Changing-out Potentiometers - UniMeasure LX-PA Series Position Transducer

/

UniMeasure LX-PA string potentiometer secured to a bespoke L-bracket and mounted on a board beneath the platform. The string connects with the rudder assembly

Potentiometers are arguably one of the most critical components in a flight simulator.  Without a potentiometer, the accurate calibration of the flight controls (or any other device) isn’t possible.  

Over the years, I have used a number of different types of potentiometers, beginning with inexpensive Chinese-made linear and rotary types, and later progressing to Bourns rotary potentiometers paired with fabricated strings. At one point, I also trialled a commercial CALTSensoR string potentiometer.

When a string broke on the potentiometer that controlled the elevator, I decided that, rather than repair the string, I would replace all the potentiometers used to calibrate the flight controls (ailerons, elevator and rudder).

Instead of using the CALTSensoR model mentioned earlier, I opted to upgrade all the potentiometers to string potentiometers made by UniMeasure.

UniMeasure Potentiometers (Transducers)

UniMeasure, an American company based in Oregon, specialises in manufacturing position and velocity sensors for the medical, defence, and industrial sectors. Renowned for their high-quality components, UniMeasure potentiometers adhere to Mil-Spec and IP/NEMA standards, and they’ve been an industry leader since 1987.

After reviewing the different types of potentiometers, I ordered three LX-PA series position transducers.

Is it a Potentiometer or a Transducer

The terms potentiometer and transducer are often used interchangeably to describe the same component.  Although the use of these terms is acceptable, there are subtle differences between a potentiometer and a transducer.  The primary difference being that a transducer will convert the input received into a different output, whereas a potentiometer will only detect and measure the movement of an object (for example, a control arm).

LX-PA Series Position Transducer

The LX-PA series transducer consists of three main parts:

  1. A potentiometer – a Bourns rotary potentiometer (type 3545).

  2. A retractable mechanism – a reel so that the string can extend and retract.

  3. A string – a 0.4 mm jacketed stainless steel length of wire with a stainless steel eyelet.

These components are housed in a compact, durable ABS plastic casing.  The small size and lightweight design make the LX-PA ideal to use in relatively tight spaces.

How the LX-PA Series Transducer Works

Before installing and using a string potentiometer, it’s important to have an understanding of how a potentiometer works.

A potentiometer is a resistive-type transducer, that converts linear or angular displacement into a variable voltage output signal. This process occurs when a sliding contact (known as the wiper) moves along the surface of a resistive element, typically a carbon track.

The potentiometer is securely mounted at a fixed position, with its string attached to a movable object. As the object moves, the string extends, rotating the internal sensing device to produce an electrical output proportional to the object’s position or velocity.  An internal spring maintains string tension and serves as the retraction mechanism for the string.

Electrical Output

The electrical output that is produced by the potentiometer is less effective as the string reaches its fully retracted or extended position, meaning that calibration is not as effective towards each end of the string’s length.  Therefore, it is important to take into consideration the length of the potentiometer’s string. 

LX-PA string potentiometer mounted to L-bracket and secured to platform. The string can be seen in a direct line connecting with the ailerons

Measuring the Throw

To ensure optimal results, measure the throw from the mounting point of the potentiometer to where the end of the string is to be attached to the moveable object, then add a few inches to your measurement.  This is the length of string that should be used with the potentiometer. 

If the string is too long or short, you will be calibrating the end points of the string, which is not as accurate as if you had calibrated the central portion of the string.  Additionally, if the string is too long and near its end point, excessive tension on the reel’s retractable mechanism may occur, leading to premature loss of tension.

An average rule of thumb is to try and use two thirds of the distance (throw), or predominately the central portion of the string, as this is where the most effective output signal is found, and where the best calibration can occur. 

When you order a potentiometer from UniMeasure, you inform the technician of the length of the string required and the potentiometer is made to your specification.

It is important to note that potentiometers are passive components, meaning they do not necessitate a power supply or additional circuitry to operate.

Important Point:

  • The length of the string is important. However, do not be overly pedantic about obtaining the exact measurement. The calibration will still be accurate provided you allow a few extra inches (a ‘dead zone’) at each end of the string, and primarily use the central portion of the string.

