E-mail Subscription

Enter your email address:

Delivered by FeedBurner

Syndicate RSS

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).


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

No advertising on this website - EVER!


Find more about Weather in Hobart, AU
Click for weather forecast






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

Journal Archive (Newest First)

Entries in Throttle Quadrant (12)


Repair Backlighting on Throttle Quadrant 

During a recent flight, I noticed that the bulbs that illuminate the backlighting for the trim and flaps lightplate (First Officer side) had failed, however, the backlighting on the Captain-side trim lightplate was illuminated.  My first thought was that the 5 volt bulbs that are integrated into the lightplate had burned out; after all, everything has an end life.

LEFT:  The rear of the First Officer side trim lightplate showing one of the two terminals that the wiring loom connects to (click to enlarge).

Backlighting - Wiring Loom

The wiring loom that supplies the power for the backlighting enters the throttle quadrant via the front firewall, and initially connects with the trim lightplate and parking brake release light on the Captain-side.  A Y-junction bifurcates the wire loom from the Captain-side to the First Officer side of the quadrant, before it snakes its way along the inside edge of the quadrant firewall to connect with the First Officer side trim lightplate, and then the flaps lightplate.  The wiring loom is attached securely to the inside edge of the throttle casing by screwed cable clamps.

The backlighting for all lightplates is powered by 5 volts and the backlighting on the throttle quadrant is turned on/off/dimmed by the pedestal lighting dimmer knob located on the center pedestal. 

Finding the Problem

Ascertaining whether the bulbs are burned out is uncomplicated, however, assessing the terminals on the rear of each lightplate, and the wiring loom the connects to the lightplates, does involve dismantling part of the throttle quadrant.

The upper section of the throttle quadrant must be dismantled (trim wheels, upper and side panels, and the saw tooth flaps arc).  This enables the inside of throttle quadrant to be inspected more easily with the aid of a torch (lamp/flashlight).  When removing the trim wheels, be especially vigilant not to accidently pull the spline shaft from its mount, as doing so will cause several cogs to fall out of position causing the trim mechanism to be inoperable.

After the lightplates have been removed, but still connected to the wiring loom, a multimeter is used to read the voltage of each respective terminal on the lightplate. If the mutlimeter indicates there is power to the terminals, then the bulbs should illuminate. 

What surprised me when this was done, was that the bulbs worked perfectly. Therefore, it was clear the problem was not bulb, but wire related.

Process of Elimination

The process of elimination is the easiest method to solve problems that may develop in complicated systems.  By reducing the components to their simplest form, a solution can readily be attained.

If you suspect that the wiring is the problem, and don't have a multimeter, then a quick and fool safe method is to connect an alligator cable from the positive terminal of the Captain-side lightplate to the respective terminal on the First Officer lightplate.  Doing this removes that portion of the wiring harness from the circuit. 

LEFT:  Alligator wire connects power from Captain-side lightplate to the First Officer lightplate.  Note the frayed outer layer of the white aircraft wire.  The gold colour is a thin layer of gold that acts as a fire retardant should the wiring overheat (click to enlarge).

In this scenario, the  bulbs illminated on both trim lightplates.  As such, the problem was not bulb related, but was associated with the wiring loom.

It must be remembered that the wire used to connect the backlighting in the throttle quadrant is OEM wire.  As such, the age of the wire is the same age as the throttle quadrant.  

Inspecting the wire loom, I noticed that one of the wires that connected to the terminal of the lightplate was severed (cut in two).   I also noted that the original aircraft wires had begun to shed their protective insulation layer. 

Aircraft Wire and Insulation Layers

The high voltage and amperages that travel through aircraft wire can generate considerable heat.  This is why aircraft wire is made to very exacting standards and incorporates several layers of insulation that surround the stranded stainless steel wire.  The use of high-grade stainless steel also provides good strength and resistance to corrosion and oxidation at elevated temperatures.  

Interestingly, one of the insulating layers is comprised of gold (Au).  The gold acts as an effective fire retardant should the wires overheat.

