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Mission Statement 

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.


Note:   I have NO affiliation with ANY manufacturer or reseller.  All reviews and content are 'frank and fearless' - I tell it as I see it.  Do not complain if you do not like what you read.

I use the words 'modules & panels' and 'CDU & FMC' interchangeably.  The definition of the acronym 'OEM' is Original Equipment Manufacturer (aka real aicraft part).


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If you see any errors or omissions, please contact me to correct the information. 

Journal Archive (Newest First)

Entries in B737-800 Flight Simulator (45)


B737 Throttle Quadrant - Flaps UP to 40; Conversion and Use

This post examines the flaps lever on the refurbished B737 throttle and how it was converted to flight simulator use..

Flaps are used to slow the aircraft by creating drag, and to apply positive lift during takeoff.  The flaps lever is located on the First Officer’s side of the throttle quadrant. 

Subsequent movement of the flaps lever is indicated by illumination of the Le Flaps Transit and Le Flaps EXT lights located on the Main Instrument Panel (MIP), movement of a needle in the flaps gauge, a change of indication in the Primary Flight Display (PFD) and illumination of the Leading Edge Device (LED) panel located on the aft overhead panel. 

There are other “less obvious” indicators, but this is not the direction of his post.

The flaps lever is an integral part of the throttle unit.  Ensuring it operates correctly and with accuracy is important.

Safety Features

Newcomers to an OEM throttle quadrant are often surprised at how difficult it is to manipulate the flaps lever; it isn't a simple pull or push of a lever - there is a reason for this. 

When flaps are extended, especially at slow air speeds the flight dynamics of the aircraft are altered.  To protect against accidental flap extension, Boeing has designed the flaps lever so that a flight crew has to physically lift the lever before moving the lever to the required flap setting.  

LEFT:  Two flap gates are observed - Flaps 1 and 15 (click for larger view).

Further safety has been designed into the system by having flaps 1 and flaps 15 guarded by a flaps gate.  The gate prevents straight-through movement of the flaps lever beyond flaps 1 and 15.  The  pilot must actually lift, push and drag the lever through the gate to the next setting.

It takes a short time to become accustomed to how to move the lever for smooth operation.

Traditional Approach used in Flaps Conversion

In most throttle conversions, a single potentiometer is used and the flaps are calibrated directly through FSUPIC.  A linear rod is attached to the potentiometer and then to the lower end of the flaps lever.  When the flaps lever is moved, the rod is moved forward or aft causing the potentiometer to turn to a defined and pre-calibrated position.  The analogue movement of the rod is converted to a digital signal that can be read by Flight Simulator.

In such a conversion, it’s important to ensure that the physical position of the flaps lever matches the flaps position used in Flight Simulator and in the flaps gauge.  It’s also vital that flaps are calibrated to ensure accurate operation.

The benefits of using this traditional method are that it’s “tried and true”, inexpensive and relatively easy to implement.  Calibration is the major key; however, using FSUPIC can be troublesome and time consuming, although once calibrated everything should operate reasonably well.  

Potentiometers - Accuracy and Longevity

Potentiometers came in a variety of sizes with differing throw values.  A throw is the length of movement that a potentiometer will allow a linear rod to move.  The larger the potentiometer the more throw allowed.  The potentiometer for the flaps must fit within the throttle unit beneath the flaps mechanism in a relatively small space.  Unfortunately, with Boeing 737 late model throttle’s there is minimal room to allow a larger than 60mm potentiometer to be installed.  Using a 60 mm potentiometer means that the device has a minimal throw.

This throw, if lucky, can be stretched to cater from flaps 0 to flaps 40, but only after facetiously calibrating with FSUPIC.  More often than not,  the throw will only reach flaps 1 or flaps 30.  Often this lack of throw goes unnoticed and many virtual pilots select flaps 40 believing they actually have flaps 40, but in reality it is flaps 30.

Longevity is another more minor issue when using potentiometers.  Most potentiometers have a +- tolerance during manufacture, are made cheaply and depending upon the type selected are open to contamination from dust and debris.  Dust on a potentimeter can affect the accurancy of the unit. At the very least, maintenance is required if the potentiometer is located in a dusty area.

Several Ways to Skin a Cat.....

To solve these potential problems two methods were assessed.  The first was using two micro- buttons at each end of the linear rod that connects the flaps lever with the potentiometer.  These buttons can be assigned directly with FSUPIC to flaps UP and flaps 40.  This theoretically would solve the shortness of throw experienced with traditional conversion and calibration.  Flaps UP and 40 are controlled by micro-buttons and everything in-between is calibrated within FSUPIC.


The second method is to replace the potentiometer with micro-buttons; thereby,  rectifying the issue of minimal throw.  Replacement will also alleviate the chance of a potentiometer being inaccurate, remove any chance of contamination, and also remove the tedious task of calibrating flaps in FSUPIC.   

LEFT:  Working through an issue with the Flaps 5 micro button, custom VGA cable and PoKeys card (see below) - it's not all fun.  Chasing problems can be frustrating and very time consuming.

The use of micro-buttons to control flaps movement is relatively novel, but the potential benefits of implementing this into the throttle unit could not be overlooked; therefore, it was decided to use this method.