Splitter ring attached to tilt elevator mechanism of the flight controls with the lobster clasp connected

Installation and Attachment

Although the LX-PA series is not the smallest potentiometer UniMeasure manufactures, it is small enough to be mounted in all but the tightest areas. 

The potentiometer requires a secure attachment point, and a bracket is the easiest way to do this.  UniMeasure sell a dedicated bracket, however, I decided to fabricate my own L-bracket.  The LX-PA has two 5 mm diameter holes in the body of the potentiometer and can be secured to the L-bracket by a suitable bolt and nut.  The position of the holes in the main body enable the potentiometer to be mounted a number of ways (up, down, inverted, sideways, flat, etc).

To attach the string to the moveable object (aileron, elevator rudder, etc) requires that there be a stable attachment point on the object.  The string does not have to be permanently attached to the object (although you can do this). 

Splitter Ring and Lobster Clasp

My preferred method is to attach to the eyelet of the string to a one way clasp (often called a lobster clasp).  This clasp is attached to a splitter ring (curtain-rail ring) that is attached to the object. To enable attachment of the splitter ring, I have drilled a hole into a moveable part of the flight control mechanism.

The advantage of using a lobster clasp is the string can easily be disconnected; for example, when servicing is required.

Line Pull

The direction of the line pull is important and UniMeasure recommend to not exceed 2 degrees in any direction.  If a further offset is required, a stand-off, such as a fly wheel, will need to be purchased or fabricated.   

Exceeding the recommended offset will more than likely shorten the life the string, because the string would retract into the spool at an angle, causing undue wear and tear with a potential loss of accuracy.

Important Points:

  • For the string to be effective, the string must be under tension.

  • Do not extend the string to its full length, as this can damage the reel mechanism.

  • Maintain lateral or vertical alignment within 2 degrees to avoid unnecessary wear on the reel.

  • Do not allow the string to come into contact with objects along its path.

Connection and Calibration

It is not difficult to connect the UniMeasure potentiometer to the computer.

The wires from the potentiometer are connected to the appropriate input on a joystick interface card, such as a Leo Bodnar BU0836A or BU0836X joystick card.  Previously I used the former, but changed-out to the larger BU0836X card as it was easier to connect the wires to the card. 

One difference between UniMeasure potentiometers and others is that the UniMeasure has one additional wire to the standard three.  The forth wire is a shield wire (naked coiled wire). 

What is a Shield Wire

A shield wire provides electromagnetic compatibility protection and serves two purposes:

  1. It prevents interfering signals from the inside of the cable from reaching the outside and being disturbed to other cables and electrical devices; and.

  2. It ensures that external interference does not reach the inside of the cable and potentiometer.

Shield wires are often connected when a potentiometer is used in a high-accuracy setting such as in medical scanners.  However, it is debatable if there is any positive benefit in using a shield wire in a flight simulator environment. If the shield wire is used, the wire should be connected to ground for optimal results.

The other three wires from the potentiometer: common (black wire), +5 Volts (red wire) and the output (white wire) connect to the respective points in the BU0836X card.

All the wires have been professionally soldered to the potentiometer’s terminals, and are insulated with PVC, except for the last 4 cm which are bare. 

Calibration of the potentiometer requires initial registration in Windows using the joystick controller interface (type JOY in the computer’s search bar).  After this has been completed, calibration can be done either directly in flight simulator, ProSim737 or FSUIPC.

Protection from Dust and Water

The LX-PA series is not fully dust-proof or waterproof.  Whilst the potentiometer itself is dust-proof due to its enclosed design, dust can accumulate where the string enters the reel.  This said, unless the string is located in a particularly dusty environment, dust ingress should not occur.  If dust does present a problem, the string can be cleaned using high-pressure air, alcohol, or a circuit-board cleaner. 

If the environment is particularly dusty or damp, an alternative UniMeasure series potentiometer should be considered that has better dust-proofing.

Interestingly, the method used by UniMeasure to dust-proof their potentiometers (not the LX-PA series) involves the use of two curtains that are installed where the string slides into the mechanism.  These curtains when dirty, can be removed for cleaning or can be replaced. 