LEFT:  The green wire has been severed.  A possible scenario was that the wiring loom had been pulled slightly loose from the throttle chassis, and had become caught in the flaps mechanism.  When the flaps lever is moved, the mechanism can easily crimp (and eventually sever) any wire in its path.  If you observe the white wire you can see the insulation that is shedding (click to enlarge).

The breakdown of the upper insulating layer is not a major cause for concern, as a 'shedding' wire still has enough insulation to not arc or short circuit.  However, the wire should be replaced if more than one layer is compromised, or the stainless threads of the wire are visible.


Possible Scenario

When inspecting the wiring loom, I noted that one of the screws that holds the cable clamp to the inside of the throttle casing was loose.  This resulted in part of the wire loom to 'hang' near the flaps arc mechanism.    It is possible that during the throttle’s operational use, the movement and vibration of the aircraft had caused the screw to become loose resulting in the wires hanging down further than normal.  It appears that the wire had been severed, because it became caught in the mechanism of the flaps lever.  

Unlike reproduction throttles, the parts used in an OEM throttle are heavy duty and very solid; they are designed to withstand considerable abuse.  The speedbrake lever, when activated can easily cut a pencil in two, and the repeated movement of the flaps lever, when moved quickly between the teeth of the flaps arc, can easily crimp or flatten a wire.

Rather than try to solder the wires together (soldering stainless wire is difficult) and possibly have the same issue re-occur, I routed the wires from both lightplates (trim and flaps) directly to the 5 volt bus bar located in the center pedestal. 

I could have removed the wire loom completely and replaced it with another loom, however, this would involve having to disassemble the complete upper structure of the throttle quadrant to access the wire loom attachment points on the inside of the throttle casing; something I was not keen to do.

Final Call

OEM parts, although used in a static and simulated environment can have drawbacks.  Apart from age, the repeated movement of mechanical parts and the vibration of the spinning trim wheels, can loosen screws and nuts that otherwise should be securely tightened. 


OEM – Original Equipment Manufacturer

Wire Loom – Several wires bundled together and attached to a fixed point by some type of clamp


Replacement Curtains - B737 OEM Throttle Dust Curtains

Interesting items can arrive in the post.  Earlier today I opened a small parcel to find a collection of grey coloured pieces of material.  To anyone else they would appear exactly as they do – pieces of material stamped with numbers.

LEFT:  OEM dust covers for the Boeing throttle.

The throttle quadrant I use is original equipment manufacture (OEM) and once plied the skies above Europe.  As such it is a used item with the usual wear and tear you expect from a well-used aircraft part. 

One item that continually caught my attention was the dust curtains or skirts that sit behind the two thrust levers.  In my throttle, the curtains had been abused at some point and were torn and the edges looked rather ragged in appearance.  Although a replacement curtain could have been made by using vinyl or another similar material it would not be the same. 

The numbered pieces of material now have a home – they are OEM dust curtains that will replace the damaged curtains on the throttle.

Installing the Dust Curtains

The B737 throttle quadrant has three dust curtains.  Two on the outer side of the thrust levers and one double-sided curtain that resides between the thrust levers.  Each curtain comprises three parts sandwiched together and held by three screws. 

The parts are:

(i)     The thin aluminium arc which is the outer face plate;
(ii)    The actual curtain; and,
(iii)   The plastic arc retainer. 

The plastic arc retainer is curve-shaped and sits flush against the bare metal of the quadrant.  The dust curtain then lies above the retainer and beneath the outer face plate.

LEFT:  Dust curtains have been removed and the plastic retainer and aluminium arc can be seen along with one of the three attachment screws (click to enlarge).

Replacing the curtains is straightforward. Remove the three screws that hold the metal arc in place to the throttle, then gentle pry loose the aluminium strip beneath which are the dust curtain and plastic arc retainer.  It’s wise to ensure that you place the parts anatomically on the workbench as each of the items must be reassembled the same way it was removed.