Problems with Micro-buttons - Design of Lower Flaps Arc Plate (LFAP)

The first initial problem encountered is that micro-buttons are small, delicate and can be easily damaged if mounted directly onto the metal flaps arc.  Manipulating the flaps lever requires considerable pressure to pull, drag and drop the lever into the correct flaps detent position. Clearly, mounting the buttons on top of the metal flaps arc for direct contact with the flaps lever was not feasible.

After much thought, it was decided to fabricate from aluminum, a plate that replicated the arc that the flaps lever moves over.  This plate has been called the Lower Flaps Arc Plate (LFAP).  The micro-buttons were then strategically mounted to the plate, each buttons’ position corresponding to a flap position.  The LFAP with the mounted buttons was then mounted directly beneath the existing flap arc plate. 

Design Considerations

Before implementing a new design, considerable thought must be taken to potential problems that may arise from the design.  In the case of using micro-buttons the issue was connectivity and the possibility of a damaged or faulty button.  The LFAP can be accessed relatively easily by removing the First Officer's side panel which allows access to the plate from behind the trim wheel.

Half-moon Provides Accuracy, Reliability and Repeatability

To enable the micro-buttons to be triggered by the flaps lever, a half-moon piece of aluminum was fabricated using the same dimensions of the lower portion of the flaps lever.  One end of the "half-moon" was  curve-shaped pointing downwards. The "half-moon" was then screwed to the lower section of the flaps lever handle   

LEFT:  Rough initial sketch of half-moon showing relationship to flaps arc and micro-buttons.

When the flaps lever is dropped into a flaps detent position, the curved side touches and depresses the micro-button mounted on the lower flaps arc plate.  When the flaps lever is moved to another flaps setting, the lever is first lifted breaking contact with the button, moved to the next setting and dropped into the detent position triggering the next button and so forth.

Interface Card

A standard PoKeys 55 interface card was used to connect the outputs from the buttons to the avionics suite software.  ProSim737 software allows easy interfacing by allowing direct connection of a button to a specific flap position.  If ProSim737 is not used and the choosen avionics suite does not support direct connection, FSUPIC can be used to assign individual buttons to flap positions.  The PoKeys card is installed in the Interface Master Module (IMM).

Advantages of Micro-buttons - It's Worth The Effort...

The benefits of using micro-buttons cannot be underestimated. 

  • 100 % accuracy of flap movement from flaps UP to flaps 40 at all times.
  • No calibration required using FSUPIC.
  • Non-reliance on FSUPIC software as the installation is mechanical.
  • Very easy configuration of flaps UP through flaps 40 using ProSim737 software configuration.
  • Removal of the potentiometer and possible inaccuracy caused by +- variation.
  • No concern regarding possible contamination of the potentiometers.
  • Enhanced reliability of operation with no maintenance required.
  • Easy removal of the Lower Flaps Arc Plate to facilitate button replacement.

Back-up Potentiometer System

Although the use of micro-buttons is successful, I still have a potentiometer installed that can be used to operate the flaps.  The reason for installing the potentiometer was in case the micro-buttons did not work correctly; it would save time installing a replacement system.  To change from buttons to the potentiometer is as easy as disconnecting one quick release connector and reconnecting it to another.

Quick Access Mounting Plate (QAMP)

The potentiometer is mounted directly onto a custom-made aluminum plate that is attached to the inside of the throttle unit by solid thumb screws. 

The reason for the plate and screws was easy access should the potentiometer need to be cleaned or be replaced. 

To access the potentiometer requires the side inspection plate of the throttle be removed (a few screws) and then removal of the thumb screws on the access plate that allows the potentiometer to be dropped from its bracket.

Unfortunately, I failed to photograph the flaps QRMP before installation; however, its design is similar to all quick release plates used within the throttle unit.  The plates are made from aluminium and are attached to the throttle unit by thumb screws rather than nuts and bolts.  This allows for easier and faster "change out" if necessary.  The above image shows the QRMP for the throttle levers - the flaps QRMP is far smaller and thinner.


During testing a problem was observed with the micro-button for flaps 5.  For an unknown reason flaps 5 would not register correctly on the PoKeys 55 card.  After several hours troubleshooting the buttons and wiring, it was determined that the PoKeys card must have a damaged circuit or connection where they flaps 5 wire was installed to the card.

The problem turned out not to be the PoKeys card, but the Belkin USB hub installed to the Interface Master Module (IMM).  I had replaced the first hub (which I damaged) with another hub that had a lower voltage.  For some reason this lower voltage was not enough to allow operation of all the functions running from the hub. 

After replacing the hub with a higher voltage device, the issue with the flaps was immediately rectified.  Of course, this was after I spent literally hours troubleshooting flaps 5!  As stated earlier, teething issues on a new design can be frustratingly time consuming...

Acronyms and Glossary

  • Flaps Arc – A curved piece of aluminum positioned directly beneath the flaps lever and corresponds to the curvature of the light plate.
  • Lower Flaps Arc Plate (LFAP) - A curved piece of aluminium that is the same size as the flaps arc and is mounted directly beneath the flaps arc.
  • Half-Moon Pencil – a custom fabricated piece of aluminum with a curved edge at one end.  Used to depress micro-buttons on flaps arc as flaps lever is moved..
  • OEM - Original Equipment Manufacturer.
  • Quick Access Mounting Plate QAMP – Quick Access Mounting Plate for the potentiometer that is a redundancy system for flaps movement.
  • Avionics Suite - Software that interacts with Flight Simulator to control avionics, gauges, etc - ProSim737, Sim Avionics, Project Magenta, etc.