To protect the potentiometer’s terminals where the wires are soldered, UniMeasure offers a removable cover.  The cover pushes over the cylindrical part of the potentiometer and is secured by three small screws.  The cover ensures that nothing will damage the terminals.  The cover is a very good fit and it is very unlikely that any dust or moisture will enter behind the cover into the potentiometer

Durability and Lifespan

The body of the LX-PA series is made from ABS plastic, and when the potentiometer is fitted with the protective cover (discussed earlier) makes for a relativity robust unit; an odd light knock here and there is not going to damage the potentiometer.

The LX-PA transducer has an impressive lifespan:

  • The potentiometer is rated for 2.5 million cycles, while

  • The lifespan of the reel and string varies and is dependent upon length of the string:

    • Up to 4.7 inches - 1,000,000 cycles.

    • 10–25 inches - 250,000 cycles.

Accuracy and User Experience

Although the LX-PA transducer uses a standard Bourns potentiometer, the accuracy when calibrating and using the potentiometer comes more from the build quality of the string and how it retracts into the reel, than the actual potentiometer. 

The slightest movement of the yoke, elevator or rudder is reflected in the avionics software and there is no noticeable acceleration or lag as the string moves in and out of the of the reel.   In comparison to its closest rival, the CALTSensoR potentiometer, the UniMeasure string’s movement is far more smooth and has no apparent binding.

Calibrating flight controls to ensure optimal accuracy has always been finicky, and considerable time can be wasted attempting to achieve optimal results. Calibration of the LX-PA potentiometers was a breeze, and since using these potentiometers I have noticed that control of the aircraft, especially during crosswind landings, is far more easy than what it previously was.

Additional Information

Final Call

The LX-PA series transducer is a premium potentiometer that combines a high-quality Bourns rotary potentiometer with a durable, precision-engineered retractable reel and string.  With proper calibration, it captures even the slightest movement with impressive accuracy, making it a perfect choice to connect to the flight controls.

Below: UniMeasure LX-PA series potentiometer

Using The Tiller To Taxi The Boeing 737

/

oem captain-side steering tiller. (737ng-info, 737controls, CC BY-SA 3.0)

To taxi an aircraft around the airport the pilot uses either the rudder pedals and/or a steering wheel device called a tiller.  The half-moon shaped tiller is mounted to the sidewall of the flight deck.  The number of tillers in an aircraft is not standard; some aircraft have one tiller while some have two.  The tiller controls the lateral movement of the aircraft’s nose wheel, located below and behind the flight deck.

The rudder pedals when pressed do provide some lateral movement, however, nose wheel steering is no more than 7 degrees.  To enable full lateral movement of the nose wheel  requires using the tiller with some forward thrust (called break-away thrust) from the aircraft’s engines.   

If the aircraft is to be moved backwards (for example from the ramp), then a push-back truck and ground controller is required.  The controller will connect a bar from the push-back truck to the main coupling of the nose wheel to lock the nose wheel in the forward position.  Once this is done, the push-back truck will lift the nose wheel enabling the aircraft to be reversed backwards.  A push-back truck can also be used to pull the aircraft forwards.

The ground controller will be in communication with the pilot and will instruct the pilot when it is safe to release the parking brake or start the engines (it is the responsibility of the ground controller, amongst other things, to check that the doors are closed and that personnel are clear of the aircraft).  Prior to the aircraft being moved, the pilot will speak with Air Traffic Control to obtain starting and push-back approval.

After ATC has given clearance, the pilot will:

  1. Check and cross check the taxi route instructions issued by ATC.

  2. Release the parking brake by pressing the upper section of the toe brakes.

  3. Apply forward thrust by advancing both thrust levers to around 32%.  The actual percentage N1 depends on the weight of the aircraft.  The forward thrust should not exceed 40%N1.

  4. Use the tiller to maneuver the aircraft.

  5. To stop the aircraft the thrust levers are brought back to idle, the toe brakes are pressed to stop any forward movement of the aircraft, and the parking brake applied.

Although not recommended, it is possible to aid in the turn by applying appropriate thrust only to one engine.

Important Points:

  • Reverse thrust should not be used to move the aircraft backwards primarily because of the likelihood on ingesting foreign material into the engine.

  • Whenever the aircraft is at a standstill the parking brake should be applied.

Taxi Speeds

Taxi speeds vary.  Generally, in good conditions the maximum permissible speeds are:

  • 10 knots – when doing turns;

  • 30 knots – when traveling in a straight-line along a runway;

  • 50 knots – when back-tracking along a runway; and,

  • If the runway is contaminated (ice, snow, etc) the taxi speeds are reduced to 5 knots.

