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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 Speedrake Logic (1)


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.