B737 Aural Warning Module (AWM) Installed and Operational

One of the recent upgrades to the simulator has been the installation of an Aural Warning Module (AWM).  This module resides on the first officer side of the flight deck and is attached to the forward bulkhead of the Main Instrument Panel directly beside the throttle quadrant.  The module replaces four of the computer-synthesised warnings with the OEM counterparts.

LEFT:  Front of the AWM.

The purpose of the module is straightforward; to provide a fail-safe mechanical device that delivers loud, clear and concise tones and bells to indicate to the flight crew that major problem or configuration issue exists.  The aural alarms activate in unison with warning lights that are located on the forward overhead panel, fire suppression panel and on the glare shield of the Main Instrument Panel (six pack annunciators and master fire warning (bell cutout) and master caution buttons).

What's in the Grey Box

The aural warning box contains three mechanical devices capable of delivering four aural warnings:

(i)    The fire bell;

(ii)   The clacker; and,

(iii)   The horn (double purpose that activates either in a continuous or intermittent tone).    

The fire bell rings when any number of events relating to a fire on the aircraft occurs.  The fire bell can be silenced by either pushing the master fire warning button located on the glare shield or bellcutout switch located on the fire suppression panel.  I will be discussing at length the fire suppression panel in a future post; therefore, will not discuss the various scenarios that the fire bell operates.

The overspeeed clacker sounds when the aircraft exceeds the maximum allowable airspeed (Vmo /Mno).  The warning clacker can only be silenced by reducing your airspeed below Vmo/Mno.

The intermittent horn is an aural cue for the takeoff configuration alert.  The horn will sound when a configuration problem exists with the aircraft.  For example, advancing the throttles with the parking break set or the flaps not set.  

When the horn is activated a takeoff config warning light (in red) illuminates on the left forward overhead panel.  Deactivation of the alarm is by retarding the throttle levers to idle and then configuring the aircraft correctly.

The continuous horn is activated when specific flight conditions are met. The following are the main scenarios that activate this alarm.

  • The aircraft is below 800 feet radio altitude with flaps set from UP to flaps 10 with either throttle thrust lever set between idle and 20 degrees forward of idle.
  • The aircraft descends below 200 feet radio altitude (any configuration)
  • The aircraft has flaps set 15 through 25 with either throttle thrust lever set between idle and 20 degrees forward of idle.
  • Flaps 15 is selected without the landing gear being in the down position.
  • The aircraft has flaps set greater than flaps 25.
  • The aircraft’s landing gear is not extended.

Silencing the Continuous Tone Horn

The horn can be silenced by depressing the horn cutout switch located on the throttle quadrant; however, if the aircraft descends below 200 feet radio altitude, or the flaps are extended greater than Flaps 15 (without landing gear extended), the horn cutout switch will not silence the horn.  

Lowering the landing gear or ascending to higher altitude will silence and reset the horn.


The grey box is not an OEM part; however, is similar to the module used in the 800 series with the exception of a toggle switch located on the upper part of the unit (the toggle switch is used by maintenance).  The box was replicated (using vacuform technology) to the identical measurements of the OEM counterpart.  The replica box will be replaced when and if I find a 800 series OEM part.

LEFT:  Inside the AWM from left to right: horn, clacker and bell.  The small box houses basic circuitry. 

The mechanical tones and bell have been acquired from a Boeing 737-200 series aircraft and retrofitted into the module. 

In time as OEM NG AWM will be procured.

Difference Between Classic Series and NG Aural Warning Modules

The AWM for the classic series (300, 400, 500) and NG are different.  The 500 is closest to the NG, however, was a transition product.

Earlier AWM were analogue and used circuits to generate (synthesize various sounds), such as chimes, navigation tunes, etc).  These AWM used mechanical devices to generate the mechanical sounds such as the horn and fire bell.

The NG AWM is 100% digital and has no physical mechanical devices to generate sound.  This said, apparently some earlier NG AWM still include the mechanical fire bell.

The 500 series AWM was transitional between analogue and digital.

The NG AWM has a maintenance toggle at the upper part of the unit.  This can be used by maintenance to check the unit and to alter the volume.  However, it’s not possible to alter the volume of an individual sound – adjust one sound’s volume and they all either increase or decrease in volume relative to each other (this is what the engineer told me).  It’s not possible for pilot’s, using the toggle, to alter the volume or to select what sounds they hear.

Table 1:  Excerpt from Boeing maintenance manual explaining conditions necessary for operation.


The aural tones are mechanical and not software generated.  To interface the warnings with ProSim737 a Phidget 0/0/4 card has been used.  This card is located within the SIM interface module (SIM) and is connected to the aural warning module by a custom wired VGA cable.  The relays on the Phidget card are triggered when a specific condition, based on the offsets set within the avionics software, are met.

Authenticity and Volume

Although FSX, ProSim737, Sim Avionics and many other avionics suites include aural warnings within their package, the clarity and volume in sound produced by a mechanical device surpasses that of a computer generated sound.  

