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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.


Please Note:   I have NO affiliation with ANY manufacturer or reseller.  I tell it as I see it!

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


All funds are used to offset the cost of server and website hosting (Thank You...)


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


Flight Controls - General Information

Second to the yoke, a throttle quadrant (TQ) is probably the next important item in the simulator.  There is more to the TQ than thrust levers.  A TQ includes functionality for thrust levers, reverse thrust, idle cut outs, flaps, speedbrakes, park brake, trim wheels spinning, horn cut out, stabilizer trim cut out and trim tab indicators.  As with the flight yoke, if possible it's best to not scrimp in this area.

I've used several  throttle set ups beginning with the keyboard (FS1), migrating to CH Products throttle before using a higher end unit manufactured by Precision Flight Controls (PFC). 

The B737-800 Project uses the following flight controls:

  • OEM B737-500 dual yokes and columns (identical to NG)
  • OEM B737-500 Throttle Quadrant (revamped to NG style)
  • OEM B737-400 Rudder Pedals with custom-made rudder pedal assembly
  • OEM B737-400 Steering Tiller (identical to NG)

OEM is an acronym for Original Equipment Manufacturer (meaning a 'real' aircraft part).

Throttle Quadrants - Historical perspective

There are many differing throttle units commercially available for use in flight simulator and selecting one is predominately based on what level of realism you are seeking.

At an early stage I was moving towards a reproduction throttle quadrant produced by one of several companies such as ThrottleTec or Revolution-Sim. Without going into great detail, I wasn't convinced that the reproduction throttles provided consistent reliable service for the amount of expenditure required (over $5000.00 USD). The more down market throttles, such as that produced by ThrottleTec did provide consistent operation, however, lacked realism and authenticity. It didn't take much imagination to finally decide on a reconditioned genuine 737 throttle from a real aircraft.

To Avoid Confusion - Throttle Conversions

To date I have converted two throttle quadrants.  The first was from a B737-300 with a two-bay center pedestal.  This was converted by Northern Flight Sim for manual use only.  The unit was subsequently sold and replaced by a B737-500 quadrant with three-bay center pedestal.  This unit was converted to an NG style unit with full automation.  It is the current throttle quadrant in the simulator.

This unit will be modified further during 2014 to replace some of the mechanism which is now antiquated, and to improve the automation.

On this website you will find mention of three (3) throttle quadrants and their conversions. 

  1. The earlier B737-300 (converted by Northern Flight Simulations and subsequently sold)
  2. The B737-500 converted to NG style (the current TQ)
  3. The advanced advanced conversion of the above-mentioned NG style quadrant.

The conversion of the throttle unit has been an 'evolutionary process' and the latest advanced conversion has incorporated high-level mechanical and software engineering.  The design and operation duplicates the methods used in the real aircraft and in high level commercial simulators used for pilot training.

The advanced conversion of the throttle will be completed by late 2014.

First TQ - B737-300 Throttle Quadrant

I was fortunate to find a throttle quadrant for sale in a tear down yard in Arizona. The throttle came from retired Boeing 737-300 series aircraft and included the full throttle quadrant and 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 wasn't overly difficult.


Historically, autothrottle use has always posed more challenges and problems than what is was worth; however, technology goes forward and the development of specialist interface cards, that superceeded the standard-issue Phidget card, and the continued development of advanced avionic suites such as ProSim737 and to a lesser extent Sim Avionics has allowed automation to be improved upon several-fold.

The first TQ, since sold, was not converted for automation as at that time (late 2012) automation was in its infancy.

Second TQ - B737-500 (Full Automation and NG Conversion)

In late 2013, I upgraded the throttle to a throttle that came from a B737-500 series aircraft.  Because of the improvements to automation, this unit was refitted for full automation and motorization.

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

Throttle Automation & 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.

ABOVE:  The second TQ - A NG style quadrant, still in the crate after delivery.  The various VGA cables and D-Sub plugs can be seen forward of the throttle unit.

A major benefit of using these cards is that it removes the need for the FS2Phidgets library.  In the past with my 300 series throttle the Phidget library caused minor glitches., this 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  & Leo Bodnar 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, 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 FSUPIC is used to assign functionality and calibrate if necessary.

Controlling Trim Wheel Spinning & Movement of Throttle Levers

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). 

LEFT:  Backlighting is impressive as the light plates are in excellent condition.

The speed that the trim wheels spin is dictated by the logic supplied by the avionics suite (ProSim737); this logic is identical to that of the real aircraft.  To change the revolutions of the trim wheels, the Phidget card has been set-up using two channels, each channel controls a specific speed.  The speed used is dependent on the level of automation set from the MCP.

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

Logic for Speed Brake and Flaps

Speed Brake

The operation of the speed brake, which replicates the logic observed in the real aircraft, can be either controlled by the logic within the Alpha Quadrant cards or by the flight avionics (ProSim 737). 

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.

Speed of Lever Movement

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 FSUPIC 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, FSUPIC 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.

Journal Posts

The posts that document the conversion of the Boeing 737 throttle to Flight Simulator have generated considerable interest.  To consolidate the posts for retrieval, I’ve provided links to those posts that deal with the conversion of the throttle quadrant.  These links only relate to the B737 NG throttle and do not relate to the B737-300 throttle sold in 2012.  Any post from 2013 onwards deals ONLY with the new quadrant.

The following links are to posts on this site concerning the B737 NG  throttle quadrant. 

Throttle Improvements / Retrofitting The last Build (evolution) - Advanced Conversion (updated Information July 2014)

The throttle unit, in its current revision, works well, but a number of small issues needed rectifying to improve upon the functionality, automation, and more importantly the accuracy of the automation.  There were also some 'niggling' issues with how the clutch assembly operated - it was somewhat loose which caused several on-going issues.

Initially, some minor improvements were to be made; however, one thing lead to another and fate would have it . the throttle units has been 'almost' rebuilt from the bottom up.

The improvements have primarily been to the autothrottle and speed brake system; however, replacement motors will also improve the movement of the trim wheels and the accuracy of the trim tab indicators, while enhancing synchronised thrust lever movement.  An updated system to operate the parking brake has also been devised which replicates the system used in the real aircraft and is more reliable.  Furthermore, the potentiometers controlling the movement of the flaps will be replaced with string potentiometers which will improve the throw of the potentiometer and improve accuracy.  Finally, the automotive fan-belt system/clutch system which was a chapter from the 'dark Ages' will be replaced with a mechanical clutch assembly that has been professionally designed to operate within the throttle unit - this will completely remove any of the  'niggles' with the clutch assembly becoming loose.

The throttle quadrant conversion has been a learning process, and the changes that are to be made in late 2014 will improve the unit's functionality, longevity and accuracy far beyond what it was previously.  It is hoped everything will be operational by very early January 2015.

The alterations will be documented in separate posts and links added to the section above.