How To Taxi

The nose wheel is located under and to the rear of the flight deck.  Therefore, to turn onto and follow the taxi lines accurately you must slightly overshoot the line prior to turning.

OEM Tiller

Another article addresses how to convert an OEM tiller and use in ProSim737 -  OEM tiller in ProSim737.

Final Call

With a little practice taxing the aircraft in the flight simulator is straightforward.  Points to consider are turning the nose wheel at the correct time (before crossing the line) and applying the correct amount of thrust based on aircraft weight.

OEM Trip Reminder Indicator

/

Trip Reminder Indicator.  A small OEM part that is easily installed to any simulator

The trip reminder indicator (TRI) is a mechanical device installed to the right hand side of the yoke; it’s an airline option.  Basically, the device is three separate digits that can be rotated in any combination, from zero to nine.

The trip indicator is a memory device from which the crew historically used to record the flight number; the pilot uses his thumb to move the three digits to indicate the flight number.  However, over time flight numbers became longer than three digits and the use of the trip indicator, for it’s intended purpose, wanned

I use the trip indicator to dial in the Vref, as it’s often easier to quickly glance at the trip indicator to remind you of the Vref speed rather than look at the PFD or CDU.  Some dial in the Vref + wind speed.

Background

The trip indicator has a very long lineage beginning with the Boeing 707 aircraft.  The device was then ported to the 717, 727 and finally the 737 Classic and Next Generation airframes.

Installation and Backlighting

Because the OEM yoke already has the correctly shaped hole, installation of the trip indicator is straightforward.  If you are using an OEM yoke, you probably will need to carefully remove the blanking cover from the hole.

If a reproduction yoke is used, and the hole is not present, a circular hole will need to be cut from aluminium or plastic to enable the trip indicator to fit snugly into the yoke.  As the three dials are mechanical, there is no requirement to connect the device to an interface card.

Each of the digits on the indicator is backlit by a 5 volt incandescent aircraft bulb. 

The design of the trip indicator is ingenious, in that after the trip indicator has been removed from the yoke (two screws at the front of the yoke secure the indicator), a transparent acrylic slide can be unlocked to slide laterally from behind the three digits (see picture).  The acrylic slide accommodates three 5 volt bulbs, each in its own compartment.

To enable the backlighting to function requires two wires (positive & negative/common) to be connected to the appropriate connection on the rear of the trip indicator, and then to a 5 volt power supply.  The amperage draw from the three bulbs is minimal.  The wiring should be run through the yoke and down the control column so that it comes out at the bottom of the column.

In the aircraft, the backlighting for the trip indicator is connected to the panel light knob located on the center pedestal.  This enables the backlighting on the trip indicator to be turned on and off or dimmed. 

Final Call

The trip reminder indicator is but a small and unobtrusive item, however, it’s often the small things which add considerable immersion and enjoyment when using the simulator.  The trip indicator is also an OEM part that can be very easily installed to a reproduction yoke with minimal experience in fabrication and wiring.

Glossary

OEM - Original Equipment Manufacture.

OEM Boeing 737 Stick Shaker - Interfacing and Operation

/

OEM 737 stick shaker installed to Captain-side column.  The lower section of device is what vibrates

The stick shaker is standard on all Boeing series aircraft; the Next Generation having two units (Captain and First Officer) and the earlier classic series having one unit.  The stick shaker is mounted directly to the control column and is designed to vibrate if air speed degrades to stall speed.  The Stick shaker I am using is manufactured by a company in New York. It is powered by 28 Volts (27.5 Volts to be exact).  

Configuration

Configuration of the stick shaker is a relatively easy task.  The electrical cable from the device is connected to 28 Volts, or if this is not available 12 Volts;  12 Volts still produces enough power for the shaker to vibrate, although the intensity is not as great as if the unit was connected to 28 Volts. 

To allow Flight Simulator to connect to the stick shaker, a relay card is required such as a Phidget 0/4/4 relay card.  A USB cable then connects from the card to the computer.  The stick shaker will vibrate when variables that relate to low air speed are met.  The variables are determined by the flight avionics software (ProSim737 or Sim Avionics).