"A word of warning".  The horns and bell are loud – very loud… They are loud for a reason – to annoy a flight crew so that will not ignore the "urgency" of the alarm.  The first time the fire bell sounded during testing made me jump out of my skin!  It also activated the “yell” button on my girlfriend…  

The devices do not have a volume control.  To quieten the aural warnings for “inside” simulation use, I’ve installed foam around the mechanical devices and bell.  This has been successful in lowering the volume by around 60%. 

Below is a short video showing the Aural Warning Module and its various sounds (turn volume up).


B737 Throttle Quadrant - Speedbrake Conversion and Use


The speedbrake serves three purposes: to slow the aircraft in flight (by incurring drag), to slow the aircraft immediately upon landing, and to assist in the stopping of the aircraft during a Rejected Takeoff (RTO).  

There are four speedbrake settings: Down (detent), Armed, Flight Detent and Up. 

In addition, there are three speedbrake condition annunciators (lights), located on the Main Instrument Panel (MIP), that annunciate speedbrake protocol.  They are: Speedbrake Armed, Speedbrakes Do Not Arm and Speedbrakes Extended.  These annunciators (lights) illuminate when certain operating conditions are triggered.

This post is rather long as I've attempted to cover quite a bit of ground.  The first part of the post relates to technical aspects while the second portion deals with conversion.  Hopefully, the video at the end of this post will help to clarify what I have written.

Technical Information

Speedbrakes consist of flight spoilers and ground spoilers. The speedbrake lever controls a 'spoiler mixer', which positions the flight spoiler power control unit (PCU) and a ground spoiler control valve.   The surfaces are actuated by hydraulic power supplied to the power control units or to actuators on each surface.

Ground spoilers operate only on the ground, due to a ground spoiler shutoff valve which remains closed until the main gear strut compresses on touchdown (this is activated by the squat switch).

In Flight Operation

Actuation of the speedbrake lever causes all flight spoiler panels to rise symmetrically to act as speedbrakes.  The lever can be raised partly or fully to the UP position.  This causes the extension of the flight spoilers to the equivelent full up (ground spoiler) position.

Ground Operation

All flight and ground spoilers automatically rise to full extension on landing, if the speedbrake lever is in the ARMED position and both throttle thrust levers are in IDLE. When spin-up occurs on any two main wheels, the speedbrake lever moves to the UP position, and the spoilers extend.

When the right main landing gear shock strut is compressed, a mechanical linkage opens a hydraulic valve to extend the ground spoilers.  If a wheel spin-up signal is not detected, the speed brake lever moves to the UP position, and all spoiler panels deploy automatically after the ground safety sensor engages in the ground mode.

After touchdown, all spoiler panels retract automatically if either throttle thrust lever is advanced. The speedbrake lever will move to the DOWN detent.

All spoiler panels will extend automatically if take-off is rejected (RTO) and either reverse thrust lever is positioned for reverse thrust. Wheel speed must be above 80 knots in order for the automatic extension of the spoilers to take place.

A failure in the automatic functions of the speedbrakes is indicated by the illumination of the SPEEDBRAKE DO NOT ARM Light. In the event the automatic system is inoperative, the speed brake lever must be selected manually placed in the UP position after landing by the pilot not flying.

Speedbrake Lever Movement

The logic relating to the position of the speedbrake lever is:

DOWN (detent)

  • All flight and ground spoiler panels are in the closed position.


  • Automatic speedbrake system armed.
  • Upon touchdown and activation of the squat switch, the speedbrake lever moves to the UP position and all flight and ground spoilers are deployed.


  • All flight spoilers are extended to their maximum position for inflight use.


  • All flight and ground spoilers are extended to their maximum position for ground use.

Illumination of Speedbrake Condition Annunciators (lights)

The logic relating to the illumination of the annunciator condition lights is:

Speedbrake Armed Annunciator

  • The light will not illuminate when the speedbrake lever is in the DOWN position.
  • The light illuminates green when the speedbrake is armed with valid automatic system inputs.

Speedbrake Do Not Arm Annunciator

  • The light will not illuminate when the speedbrake lever is in the DOWN position.
  • The light indicates AMBER if there is a problem (abnormal condition).
  • The light will illuminate during the landing roll following through 64 KIAS provided the speedbrake lever has not been stowed.  The light will extinguish when the aircraft stops or when the speedbrake lever is stowed.

Speedbrakes Extended Light

  • The annunciator illuminates AMBER pursuant to the following conditions.

In Flight

  • Amber light illuminates if speedbrake lever is positioned beyond the ARMED position, and
  • TE flaps are extended more than flaps 10, or
  • Aircraft has a radio altitude (RA) of less than 800 feet .

On The Ground

  • Amber light if the speedbrake is in the DOWN (detent) position.
  • Amber light if the ground spoilers are not stowed.

It is important to remember that the speedbrakes extended annunciator will not illuminate when the hydraulic systems A pressure is less than 750 psi.

Simulator Operation - What Works

Note that the following has since been replaced with more reliable system.

Throttle Quadrant Rebuild - Speedbrake Motor and Clutch Assembly Replacement.