One aspect of Boeing philosophy which makes building a flight simulator much easier is their reuse of parts from earlier airframes.  Boeing do not always redesign a part from scratch, but add to or change existing parts.  This philosophy can save the company millions of dollars.

For those who study this type of thing, you will know that dust curtains can come in differing colour shades.  In general, the older classic style throttle used a paler grey/cream coloured skirt whilst the Next Generation airframes use a standard light grey colour.  But, I would not get too concerned if the colour does not exactly match.

Why are the Dust Curtains Important

The main purpose of the dust curtainsis to minimise the chance of foreign bodies falling into the throttle mechanism.  Think pens, rubbers, straws, paper clips and coke can pull tabs (or anything else pilots play with in the flight deck).  The dust curtains are made from a fire retardant material (not asbestos) which minimises the chance of any fire/sparks from licking up the sides of the thrust levers in the unlikely event that a fire devlops inside the throttle quadrant.

For those keen to find replacement OEM dust curtains the stock numbers are: 69-33918-8 REF, 69-33918-9 REF-F and 69-33918-10 REF-F.


Anatomically – Meaning items removed are placed on a table in the same position as they were when they were in place.
Curtain Arc – the semi circular arc that the dust curtains are attached to.
OEM – Original Equipment Manufacture (aka real aircraft part).
Plastic Arc Retainer – A piece of heavy duty plastic shaped as a curve (arc).


Major Differences Between Classic and Next Generation Throttle Quadrants

The advent of high quality reproduction parts in association with advanced avionics suites produced by companies such as ProSim-AR and Sim Avionics, has led many flight simulator enthusiasts to strive closer to Microsoft’s claim ‘as real as it gets’.

LEFT:  There is little mistaking the tell-tale white-coloured handles and skirts of the Next Generation Throttle. (click to enlarge).

The availability of real parts formally used in classic airframes has never been greater, and many enthusiasts are purchasing various parts and converting them to flight simulator use.

The ‘holy grail’ of conversion has always been the Boeing throttle unit, and depending upon individual requirements, many older style throttle units have been retrofitted to appear very similar, if not near-identical, to their Next Generation counterparts.

This article will compare and contrast the major differences between the Boeing 737 classic throttle and the Next Generation throttle.  The word classic is usually used to refer to airframes belonging to the 200, 300, 400 and 500 series.  The Next Generation (NG) refers to the Boeing 600, 700, 800 and 900 series.

Historical Context

The throttle quadrant observed in a modern airliner has relatively old roots. 

LEFT:  Boeing 727-100 throttle quadrant.  Although there are obvious differences in that the 727 has three engines, the overall design and appearance of the quadrant is very similar to its modern counterpart.  Image copyright to Keven Walchler (click to enlarge).

The forbearer of the NG throttle was designed in the late 50's and early 60's and was initially used in the B707.  As aircraft types evolved, throttle design remained relatively static with similar-designed throttles being used in the Boeing 727, 717 and 737 series aircraft.

The B737-100 made its debut in April 1968, to be followed shortly by the 200 series with a slightly longer fuselage.  During the 1980’s Boeing released the classic series of airframes from the 300 through to the 500 series. 

During this time, the technology altered little and the design of the throttle quadrant reflected the ability of Boeing to reuse existing technology with minimal alterations.  This principle of reuse can save a company millions of dollars in redesign and development costs.

This Goes With That (Compare and Contrast)

The Boeing 737-800 NG is the airframe that many enthusiasts strive to duplicate in a flight simulator.  However, Next Generation parts are difficult to find and when found are expensive to procure.  Fortunately, for the simulation community, a throttle unit will function correctly within flight simulator no matter what airframe the throttle originated.

Many of the nuances between a classic and NG throttle quadrant are subtle and for the most part only the more knowledgeable will notice.  

The more obvious highlights of the NG are the white-coloured thrust lever shrouds, TOGA button assembly, flaps arc, speedbrake lever knob, and the moulded white-coloured side panels and panniers.  Whilst it is possible to alter many of the attributes of a classic throttle to conform with those of an NG, not every part can be easily transformed.  For example, the flaps arc between the classic and NG is very different in design and appearance and cannot be retrofitted.

