needs to be maintained on the fan belt connecting the 12 Volt motor to the mechanical system beneath the thrust levers.

LEFT:  Autothrottle activation will advance both thrust levers in unison to a defined %N1 output.

Another aspect to note is that the position of the thrust levers during automation is arbitrary and is a visual representation of the %N1 output; it may or may not reflect the exact position on the throttle arc that the thrust lever would be placed if moved manually (by hand with Autothrottle turned off).  

Although the TQ is automated, manual override (moving the thrust levers by hand) is possible at any time as long as the override is within the constraints of the real aircraft logic and that provided by the flight avionics (ProSim737).  

Power Requirements and Mechanics

To provide the power to move the throttle thrust levers, a 12 Volt motor previously used to power electric automobile windows, is mounted forward of the throttle bulkhead (see image at bottom of post).  Connected to the motor's pulley is a fan belt that connects to the main pulley located beneath the thrust levers.  To enable the thrust levers to move in unison, a slip clutch, which is part of the main pulley assembly, is used.  

ProSim737 Limitations - TO/GA and Auto Throttle Override

Unfortunately, concerning automation the ProSim737 is deficient in two areas: TO/GA and A/T Override (see postscript below).

LEFT:  Captain-side TO/GA button is clearly seen below lever handles.  The button at the end of the handle is the Auto Throttle disconnect button.

(A)  TO/GA

In the real aircraft, the flight crew advances the thrust levers to power 40%N1 (or to whatever the airline policy dictates), allows the engines to spool, and then pushes the TO/GA button/s.  Pressing TO/GA causes the throttle to go on-line and to be controlled by the AFDS logic.  The throttle levers then advance automatically to whatever %N1 the logic deems appropriate based on takeoff calculations.

As at the time of writing. If you're using ProSim737, this will NOT occur.  Rather, you will observe the thrust levers retard before they advance (assuming you have moved the thrust levers to %40 N1).  The reason for this is nothing to do with how the throttle is calibrated, FSUPIC or anything else.  ProSim737 software controls the %N1 outputs for the automation of the thrust levers and the developer of the software has not fine-tuned the calibration in the software to take into account real-world avionics logic.  This thread located on the ProSim737 forum provides additional information. 

I have not tested Sim Avionics, but have been told this issue doesn't occur in their avionics suite.

There are two workarounds:  Engage TO/GA from idle (hardly realistic) or push the thrust levers to around 80% N1, allow the engines to spool, then push TO/GA.  Anything less that around 80% N1 will cause the thrust levers to retard before advancing.


The latest version of ProSim (V-133) has provided improvement to the above issue.  Throttles can now be advanced to ~60% N1 and TOGA engaged without the throttle levers retarding.  This is possible ONLY if you calibrate the throttle levers within ProSim and allow ProSim to control the throttle output logic.  if you calibrate within FSUPIC then the same issue will apply.

According to ProSim developers, this issue is probably related to the calibration of the ProSim servo output. When you press TO/GA, the current N1 is taken and calculated back to a throttle percentage. This throttle percentage, when combined with the servo calibration data from ProSim results in a servo output. The servo calibration at the moment only has 2 calibration points, which are 0 and 100%. This results in a linear behavior between the two points, while depending on the construction of the throttle, the relationship might be non-linear. This would require a multi point calibration which is hard to do at the moment, because a throttle does not have exact readouts of the current position, so it will be hard to calibrate a 50% point.

This may need improvement in the code to auto calibrate the throttle system.

It's hoped that fuutre relase of ProSim will rectify this issue.

(B) Autothrottle Manual Override

In the real aircraft, manual override is available to a flight crew and the thrust levers can be retarded with the Autothrottle engaged.  When the flight crew release pressure on the thrust levers the Autothrottle will take control again and return the thrust levers to the appropriate position on the throttle arc dependent upon the speed indicated in the speed window of the MCP.

ProSim737 will not temporarily disconnect (manual override) the Autothrottle.  

At the time of writing, there is no workaround to solve this.

Potentiometers - Two Types; Which is Best

There are two types of potentiometers.  The first type, (I will call them standard potentiometers) are inexpensive, often have a +- percentage variance, are compact, have a minimal throw depending upon the size of the device and are not contaminate free.  

The last point is worth mentioning as it is wrongly assumed that a potentiometer will remain correctly calibrated for the life of the unit.  General wear and tear, dust and other debris will accumulate on the potentiometer; any of which may cause calibration and accuracy problems.  Keeping the potentiometers free of dust is important.

The second type of potentiometer is called a string potentiometer (strings).  Contrary to the standard type, strings are very accurate, are in a sealed unit presenting zero contamination, are manufactured to exacting standards, are larger in size and are expensive.

The difference in size between the two potentiometer types is often the reason for using the smaller standard type.  The strings are very long requiring quite a bit of real estate either forward of the throttle bulkhead or within the center pedestal.  In contrast, the standard potentiometers are quite compact; finding a position to install them is not problematic.

Calibration of Potentiometers

The main method of calibrating the position of the thrust levers is by calibrating the potentiometer in Windows, then in FSX followed by fine-tuning in FSUPIC (if needed).  

Standard potentiometers are used in the simulator; therefore, at some stage cleaning or replacement of a potentiometer maybe necessary.   The 737 throttle quadrant is not cavernous and only certain sized potentiometers will fit into the unit; this combined with other parts and wiring means that the potentiometers are often inaccessible without removing other components.  