  1. Rejected Take Off (RTO) after 80 knots called - Activation of either reverse thrust lever and throttle to idle will extend spoilers (if RTO armed).  Lever moves to UP position on throttle quadrant.
  2. Spoilers extend on landing when squat switch activated, throttles are at idle and lever is in armed position (3 requirements).  Lever moves to UP position on throttle quadrant automatically
  3. Spoilers extend automatically when reverse thrust is applied (if not previously armed - see above)
  4. Engaging thrust after landing automatically closes spoilers.  Lever moves to DOWN position on throttle quadrant.
  5. Speedbrakes extend incrementally in air dependent upon lever position (flight detent).

Speedbrake Logic - Alpha Quadrant Card and Closed System

The logic for the speedbrake, which is identical to the real B737 aircraft, is “hardwired” into the Alpha Quadrant card which is located in the Interface Master Module (IMM) and connected to the throttle quadrant by a custom wired VGA cable.  Programming the Alpha Quadrant card is by stand-alone software.

The speedbrake system is a closed system, meaning it does not require any interaction with the avionics suite software (ProSim737); however, the illumination of the condition lights does require configuration within the avionics suite as they are not part of the closed system (a future update will include all annunciators within the system). 


A common method to convert the speedbrake is to use a potentiometer and then calibrate using FSUIPC ( Flight Simulator Universal Inter-Process Communication).   Whilst this method is valid, it relies very much on FSUIPC to determine the accuracy of the visual position of the speedbrake lever.  The longevity of the system also very much depends upon the potentiometer used, its +- variance at time of manufacture and its cleanliness.  I wanted to move away from potentiometers and FSUIPC and develop a more reliable and robust system.

Micro-buttons Replace Potentimeter - Goodbye FSUIPC

A series of micro buttons are attached to a half-moon shaped arc made from aluminum.  The arc is installed directly beneath the speedbrake lever’s range of movement.  There are six micro buttons installed and each button corresponds with the exact point that a function will be activated when the speedbrake lever moves over the button.  A further two buttons are used forward of the throttle bulkhead and are associated with arming of the speedbrake.

The benefit of using buttons rather than a potentiometer is accuracy and reliability.  A button is on or off and there is little variance.  A potentiometer on the other hand has considerable variance in both accuracy and reliability.

Relay Card

The micro buttons are connected to a Phidget 0/0/8 relay card (4 relays) that, depending upon the position of the speedbrake lever, turn on or off the programmed speedbrake logic.  The Phidget 0/0/8 relay card is located in the Interface Master Module (IMM). 

Basically, the system is a mechanical circuit controlled by micro switches that reads logic programmed into the Alpha Quadrant cards.  Because it’s a closed system, the logic from the avionics suite (ProSim737) software is not required.  Nor, is calibration by FSUPIC.

Arming the Speedbrake - The Detail

To arm the speedbrake, two micro-buttons, located forward of the throttle bulkhead and attached to a solid piece of metal are used.  Connecting the lower end of the speedbrake lever to the clutch assembly is a green coloured rod.  The rod is the linkage that moves the speedbrake lever.  Adjacent to this rod is a cylinder made from aluminum used to open or close the arming circuit.

As the speedbrake lever is brought into the arm position, the cylinder is moved until it touches either of the arming on/off button-switches.  

The cylinder will stay in the armed position until voltage is provided to the motor to move the speedbrake lever, which in turn moves the rod and cylinder. 

Power is sent to the motor in only two circumstances: when the aircraft lands and the squat switch is activated, or during a Rejected Takeoff (RTO).

LEFT & BELOW LEFTDetail of the speedbrake mechanism (click picture for larger view).  

The motor powering the movement of the lever is the angled motor. The two arming button switches can be seen, one is red the other black, while the rod, clutch assembly and cylinder can easily be identified.


Most enthusiasts use a servo motor to control the movement of the speedbrake lever.  I used a servo motor on my first TQ and was never satisfied at the speed the lever moved; it was always VERY slow and seemed to lack consistent power.

In this conversion a DC electric motor, previously used to automobile power electric windows was used.   The advantage in using a motor of this type is its small size, strong build quality and high torque output.  This translates to more than enough power to mobilize the speedbrake lever.  The motor is mounted to the front of the throttle bulkhead.

Clutch Assembly

The purpose of the clutch is to enable the movement of the motor’s internal shaft to be transferred to the rod which moves the speedbrake lever.  The clutch is fitted with a synthetic washer and a torque nut either loosens or tightens the clutch to either increase or decrease the drag pressure on the speedbrake lever (see photograph).  

Speedbrake Lever Movement - Variable Voltage to Control Speed

The speedbrake lever in the real B737 moves rather slowly when the lever is powered.  Traditionally, this slow movement has been cumbersome to replicate, the movement of the lever either being too slow or too fast.  

Below is a short video showing the speed that the speedbrake lever moves on a real Boeing 737-800 (courtesy & copyright to 737maint U-Tube).  Apologies for the adverts which I can not remove from the embed code.

Altering the Speed of Lever Movement

You will note that the lever movement is speed-controlled in both directions (forward and aft).  Whilst controlling the speed of the lever during landing is relatively easy, controlling the speed of the lever as it is stowed (down) is more difficult.  At this time I have not attempted to control the later speed.