TABLE 1 provides an overview to the main visual differences between the classic and NG throttle quadrants (courtesy Karl Penrose who kindly allowed the use of photographs taken of his 600 series throttle).  Note that there may be other subtle differences, some visual and others in design/operation.  The table does not address the center pedestal as pedestals vary greatly between airframes.  Retrofit 1 refers to the level of difficulty it is to make the classic throttle appear similar to the NG unit.

1 The words 'level of difficulty' is subjective; it depends on numerous factors such as experience and knowledge – neither of which is identical between individuals.

Final Call

The differences between a classic and NG throttle unit are largely cosmetic with some subtle design and operational differences.  Retrofitting a classic unit to appear similar to a Next Generation throttle is possible, however, there will be some aesthetics that will probably not be altered, such as the speedbrake lever knob, stab trim indicator tabs, side mouldings, paniers and flaps arc.  

This said, the ability to use an OEM throttle unit, no matter from which airframe, far supersedes any reproduction unit on the market.  OEM throttles are sturdy, robust and well-built.  Unless you do something particularly foolish, you will not damage an OEM throttle.

BELOW:  Two image galleries showing the various differences between the classic and Next Generation throttle quadrants.  Thanks to Karl Penrose who kindly allowed the use of photographs taken of his 600 series throttle.  To stop the slideshow, click the image and navigate by the numbered squares beneath the image.

B737 Classic Series Throttle Quadrant

B737 Next Generation (NG) Series Throttle Quadrant


Throttle Quadrant Rebuild - Evolution Has Led to Major Alterations 

Two major changes to the simulator have occurred.  The first concerns the throttle quadrant and the second is the replacement of the trial Interface Master Module with a more permanent modular solution.  The changes will be documented in the near future after final testing is complete.

LEFT:  The throttle quadrant has been completely rebuilt from the ground up.  Although the outside may appear identical to the earlier quadrant, the rebuild has replaced nearly everything inside the quadrant and the end product is far more reliable than its predecessor.

The throttle unit, in its previous revision, worked well, but there were several matters which needed attention.  The automation and functionality was adequate, but could be improved upon.  There were also 'niggling' issues with how the clutch assembly operated - it was somewhat loose which caused several flow-on problems.

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


The improvements have primarily been to the automation, the autothrottle and the speedbrake system.  However, during the rebuild other functionality have been improved: the synchronised tracking movement of the thrust levers is now more consistent and reliable,  and an updated system to operate the parking brake has also been devised.  This system replicates the system used in the real aircraft in which the toe brakes must be depressed before the parking lever can set or disengaged.

Furthermore, the potentiometers controlling the movement of the flaps and thrust levers have been replaced with string potentiometers which increases the throw of the potentiometer and improves accuracy.  The calibration of the flaps and speedbrake is now done within the system, removing the need for 'tricky' calibration in FSUIPIC. 

In the previous throttle version there was an issue with the speedbrake not reliably engaging on landing.  This in part was caused by a motor that was not powerful enough to push the lever to the UP position with consistent reliability.  This motor has been replaced with a motor more suitable to the power requirement needed.  The speedbrake is mechanical, mimics the real counterpart in functionality, and does not require software to operate.

This throttle conversion has maintained the advanced servo card and motor that was used to control the movement of the stab trim tabs (trim indicators); however, the motor that provides the power to rotate the trim wheels has been replaced with a more reliable motor with greater power and torque.  The replacement motor, in conjunction with three speed controller interface cards, have enabled the trim wheels to be rotated at four independent speeds.  This replicates the four speeds that the wheels rotate in the real B737.

Finally, the automotive fan-belt system/clutch system which was a chapter from the 'Dark Ages' has been replaced with two mechanical clutch assemblies that has been professionally designed to operate within the throttle unit - this will completely remove any of the  'niggles' with the previous clutch assembly becoming loose and the fan belt slipping.  Each thrust lever has a dedicated poly-clutch and separate high powered motor. 