To allow speedier access to the potentiometers, a Quick Assess Mounting Plate was designed.

Quick Access Mounting Plate (QAMP)

The potentiometers are mounted directly onto a custom-made aluminum plate that is attached to the inside of the throttle unit by solid thumb screws.   To access the plate, the side inspection cover of the throttle is removed (a few screws) followed by turning the thumb screws on the access plate.  This releases the plate.

LEFT:  QAMP secured to base of throttle unit.  Thumb screws are visible on each corner of the plate.  A possible add on modification to reduce the risk of dust contamination to the potentiometers is a plastic cover that fits over the plate (a lunch box).

A similar plate has been designed and constructed for use with the stand-by potentiometer that controls the flaps.  A more detailed picture of the QAMP can be seen here in an earlier post.

Below is a video showing the movement of the thrust levers with the Autothrottle (A/T) engaged.  The movement of the thrust levers is in real time according to flight parameters during the test flight and has not been instigated by overriding the servo. 

Teething Issues with the Throttle Conversion

It was envisaged that more problems would have surfaced than have occurred.  The major issues are outlined below:

(A) Trim Wheels

An early problem encountered was that the trim wheels when engaging generated considerable noise.  After checking through the system, it was discovered that the two-speed rotation of the trim wheels were causing the two nuts that hold each of the trim wheels in place to become loose.  This in turn caused the trim wheels to wobble  slightly generating undue noise.  


Tighten the two nuts at the end of the rod that holds the two trim wheels in place.

(B) Flaps 5 Not Engaging

The problem with the flaps 5 micro-button has been discussed in an earlier post.  To summarize, when you moved the flaps lever to flaps 5 the correct flaps were not selected on the aircraft or registered on the PoKeys 55 interface card.  Several hours were spent checking connections, micro-buttons, wiring and the custom VGA cables that connect the flaps section of the quadrant to the appropiate interface module; the problem could not be discovered.  


One of the two Belkin powered hubs located within the IMM had been replaced with another powered unit.  It appears the problem was that the replacement hub had too low a voltage, as a replacement with a higher voltage solved the problem.

(C) Throttle Thrust Levers Not Synchronizing (A/T on)

The two thrust levers of the quadrant did not synchronize when the Auto Throttle (A/T) was engaged; one lever would always be ahead or behind of the other.  At other times they would split apart (do the splits) when A/T was engaged.  


The problem was easily solved by altering the tension on the slip clutch nut.  When the nut was  tightened, both levers moved together as one unit.  The secret was finding the appropriate torque.

(D) Throttle Thrust Levers Difficult To Move in Manual Mode (A/T Off)

The ability to move the thrust levers in manual mode (Autothrottle turned off) was not fluid and the levers occasionally snagged or were sticky when trying to move them.  

This is caused by the fan belt not moving smoothly through the groove of the pulley wheel.   The Autothrottle when engaged overrides any stickness due to the power and torque of the Auto Throttle motor.


Unfortunately, there isn’t a lot you can do to rectify this issue as it’s a by-product of using a mechanical system in which the fan belt is central to the consistent operation of the unit.  

LEFT: The fan belt is barely visible linking the pulley of the motor to the main pulley inside the quadrant.

The conundrum is that if you tighten the fan belt too much you will be unable to manually move the thrust levers as they will be exceptionally stiff and difficult to move (as you are pushing against the tension of the fan belt); however, if you loosen the fan belt too much, although the levers will move fluidly by hand, the fan belt may not have enough tension to move the levers when Auto throttle is engaged.  It’s a matter of compromise; selecting an appropriate in-between tension to allow acceptable manual and Autothrottle operation.

A more reliable method is to use a small gearbox, a simple slip clutch and a coupler to connect to the spur gear.  Another option is to use an electrical system.

Further thought needs to be done in this area before a decision is made to replace the fan belt system.  If a new system is incorporated, the change-out will be documented in a future post.


This brings us to the end of the throttle conversion.  The following links will take you to other posts regarding the conversion.  

B737 TQ - General Overview
B737 TQ - Speedbrake Conversion and Use
B737 TQ - Flaps UP to 40; Conversion and Use
B737 TQ - Trim Wheels and Trim Indicator Tabs
B737 TQ - Parking Brake Mechanism

Despite some of the shortcomings to this conversion, in particular the mechanical fan belt system, the throttle unit shows a marked improvement on the earlier 300 series conversion.

Technology and innovation rarely stand still and there is little doubt other ways will evolve to achieve similar results with greater efficiency.

Acronyms and Glossary 

AFDS - Autopilot Flight Director system
A/T – Autothrottle
CMD A/B - Autopilot on/off for system A or system B
Flight Avionics Software - Sim Avionics, ProSim737 or similar
FMC - Flight Management Computer
MCP - Main Control Panel
QAMP – Quick Access Mounting Plate
Throttle Arc – The arc of the thrust levers from the end of the blocks to fully forward.  The term refers to the curved piece of aluminum that the throttle levers are moved along
TO/GA - Takeoff Go-around switch
%N1 -  Very simply explained, %N1 is throttle demand and as N1 (and N2) spin at absurdly high speeds, it is easier to simply reference a percentage and display that to the crew. It's much easier for our brains to interpret a value on a scale of 0-100% rather than tens of thousands of RPM