Variable Voltage - 12 Volts

If you provide 12 volts directly to the motor, the lever will move very fast in a movement I have coined the 'biscuit cutter'.  However, if you lower the voltage that is provided to the motor, the speed of the lever will slow.  The crux of the issue is if you provide a voltage that is too low the lever will not move and if the voltage is too high you have a 'biscuit cutter'.   There has to be enough voltage for the motor to provide power to start the movement of the lever and rod. Further, the power must be strong enough to be able to push the cylinder past the on/off switch when the speedbrake is armed and deployed (down), or is being closed (up) when throttles are advanced (after touchdown).

Two Methods  & Troubleshooting Potemtial Problems

I examined two methods to reduce the speed of the lever movement.

The first method uses a commercially manufactured reducer to lower the voltage, to a level that allows the lever to move more slowly than if full voltage was supplied to the motor.  This is the more expensive, but probably the better method to use, as you know exactly what voltage the motor is receiving after the reducer is connected.  Reducers can be purchased that reduce voltage by a known amount.

The second method takes advantage of voltage-reducing diodes and resisters to minimize the voltage coming directly from the relay card (the power connects directly to the relay card).  Although simplistic and less expensive than a reducer, it can be troublesome to determine the correct voltage reduction after the diodes or resisters are installed.  

As stated above, 'too little voltage and the lever will not move or move at a snail’s pace; too fast and your cutting biscuits… '

Although diodes and resisters were used, I believe using a reducer is probably more effective.  Using the former method involves educated "guesswork"  to how much voltage is needed to start the movement of the lever.  I believe a reducer may provide a more measurable approach.

The speed that the lever moves is not "perfect", but is adequate in comparison to the speed that the lever moves in the real aircraft.  I'd like to implement the correct noise that can be heard when the speedbrake is moving.  The noise (heard in the above video) emanates from the hydraulic actuator that pushes the lever mechanism.

Illumination of Speedbrake Condition Annunciators on MIP

As outlined earlier, there are specific operational conditions that dictate the illumination of annunciators on the MIP

It’s not difficult to connect the condition lights on the MIP, to the actual position that the speedbrake lever is in.  To do so requires re-routing the wiring from the lights so that they illuminate at the correct setting as determined by the on/off micro buttons.   Connecting the condition lights completes the speedbrake circuit (movement and illumination) in a closed system separate to the avionics suite.

I have chosen not to do this; therefore, whilst the movement of the lever is a closed system the illumination of condition lights is dictated by the flight avionics.  This said, if the micro buttons have been positioned correctly, synchronization between illumination of the condition lights and the speedbrake lever position will not be problematic.

Upgrading Condtion Annunciators to Closed System

Sometime in the future, I’ll probably opt to attach the condition lights to the speedbrake closed system.   The advantage of this being, that if the developers of the avionics suite alter their speedbrake logic, it will not interfere with the closed system logic I am using.

Power Supply

The speedbrake motor is powered by a Meanwell S150 12 Volt 12.5 amp power supply.

Below is a video showing the movement and speed of the speedbrake lever.  The video also shows how the mechanism operates.  Please ignore the lack of lower display panel and GoFlight panel.  The later is for testing purposes until I have installed a fully functional overhead panel.


B737-500 Throttle Conversion to NG Style - Overview

This is the second throttle unit I’ve owned and based on experience, there are many changes that have been implemented that are different to the earlier unit.

The throttle quadrant is a relatively complicated piece of kit.  To do it justice, rather than write about everything in one very long post, I’ve decided to divide the posts into sections.  

This is the first post that will deal with the general attributes of the throttle unit, interface cards used and touch on the automation and motorization of the unit.  Further detailed posts will address individual functionality, conversion and troubleshooting.

Historical Perspective and Conversion

The TQ and center pedestal were removed from an Alaskan Air Boeing 737-500 airframe.  I purchased the unit directly from the teardown yard in Arizona (via a finder).  

The conversion to full automation and motorization was not done by myself, but by a good friend of mine who is well versed in the intricacies of the B737 and in the various methods used to install automation to a throttle unit.  I am very fortunate to be friends with this individual as in addition to being an excellent craftsman with a though understanding of electronics; he is also a retired Boeing 737 Training Captain.

New Design

The new throttle unit has been converted to Flight Simulator use based on a new design.  The interface cards, rather than being mounted on the forward bulkhead have been mounted within the Interface Master Module (IMM) which is separate to the actual throttle unit.  The DC motors required for throttle and speed brake motorization are mounted forward of the throttle unit (in the traditional location).  

Connection from the throttle to the IMM is via specially-adapted VGA cables and D-Sub plugs.  This keeps the unit clean of unsightly wiring and interface cards.  it also keeps loose cables and wires to a bare minimum on the outside of, and inside the unit; automation and motorization means that there are now moving parts and it’s important to separate delicate cards and wiring away from mechanically moving parts

This is in stark contrast to my first throttle that had the interface cards mounted directly on the forward bulkhead and within the unit.

To read about the Interface Master Module (IMM) navigate to the main tabs at the top of the website.

In addition, micro buttons have been used in some circumstances to counter the traditional method of using potentiometers to control calibration of the speed brake, flaps and throttles.

Center Pedestal - Cabling and Wiring

The three-bay center pedestal, mounted directly behind the throttle unit, has a number of cables and connections required for individual panel operation.  Rather than have these cables weave through the mechanism of the throttle (remember this is an automated throttle and there is considerable movement inside the unit), I’ve opened a hole into the platform directly under the pedestal.  