A brief list of improvements and changes is listed below:

  • NG skirt replaced with more accurate skirt (prototype);
  • Reproduction TO/GA buttons replaced with OEM square TO/GA buttons;
  • Fanbelt driven clutch system replaced with slipper clutch system;
  • motors replaced that control lever movement and trim wheels;
  • 95% of wiring re-done to incorporate new interface modules;
  • Replacement interface alert system;
  • Flap potentiometers replaced by string potentiometers;
  • Speedbrake potentiometer replaced by linear potentiometer;
  • Thrust levers potentiometers replaced by dual string potentiometers;
  • Internal mechanism altered to stop noise of chain hitting throttle frame;
  • Thrust lever tracking movement accuracy improved;
  • Thrust reversers now have proportional thrust for each lever 1 and 2; and
  • The parking brake mechanism replaced with a more accurate system that reflects that used in the real aircraft

The conversion of the throttle quadrant has been a learning process, and the changes that have been done improve the unit's functionality and longevity - not too mention accuracy, far beyond what it was previously.

Dedicated Interface Modules

The throttle previously interfaced with the Interface Master Module (IMM).  The IMM was developed as a trial module to evaluate the modular concept.

The throttle quadrant will now directly interface with two dedicated modules called the Throttle Interface Module (TIM) and Throttle Communication Module (TCM).  Both of these modules contain only the interface cards, relays and other components required to operate the throttle and automation.  Additionally, the system incorporates a revised Interface Alert System which evolved from the original concept used in the IMM.

To read more concerning the various interface modules, a new website section  has been produced named Interface Modules.  This section is found in the main menu tabs at the top of each page.

Flight Testing (March 2015)

The throttle and replacement interface modules are currently being evaluated and minor issues rectified.

Once testing is complete, the alterations undertaken during the rebuild process will be documented in separate posts and, to facilitate ease of searching, links will be added to the flight controls/throttle quadrant section.

It should be noted that the work done to rebuild the throttle was done with the help a friend, who has a through knowledge of electronics and robotics.


B737 Throttle Quadrant - Automated Thrust Lever Movement

In this final post dealing with the conversion of the throttle quadrant, we will discuss the automation and movement of the throttle thrust levers and look at some of the teething problems encountered during the throttle conversion.  We will also briefly discuss the use of potentiometers.  Part of this post will be repetitive as I briefly discussed automation in an earlier post.

LEFT:  The Auto Throttle arming switch is a solenoid operated switch clearly identified on the main Instrument Panel (MCP).  The switch is linked to the IAS/MACH speed window (adjacent) and to two A/T disconnect buttons located either side of the throttle lever handles.

Avoiding Confusion - Automation

To avoid confusion, automation refers only to the movement of the two throttle thrust levers in relation to the %N1 output.  These N1 limits and targets are provided by the Flight Management Computer (FMC) and normally are used by the Autopilot Flight Director System (AFDS) and the Auto Throttle (A/T) to maintain airspeed and/or thrust setting.  

Automation and Movement - Interface Cards

Essentially, automation is the use of CMD A or CMD B (autopilot) to control the %N1 outputs from the Auto Throttle (logic), and motorization is the moving of the throttle levers in unison with %N1 output.  A number of interface cards are used to acheive this seemlessly.

Alpha Quadrant Cards (2):  Each  motor controller cards has the automation logic programmed directly to the card.  One card controls Auto Pilot CMD A while the other card controls CMD B.

Phidget Advanced Servo Card (2):  This card acts as an interface and bridge between the Alpha Quadrant cards and FSX. 

The card does not provide movement for the throttle thrust levers; this is controlled by a Phidget Motor Controller card.

Leo Bodnar BUO 836 A Joystick Controller Card:  This card will register in Windows the movement of levers, buttons and switches on the TQ.  Calibration of this card is done first in Windows then in Flight Simulator (FSX), FSUPIC or the avionics suite used; for example, ProSim737.

