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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.

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Journal Archive (Newest First)

Entries in Speedbrake (2)


Throttle Quadrant Rebuild - Speedbrake Motor and Clutch Assembly Replacement 

The speedbrake system has been completely overhauled. 

In the previous conversion a number of problems developed.  In particular, the speed of the speedbrake lever when deployed was either too fast, too slow, or did not move at all.  Furthermore, the clutch mechanism frequently became loose.

Rather than continually ‘tweak’ a problematic system, the motor and clutch assembly was removed and replaced with a more advanced and reliable system.

LEFT:  The motor that provides the power to move the speedbrake lever is attached via a slipper clutch to the speedbrake control rod (click to enlarge).  Below the motor is the Throttle Communication Module (TCM).

To read about the first conversion and learn a little more about closed-loop systems and how the speedbrake works I recommend you read the first portion of my earlier article.

B737 Throttle Quadrant  Speedbrake Conversion and Use

Motor and Clutch Assembly

A 12 volt motor is used to power the speed brake.  The motor is mounted forward of the throttle unit above the Throttle Communication Module (TCM).  The wiring from the motor is routed, in a lumen through the throttle firewall to a 12 volt busbar and relays.  The relays, mounted inside the TCM, are dedicated to the speedbrake. 

Attached to the 12 volt motor is a clutch assembly, similar in design to the slipper clutches used in the movement of the two throttle thrust levers.  The clutch can easily be loosed or tightened to provide the correct torque on speedbrake lever, and once set will not become loose.

The slipper clutch was commercially made.

LEFT:  A linear throw potentiometer has been mounted on the Captain-side of the quadrant.  The potentiometer enables the movement of the speedbrake lever to be finely calibrated (click to enlarge).

Speedbrake Mechanics and Logic

The logic that controls the speedbrake has not be changed from what it was in the previous conversion. 

Micro-buttons have been strategically placed beneath the speedbrake lever arc.  As the speedbrake lever moves over one of the buttons, the button will trigger a relay to either open or close (on/off).  The four relays, which are mounted in the Throttle Communication Module (TCM) trigger whether the speedbrake will be armed, stowed, engaged on landing, or placed in the UP position.

To improve the positional accuracy of the speedbrake lever, a linear throw potentiometer has been mounted to the throttle superstructure on the Captain-side. 

Speedbrake Operation

The speedbrake system is a mechanical closed-loop system, meaning it does not require calibration using FSUIPC or from the avionics suite (ProSim737).  The speedbrake lever, however, does need to be registered and calibrated  as a joystick controller in the Windows operating system.  This enables the avionics suite to see the potentiometer.

The following condtions will cause the speedbrake lever to deploy from the DOWN to the UP position.

(i)  When the aircraft lands and the squat switch is activated;

(ii)  During a Rejected Takeoff (RTO).  Assuming the autobrake selector switch has been set to RTO, there is active wheel spin, and the groundspeed exceeds 80 knots; and,

(iii)  If the reverse thrust (reversers) are engaged with a positive wheel spin and a ground speed in excess of 100 knots.

Point (iii) is worth expanding upon.  The speedbrake system (in the real aircraft) has a built-in redundancy in that if the flight crew forget to arm the speedbrake system and make a landing, the system will automatically engage the spoilers when reverse thrust is engaged.  This redundant system was installed into the Next Generation airframe after several occurrences of pilots forgetting to arm the speedbrake prior to landing.  

Therefore, the speedbrake will deploy on landing either by activation of the squat switch (if the speedbrake was armed), or when reverse thrust is applied.

The illumination of the speedbrake annunciators (condition lights) is not part of the system and must be programmed directly within ProSim737 (switches/indicators).

Programmed Variables

The following variables have been programmed into the logic that controls the operation of the speedbrake.

(i)    Rejected Take Off (RTO).  This will occur after 80 knots call-out.  Spoilers will extend to the UP position  when reverse thrust is applied.  The speedbrake lever moves to UP position on throttle quadrant.  RTO must be armed prior to takeoff roll;

(ii)    Spoilers extend on landing when the squat switch is activated.  For this to occur, both throttle thrust levers must be at idle (at the stops).  The speedbrake lever also must be in the armed position prior to landing.  The speedbrake lever moves to UP position on throttle quadrant;

(iii)    Spoilers extend automatically and the speedbrake lever moves to the UP position when reverse thrust is applied;

(iv)    Spoilers close and the speedbrake lever moves to the DOWN position on throttle quadrant when the thrust levers are advanced after landing (auto-stow); and,

(v)    Speedbrakes extend incrementally in the air dependent on lever position (flight detent).

Speedbrake Lever Speed

When the speedbrake lever is engaged, the speed at which lever moves is quite fast.  The term ‘biscuit cutter’ best describes the energy that is generated when the lever is moving; it certainly will break a biscuit in two as well as a lead pencil.  Speaking of lead pencils, I have been told a favorite trick of pilots from yesteryear, was to rest a pencil on the throttle so that when the speedbrake engaged the pencil would be snapped in two by the lever!

In the real Boeing aircraft the movement of the lever is marginally slower and is controlled by an electrically operated actuator (28 volts DC). 

LEFT:  The actuator that controls the movement of the speedbrake.  This image was taken from beneath the floor structure of a Boeing 600 aircraft.  Image copyright to Karl Penrose (click to enlarge).

In theory, the moderate speed that the speedbrake lever moves in the real aircraft should be able to be duplicated; for example, by suppressing the voltage from the 12 volt motor by the use of a capacitor, using a power supply lower than 12 volts, or by using speed controllers.  These alternatives have yet to be trailed.

It is unfortunate, that most throttle quadrants for sale do not include the actuator.  The actuator is not part of the throttle unit itself, but is located in the forward section under the flight deck.  The actuator is then connected to the speedbrake mechanism unit via a mechanical linkage.

In the real Boeing aircraft, the speedbrake lever and actuator provide the input via cables, that in-turn actuate the speedbrakes.  There is no feedback directly from the hydraulics and all operation is achieved via the manual or electric input of the speedbrake lever.

Actuator Sound

The sound of the actuator engaging can easily heard in the flight deck when the speedbrake engages (listen to the below video).  To replicate this sound, a recording of the actuator engaging was acquired.  The .wav sound file was then uploaded into the ProSim737 audio file library and configured to play when the speedbrake is commanded to move.  

The .wav file can be shortened or lengthened to match the speed that the lever moves (download .wav file).


The upper video demonstrates the movement of the speedbrake lever.    The lower video, courtesy of U-Tube, shows the actual movement of the lever in a real Boeing aircraft.

Notice: Not for operational use; video is intended to present the features and functions of the unit in question and not procedures.

If you listen carefully to both videos, you will note a difference in the noise that the actuator generates.  I am lead to believe that the 'whine' noise is slightly different depending upon the aircraft frame.  The actuator in the older classic series Boeing being more of a high whine in comparison to the actuator in the NG airframes.


Condition(s) - A term referring to a specific parameter that is required to enable an action to occur.
FSUIPC - Flight Simulator Universal Inter-Process Communication.  A fancy term for software that interfaces between the flight simulator programme and other outside programmes.
Speedbrake Lever Arc - The curved arc that the speedbrake lever moves along.


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.