Any wiring or cabling is routed through this hole into a piece of round flexible conduit tubing (it’s actually the hose from a disused washing machine). The cables, after making their way to the front of the platform, then connect either to the computer or the Interface Master Module.

The use of flexible tubing is not to be underestimated as any cabling must be protected to avoid the chance of snagging on the under-floor yoke and rudder mechanisms which are continually moving.  

Interface Cards

Conversion of any OEM part to operate within Flight Simulator requires interface cards.  The following cards are used to convert analogue outputs to digital inputs for the throttle unit.  The cards also provide functionality for the fire panel, landing gear, yaw dampener, flaps and brake pressure gauges on the Main Instrument Panel (MIP).  All cards are mounted on the separate Interface Master Module (IMM).

  • Alpha Quadrant Motor Controller card A - TQ automation & logic CMD A channel
  • Alpha Quadrant Motor Controller card B - TQ automation & logic CMD B channel
  • Phidget High Current AC Motor Controller card – Provides two channels for trim wheel speeds and trim wheel movement
  • Phidget Motor Controller Advanced Servo card – Provides the interface or bridging between the Alpha Quadrant cards and flight avionics and CMD A
  • Phidget Motor Controller Advanced Servo card - Provides the interface or bridging between the Alpha Quadrant cards and flight avionics and CMD B
  • Phidget Motor Controller Advanced Servo card – Movement of flaps gauge
  • Phidget Motor Controller Advanced Servo card – Movement of trim indicator tabs
  • Leo Bodnar BUO 836 A Joystick Contoller card – Controls all switches & buttons on TQ
  • PoKeys 55 card - Flaps (buttons)
  • Phidget 0/0/8 relay card – Speed brake, auto throttle relays CMD B, fire panels, trim wheel revolution speed on CMD B
  • Belkin 7 input USB 6.5 amp powered mini hub (2) – TQ

Phidget Cards

Phidgets cards provide the necessary interface between the throttle and flight simulator.  I believe that Phidget cards are probably one of the more reliable cards on the market that can be used to directly interface OEM parts to flight simulator.

In addition to the two Alpha Quadrant cards mentioned above, a Phidget High Current AC Controller card acts as a "bridge" to allow communication between the Alpha Quadrant cards and the avionics suite (in this case ProSim737).  This card also provides the connectivity to allow the trim wheels to spin when CMD A or B is selected on the Main Control Panel (MCP).

Trim Tab Indicators and Throttle Buttons

To control the movement of the two trim tab indicators, a Phidget Motor Controller Advanced Servo card is used to control the output to two, two-stage DC motors.  These motors, which are normally used to power water pumps, control the variable speed of the trim indicators and the revolution of the trim wheels.  The speed which the indicator moves is reliant on the user setting within the “trim section” in the configuration page of the flight avionics software.

A Leo Bodnar 386X Joystick Controller card is used to control all switches and buttons on the throttle unit, while a Phidget 0/0/8 relay card is used to turn logic on and off that controls the actions of the speed brake.  


Essentially, automation is the use of CMD A or CMD B (auto pilot) to control the N1 outputs of the throttle, and motorization is the moving of the throttle levers in unison with N1 output.  

Automation is achieved by the use of two main motor controller cards (Alpha Quadrant cards); one for CMD A and another card for CMD B.   Each card operates separately to each other and is dependent upon whether you have CMD A or CMD B selected on the Main Control Panel (MCP).

The Alpha Quadrant cards provide the logic from which the automation of the throttle unit operates.  

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..

Main Controller Cards (thanks NASA)

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 that can be easily replaced, upgraded or changed.  

CMD A/B Auto Pilot - Two Independent Systems

Most throttle units only use one motor controller card which controls either CMD A or CMD B; whichever auto pilot 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 is selected.

Synchronized or Independent 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, the levers were synchronized; however, the NG has a computer-operated fuel control system which can minutely adjust the N1 of each engine.  This 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.  This inaccuracy can be seen on reproduction throttle units that show huge gap between lever one and lever two when automating N1 outputs.  

I decided to maintain the older system and have both levers synchronized.  If at some stage in the future I wish to change this, then it’s a matter of adding another motor to the front of the throttle bulkhead to power the second thrust lever.

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).


Four motors are used in the throttle unit.

Two electric motors are mounted forward of the bulkhead.  These motors power the movement of the throttle levers and speed brake.  Two DC “pump” motors are installed directly within the throttle unit and power the movement of the trim wheels and trim tab indicators.

A clutch system is also mounted to a solidly mounted frame on the forward bulkhead.  The clutch system is used by the speed brake.  The method of locomotion between clutch and thrust levers is a standard automobile style fan belt.  

To allow both thrust levers to move in unison, a bar linking the lever which is motorized to the non-motorized lever was fabricated and attached to the main shaft of the motor.  

The motors chosen were automobile electric window motors.  These motors are powerful, provide excellent torque and were selected due to their reliability and ease of use.

Trim Wheel Spinning

The trim wheels can spin at two different speeds dependent upon whether the auto pilot is engaged or whether automation is turned off (manual flying).  A Phidget High Current AC Controller card is used to interface the spinning of the trim wheels.  The Phidget card has two channels and each channel can be programmed to a different revolution speed.  The speed of the revolutions is controlled directly within the Phidget Advanced menu within the ProSim737 software.   