The interface cards are mounted forward of the MIP within the Interface Master Module (IMM) and are connected to the throttle unit by custom VGA cables and to the computer by a single USB cable.

Main Controller Cards

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 (firmware) that can be easily upgraded ore replaced.  

The Alpha Quadrant cards provide the logic from which the automation of the throttle unit operates.  The cards act a "bridge" between the card and the avionics suite - "call it a language transfer if you will."

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.

CMD A/B Autopilot - 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 A or B is selected on the MCP.

Synchronized or Independent Lever 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 (B707, B727 & some B737 classics), the levers were synchronized; however, the NG has a computer-operated fuel control system which can minutely adjust the %N1 of each engine.  This advanced 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 (for varying reasons).  This inaccuracy can often be observed on reproduction throttle units that exhibit a gap between lever one and lever two when automating %N1 outputs.  

It was decided to maintain the older system and have both levers synchronized.  Although this is not replicating the NG system, it does make calibration easier.  If in the future incremental thrust lever movement is required, then it’s a matter of adding another 12 Volt motor to the front of the throttle bulkhead to power the second thrust lever.  

Be aware that although both thrust levers are synchronized, the throttle handles may still show a slight difference in position in relation to each other.  This is caused by the varying tension that needs to be maintained on the fan belt connecting the 12 Volt motor to the mechanical system beneath the thrust levers.

LEFT:  Auto Throttle activation will advance both thrust levers in unison to a defined %N1 output.

Another aspect to note is that the position of the thrust levers during automation is arbitrary and is a visual representation of the %N1 output; it may or may not reflect the exact position on the throttle arc that the thrust lever would be placed if moved manually (by hand with Auto Throttle turned off).  

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).  

Power Requirements and Mechanics

To provide the power to move the throttle thrust levers, a 12 Volt motor previously used to power electric automobile windows, is mounted forward of the throttle bulkhead (see image at bottom of post).  Connected to the motor's pulley is a fan belt that connects to the main pulley located beneath the thrust levers.  To enable the thrust levers to move in unison, a slip clutch, which is part of the main pulley assembly, is used.  

ProSim737 Limitations - TO/GA and Auto Throttle Override

Unfortunately, concerning automation the ProSim737 is deficient in two areas: TO/GA and A/T Override (see postscript below).

LEFT:  Captain-side TO/GA button is clearly seen below lever handles.  The button at the end of the handle is the Auto Throttle disconnect button.

(A)  TO/GA

In the real aircraft, the flight crew advances the thrust levers to power 40%N1 (or to whatever the airline policy dictates), allows the engines to spool, then pushes the TO/GA button (s).  Pressing TO/GA causes the throttle to go on-line and to be controlled by the AFDS logic.  The throttle levers then advance automatically to whatever %N1 the logic deems appropriate based on takeoff calculations.

If you're are using ProSim737, this will NOT occur.  Rather, you will observe the thrust levers retard before they advance (assuming you have moved the thrust levers to %40 N1).  The reason for this is nothing to do with how the throttle is calibrated, FSUPIC or anything else.  ProSim737 software controls the %N1 outputs for the automation of the thrust levers and the developer of the software has not fine-tuned the calibration in the software to take into account real-world avionics logic.  This thread located on the ProSim737 forum provides additional information. 

I have not tested Sim Avionics, but have been told this issue is not reflected in their avionics suite.

There are two workarounds:  Engage TO/GA from idle (hardly realistic) or push the thrust levers to around 80% N1, allow the engines to spool, then push TO/GA.  Anything less that around 80% N1 will cause the thrust levers to retard before advancing.


The latest version of ProSim (V-133) has provided improvement to the above issue.  Throttles can now be advanced to ~60% N1 and TOGA engaged without the throttle levers retarding.  This is possible ONLY if you calibrate the throttle levers within ProSim and allow ProSim to control the throttle output logic.  if you calibrate within FSUPIC then the same issue will apply.

