B737-300 & 500 SERIES THROTTLE QUADRANTS (updated 22 August 2015)

This page discusses two throttle quadrants.  The first unit was purchased in early 2011 and sold.  The second unit was purchased in 2013.  The second throttle unit was subsequently rebuilt after several issues persisted with the first conversion.

For the purpose of this post, a throttle quadrant is the actual quadrant and does not include the attached center pedestal.

LEFT:  B737-300 series throttle quadrant.  The first unit used in the simulator (click to enlarge).

First Throttle Quadrant - B737-300 (2011)

The throttle quadrant was discovered for sale in an aviation tear-down yard in Arizona. The throttle came from retired Boeing 737-300 series aircraft and included the full throttle quadrant and two-bay center pedestal complete with DZUS rails. The TQ belonged to a South West 737 that plied the continental US for many years.

LEFT:  First TQ - B737-300 throttle quadrant.  This quadrant was not converted to full automation and motorization; therefore, conversion after basic refurbishing was not overly complex.

The throttle was only converted for manual use and the conversion did not include any form of automation.  The reason being that automation and motorization at this time was in its infancy, and more often than not when implemented presented with inconsistent outputs.
At the time, my knowledge of how to convert a throttle quadrant was minimal; therefore, I accepted guidance from Art May and asked Art from Northern Flight Simulations in Florida, USA, to convert the throttle.

The conversion was extremely basic and many of the parts used were inexpensive and rather rudimentary in nature.  The operation of the unit, although adequate, was not to a standard that I was happy with.  The unit was sold in 2012.

The following journal posts document the conversion process and some of the issues associated with the first throttle unit:

(more links soon)

Second Throttle Quadrant - B737-500 (2013)

In late 2013, I purchased another throttle quadrant that has been used in a B737-500 series aircraft. 

LEFT:  The second TQ - A 500 series quadrant, with NG conversion awaiting unpacking from the delivery crate.  The various VGA cables and D-Sub plugs can be seen forward of the throttle unit.  These cables connect directly with the trail Interface Master Module (IMM).

Because of the improvements to automation at this time, this unit was refitted for full automation and motorization.  The throttle unit was also converted to an NG style.

The replacement throttle unit coincided with the development of the interface modules that are used in the simulator to control various aircraft systems.  The trial interface module was called the Interface Master Module (IMM) and this module, amongst other things, accommodated several interface cards and relays that were used to control the automation and motorization of the throttle unit.

The following information provides an overview of the conversion process.

Controlling Throttle Automation - Controller Cards, Phidgets, DC & Electric Motors

Automation and Bridging

Automation of the throttle unit is controlled by two Alpha Quadrant motor controller interface cards.  The cards are programmed with logic to automate the operation of the thrust levers when either CMD A or CMD B is selected on the MCP.   The software used is very similar, if not identical to that used to program robotic engineering for mass production as observed in automobile and other factories.  NASA also use similar technology to control the lunar robots used in the space industry.

A major benefit of using these controller cards is that it removes the need for the FS2Phidgets library.  In the past with my 300 series throttle, the Phidget library had caused minor glitches.  These glitches being especially evident when using a USB hub.

A high current Phidget motor controller servo card is then used to act as a ‘bridge’ between the two Alpha Quadrant motor controller interface cards and the avionics suite (in this case ProSim737).
The controller cards are the software behind the automation.

Phidget, Relay and Controller Cards

Phidget and relay cards have been used to control many of the functions of the throttle unit, such as the accurate movement of the trim tab indicators, Runaway stab trim toggles, and movement of the flaps, fire handle operation, and several other functions.  Other buttons and switches located on the throttle are controlled by a Leo Bodnar BU0836X joystick controller card, and FSUIPC is used to assign functionality and calibrate if necessary.

Stab Trim Wheel Rotation and Thrust Lever Movement

A Phidget motor controller advanced servo card and high current motor controller card is used to power two DC pump motors that control the movement of the trim wheels (when the aircraft is being trimmed either manually or by the autopilot CMD A or CMD B).  Two speeds have been used to rotate the trim wheels.  The speed that the trim wheels rotate is dictated by the logic supplied by the avionics suite (ProSim737) and by a Phidget card which has been configured to read two channels; each channel controling a specific speed.  

An electric motor, previously used to power and move automobile electric windows, is used to power the smooth movement of the throttle levers.   

Speedbrake and Speedbrake Lever Movement

The operation of the speed brake, which replicates the logic found in the real aircraft, can be controlled either by the logic within the Alpha Quadrant cards or by the flight avionics (ProSim737).

LEFT:  New TQ installed and operational.

To enable the speed brake lever to engage at the correct position, a number of micro buttons have been strategically located beneath the plate of the speed brake lever.  Movement of the lever over the switch sends a signal which activates or deactivates four relays that trigger the speed brake logic.

The mechanical circuit is simplistic in design and allows accurate and trouble free operation that is a mechanical solution to a normally software driven operation.

Speedbrake Lever Speed

The speed that the lever moved when the spoilers are deployed was an issue on my earlier throttle (it was snail slow).  To solve this problem, a high torque DC motor identical to that used for the movement of the throttle levers has been used.  The motor provides enough power to move the lever at a similar speed observed in the real aircraft.

The DC motors for both the speed brake and the throttle lever movement is mounted forward of the throttle unit.


The flaps lever in most throttle units is controlled by a potentiometer that is calibrated via FSUPIC.  Calibrating with FSUIPC can be troublesome and often the calibration is not very accurate due to the minimal throw of the potentiometer.  Contamination and variance in potentiometer manufacturer (+- tolerances) can also be problematic.

A solution was developed that replaces the potentiometer with a series of micro buttons.  The micro buttons have been correctly positioned on a custom-made bracket that is mounted immediately below the flap arc beneath the flaps lever. The micro buttons are on/off and are activated as the flaps lever is lifted or dropped into the flaps detent position.

Calibration of the flaps is done directly through ProSim737; FSUIPC is not required.  This method is very accurate and does not require any calibration, tweaking or maintenance.  The use of buttons to replace the minimal throw of a potentiometer allows a complete range of movement from flaps up to flaps 40.

The following journal posts address the conversion of this throttle unit:

NOTE - This throttle has been rebuilt from the ground-up.  Navigate to the Flight Controls/Throttle Quadrant tab in the main menu to read about the advanced rebuild of this throttle unit.