The system was duplicated using a second Phidget card to ensure that both CMD A and CMD B operated identically.

In the real aircraft there are four different revolution speeds dependent upon the level of automation and the radio altitude above the ground.  Although it is possible to program this logic into the Alpha Quadrant cards and bypass ProSim737 software, it was decided not to as the difference in two of the four speeds is marginal and probably unnoticeable.  Further, the level of complexity increases somewhat programming four speeds.

Trim Wheel Braking

In the real aircraft, the trim wheels have an effective braking mechanism that stops the trim wheels from spinning down; basically it’s a brake.  Testing of a military specification motor with brakes to stop wheel movement was done; however, the motors were too powerful and whilst the trim wheels did stop spinning, the noise and jolt of the brake activating was not acceptable.

Functionality and Configuration

The TQ has been converted to allow full functionality, meaning all functions operate as they do in the real Boeing aircraft. Speed brake, flaps, parking break, reverser levers, thrust levers, trim stabilizer runaway toggles, trim tab indications, TOGA and A/T buttons, horn cut out, fuel levers and two speed trim wheel spinning have been implemented.

These functions and the process of conversion and calibration (potentiometers and micro buttons) will be addressed in separate posts.

Configuration, if not directly to the Alpha Quadrant cards via an external software program is either directly through the avionics suite (ProSim737) and the Phidgets card software or through FSUPIC.  Where possible, direct calibration and assignments via FSX have not been used. 


The throttle unit's light plates, with the exception of the parking brake which is illuminated by a 28 Volt bulb, are back lit by 5 Volt aircraft bulbs.  A dedciated S150 5 Volt 30 amp power supply is used to supply power to the bulbs.

Stab Trim T-Locker Toggles

The only function which is different from the real aircraft is the stab trim switch.  The left hand toggle operates correctly for runaway trim; however, the right hand toggle has been configured that, if toggled to the down position, the trim wheels will stop spinning.  The toggle is a basic on/off circuit and stops current going to the motors that move the trim wheels.  

The reason for doing this is that I often fly at night and spinning trim wheels can be quite loud and annoying to non-flyers…  The toggle provides a simple and easy way to turn them on or off at the flick of a switch.

Finding T-Lockers

Finding T-Locker toggles that are used in the NG series airframes is not easy.  Reproduction units are available but they appear cheesy and rarely operate effectively as an OEM toggle.  Earlier airframes used metal paddles (my earlier 300 series throttle used these type of trim switches) while the 400 series uses a different style again.  Trim switches are usually removed and reinstalled into an aircraft; therefore, I was fortunate that the throttle unit I secured had the later model T-Lockers.

The switches are called T-Lockers as you must manually pull down the cover from each switch before pulling the toggle downwards.  This is a safety feature to ensure that the toggles are not inadvertently pushed by the flight crew.

More Pictures (less words...)

To see further pictures, navigate to the Image Gallery (tabs on main menu at top of page).

In this post we have discussed a general overview of the throttle quadrant and examined the automation and motorization.  We also have looked at the interface cards used and studied the stab trim T-Lockers in more detail.  In future posts we will examine the different parts of the throttle unit and learn how they were converted and calibrated to operate with Flight Simulator.

Click any mage to make it larger.


Full-time Construction - Light at the end of the Tunnel

It's been three weeks since my oversized box arrived from the United States and the time has not been spent idle. 

LEFT:  Revealed after removing the lid of the crate - an OEM NG style throttle unit.   The three bay center pedestal was packed to the gunnels with OEM parts!

The first morning was spent attempting to drag, carry and push a rather large and heavy (110 kilos) crate from the side garden, up five sets of cement stairs, through the door and then into the flight simulator room. 

The only way one person could move the crate was to unpack whatever was possible into the garden, then construct a  pulley system to drag the crate and its remaining contents up the stairs.  The crate then had to be pushed along the carpet, using cardboard as a slide (to protect the carpet).  It was a relief to note that the crate had a few centimeters clearance between the sides of the crate and the door edges! 

This worked out well, although it took most of the morning, as unpacking the throttle unit outside the simulator room and  moving it to the room would have been problematic.

Fork Lift Damage

My concerns about fork lifts and delicate cargo came to fruition.  A fork lift had rammed one side of the crate leaving the tell-tale evidence - a fork shaped hole!  Fortunately, most of the delicate items were not damaged and for the most part the fork only pushed air.  A book that was included in the crate received much of the brunt and saved the fork from travelling further.  But, so much for my book which now has a hole in it....

Construction Mode

I've been in construction mode attempting to get as much done before I return to my job.  The days have been long and the wire clippers are becoming blunt from endless use!  Many hours have been spent thinking how to do things and then implementing decisions - some successful and others requiring a re-think.  The telephone has been "running hot" as I discuss options with my friend (who also has a B737 simulator) on the best methods to use.

There has been  challenges both in construction and in software development; however, after almost three solid weeks, the light can now be seen at the end of the tunnel.  Hopefully, I'll have some time spare soon to collate some photographs  with words and make a few detailed posts.

I have uploaded several photographs to the Image Gallery (Interface Master Module, Throttle Unit & Conversion of Real B737 Parts).  You will also note a new tab in the main menu called Interface Master Module.

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