According to ProSim developers, this issue is probably related to the calibration of the ProSim servo output. When you press TO/GA, the current N1 is taken and calculated back to a throttle percentage. This throttle percentage, when combined with the servo calibration data from ProSim results in a servo output. The servo calibration at the moment only has 2 calibration points, which are 0 and 100%. This results in a linear behavior between the two points, while depending on the construction of the throttle, the relationship might be non-linear. This would require a multi point calibration which is hard to do at the moment, because a throttle does not have exact readouts of the current position, so it will be hard to calibrate a 50% point.

This may need improvement in the code to auto calibrate the throttle system.

It's hoped that fuutre relase of ProSim will rectify this issue.

(B) Auto Throttle Manual Override

In the real aircraft, manual override is available to a flight crew and the thrust levers can be retarded with the Auto Throttle engaged.  When the flight crew release pressure on the thrust levers the Auto Throttle will take control again and return the thrust levers to the appropriate position on the throttle arc dependent upon the speed indicated in the speed window of the MCP.

ProSim737 will not temporarily disconnect (manual override) the Auto Throttle.  

At the time of writing, there is no workaround to solve this.

Potentiometers - Two Types; Which is Best

There are two types of potentiometers.  The first type, (I will call them standard potentiometers) are inexpensive, often have a +- percentage variance, are compact, have a minimal throw depending upon the size of the device and are not contaminate free.  

The last point is worth mentioning as it is wrongly assumed that a potentiometer will remain correctly calibrated for the life of the unit.  General wear and tear, dust and other debris will accumulate on the potentiometer; any of which may cause calibration and accuracy problems.  Keeping the potentiometers free of dust is important.

The second type of potentiometer is called a string potentiometer (strings).  Contrary to the standard type, strings are very accurate, are in a sealed unit presenting zero contamination, are manufactured to exacting standards, are larger in size and are expensive.

The difference in size between the two potentiometer types is often the reason for using the smaller standard type.  The strings are very long requiring quite a bit of real estate either forward of the throttle bulkhead or within the center pedestal.  In contrast, the standard potentiometers are quite compact; finding a position to install them is not problematic.

Calibration of Potentiometers

The main method of calibrating the position of the thrust levers is by calibrating the potentiometer in Windows, then in FSX followed by fine-tuning in FSUPIC (if needed).  

Standard potentiometers are used in the simulator; therefore, at some stage cleaning or replacement of a potentiometer maybe necessary.   The 737 throttle quadrant is not cavernous and only certain sized potentiometers will fit into the unit; this combined with other parts and wiring means that the potentiometers are often inaccessible without removing other components.  

To allow speedier access to the potentiometers, a Quick Assess Mounting Plate was designed.

Quick Access Mounting Plate (QAMP)

The potentiometers are mounted directly onto a custom-made aluminum plate that is attached to the inside of the throttle unit by solid thumb screws.   To access the plate, the side inspection cover of the throttle is removed (a few screws) followed by turning the thumb screws on the access plate.  This releases the plate.

LEFT:  QAMP secured to base of throttle unit.  Thumb screws are visible on each corner of the plate.  A possible add on modification to reduce the risk of dust contamination to the potentiometers is a plastic cover that fits over the plate (a lunch box).

A similar plate has been designed and constructed for use with the stand-by potentiometer that controls the flaps.  A more detailed picture of the QAMP can be seen here in an earlier post.

Below is a video showing the movement of the thrust levers with the Auto Throttle (A/T) engaged.  The movement of the thrust levers is in real time according to flight parameters during the test flight and has not been instigated by overriding the servo. 

Teething Issues with the Throttle Conversion

It was envisaged that more problems would have surfaced than have occurred.  The major issues are outlined below:

(A) Trim Wheels

An early problem encountered was that the trim wheels when engaging generated considerable noise.  After checking through the system, it was discovered that the two-speed rotation of the trim wheels were causing the two nuts that hold each of the trim wheels in place to become loose.  This in turn caused the trim wheels to wobble  slightly generating undue noise.  


