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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)
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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.

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Reader Comments (2)

Nice article.

I've wondered--can you trim by rotating the trim wheels? And if so, is it handled with an encoder or what?

September 29, 2013 | Unregistered CommenterNed Hamilton

Hi Ned - I had intended to implement trimming of the wheels by hand; this certainly is possible.

However, the level of complexity increases dramatically for such a small thing that I rarely would use. Therefore, I decided to not do this.

To alter the trim by hand requires the trim wheels to be rotated many times - something I'd never do when I have an electric trim switch :) I'll be discussing the trim wheels and trim tab indicators in a little more detail in a specific post (too much for one post....)

I am trying where possible to keep things as simple and clean as possible. I've learned through experience that the more complexity and duplicate systems you incorporate, the more opportunity you have for glitches and problems. Cheers, F2A

September 29, 2013 | Registered CommenterFLAPS 2 APPROACH
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