Tighten the two nuts at the end of the rod that holds the two trim wheels in place.

(B) Flaps 5 Not Engaging

The problem with the flaps 5 micro-button has been discussed in an earlier post.  To summarize, when you moved the flaps lever to flaps 5 the correct flaps were not selected on the aircraft or registered on the PoKeys 55 interface card.  Several hours were spent checking connections, micro-buttons, wiring and the custom VGA cables that connect the flaps section of the quadrant to the Interface Master Module (IMM); the problem could not be discovered.  


One of the two Belkin powered hubs located within the IMM had been replaced with another powered unit.  It appears the problem was that the replacement hub had too low a voltage, as a replacement with a higher voltage solved the problem.

(C) Throttle Thrust Levers Not Synchronizing (A/T on)

The two thrust levers of the quadrant did not synchronize when the Auto Throttle (A/T) was engaged; one lever would always be ahead or behind of the other.  At other times they would split apart (do the splits) when A/T was engaged.  


The problem was easily solved by altering the tension on the slip clutch nut.  When the nut was  tightened, both levers moved together as one unit.  The secret was finding the appropriate torque.

(D) Throttle Thrust Levers Difficult To Move in Manual Mode (A/T Off)

The ability to move the thrust levers in manual mode (Auto Throttle turned off) was not fluid and the levers occasionally snagged or were sticky when trying to move them.  

This is caused by the fan belt not moving smoothly through the groove of the pulley wheel.   The Auto Throttle when engaged overrides any stickness due to the power and torque of the Auto Throttle motor.


Unfortunately, there isn’t a lot you can do to rectify this issue as it’s a by-product of using a mechanical system in which the fan belt is central to the consistent operation of the unit.  

LEFT: The fan belt is barely visible linking the pulley of the motor to the main pulley inside the quadrant.

The conundrum is that if you tighten the fan belt too much you will be unable to manually move the thrust levers as they will be exceptionally stiff and difficult to move (as you are pushing against the tension of the fan belt); however, if you loosen the fan belt too much, although the levers will move fluidly by hand, the fan belt may not have enough tension to move the levers when Auto throttle is engaged.  It’s a matter of compromise; selecting an appropriate in-between tension to allow acceptable manual and Auto Throttle operation.

A more reliable method is to use a small gearbox, a simple slip clutch and a coupler to connect to the spur gear.  Another option is to use an electrical system.

Further thought needs to be done in this area before a decision is made to replace the fan belt system.  If a new system is incorporated, the change-out will be documented in a future post.


This brings us to the end of the throttle conversion.  The following links will take you to other posts regarding the conversion.  

B737 TQ - General Overview
B737 TQ - Speedbrake Conversion and Use
B737 TQ - Flaps UP to 40; Conversion and Use
B737 TQ - Trim Wheels and Trim Indicator Tabs
B737 TQ - Parking Brake Mechanism

Despite some of the shortcomings to this conversion, in particular the mechanical fan belt system, the throttle unit shows a marked improvement on the earlier 300 series conversion.

Technology and innovation rarely stand still and there is little doubt other ways will evolve to achieve similar results with greater efficiency.

Acronyms and Glossary 

AFDS - Autopilot Flight Director system
A/T – Auto Throttle
CMD A/B - Autopilot on/off for system A or system B
Flight Avionics Software - Sim Avionics, ProSim737 or similar
FMC - Flight Management Computer
MCP - Main Control Panel
QAMP – Quick Access Mounting Plate
Throttle Arc – The arc of the thrust levers from the end of the blocks to fully forward.  The term refers to the curved piece of aluminum that the throttle levers are moved along
TO/GA - Takeoff Go-around switch
%N1 -  Very simply explained, %N1 is throttle demand and as N1 (and N2) spin at absurdly high speeds, it is easier to simply reference a percentage and display that to the crew. It's much easier for our brains to interpret a value on a scale of 0-100% rather than tens of thousands of RPM