Phidget 0/0/4 relay card showing the main positive wire (red wire) cut with each end inserted into the correct terminals of the relay card

Interfacing and Wiring

The Phidget 0/0/4 relay card is mounted in-line between the 28 Volt power supply and the stick shaker.    Either of the two wires (+-) from the power supply can be cut to install the in-line relay; however, only one wire is cut; the other remaining unbroken from the power supply to the stick shaker.

The 0/0/4 relay card has four relays of which one is required.  Each relay has three terminals: normally open (NO), common (C) and normally closed (NC).  For the stick shaker the common and normally open terminals are used.

Carefully cut one of the two wires leading from the 28 Volt power supply.  Insert the wire coming directly from the power supply into the terminal marked common (1C, 2C, 3C or 4C).  The other end of the cut wire, which comes from the stick shaker is inserted into the terminal marked NO (normally open) of the same terminal.

If the wires have been inserted into the correct terminal of the relay card, the circuit will be complete only when the parameters established within the flight avionics software are valid.  At all other times the relay will break the circuit by not allowing the power to reach the stick shaker. If you have made a mistake, the stick shaker will vibrate continuously.

Protection

When connecting the stick shaker, it is a good idea to include a diode to protect your computer from any magnetic return signal should the relay fail.  A return signal to the computer may cause problems with the computer, and in it worse instance allow 28 volts to surge into the computer destroying your mother board. 

Positive and negative wires from the stick shaker enter the terminal block on the right.  A diode is placed on the corresponding end of the terminal block prior to the two wires running to the relay (not shown)

A high-end relay, such as a Phidget 0/0/4 relay should not fail, and if it does it should fail in the closed position.  However, if 'Murphy' or 'Sod' is your First Officer then it is better to be safe than sorry, so best install a diode.

A diode is an inexpensive and very simple device that behaves in a similar way to a black hole (astronomy).  In a black hole all matter is sucked into the hole and no matter, including light leaves the hole; it is one way trap.  A diode behaves in exactly the same way.  If a failure of the relay occurs, any power that is being transmitted through the wire from the stick shaker (28 volts) will enter the diode and be trapped.  No current will leave the diode.

Three 6 cm diodes.  The silver spirals indicate the positive side (red tape) while the opposite end is the negative (white tape).  Diodes come in an array of differing shapes, sizes and trapping capacities

The heavy duty diode should be placed in parallel between the stick shaker and the relay card.  It is best to try and place the diode as close to the stick shaker as possible.  Place the positive side of the diode (usually appropriately marked) on the positive side.  The other end of the diode place on the negative side.  If you use a terminal block it is very easy to connect a diode into the circuit (see photograph).

Incorrect Wiring

Do not become concerned if you have connected the wire to the wrong terminal - the stick shaker will not be destroyed.  It will be obvious if you have inserted the wires incorrectly, as the stick shaker will operate continuously as it has unbroken power.

I was debating to re-paint the stick shaker, however, decided to keep it as it is.  I like the used look rather than the pristine 'never been there' look.

Although the stick shaker is not essential, it’s often the smaller things and attention to detail which help bring the simulator to the next level.  I am using OEM control columns and adding a stick shaker enhances the immersion.

OEM and Reproduction

When an OEM stick shaker vibrates, especially when you are not expecting it, the vibrations startle you . The yoke vibrates and the noise of the vibrations is quite loud. In contrast, reproduction stick shakers generate lower vibrations and noise.

Acronyms 

  • OEM - Original Equipment Manufacture (real aviation parts)

BELOW:  A short video demonstrating the noise and vibration made to the control column and yoke by the stick shaker when approaching stall speed.

 
 

Boeing 737 OEM Steering Tiller Installed

/

oem 737-400 steering tiller mounted to bespoke aluminium plate

The steering tiller is an often overlooked piece of hardware for many virtual flyers.  The steering tiller provides greater control of the aircraft during taxi operations, and if calibrated correctly works very well.

OEM B737-400 Steering Tiller

The tiller has been salvaged from a 737-400 series aircraft and is identical to the tiller used in the Next Generation aircraft.  The actual OEM part is only the black handle and white arrow.  The remainder of the unit has been custom fabricated to allow easy attachment to the inside wall liners of the flight deck.

The simulator does not have a shell and liner at the moment; therefore, I've attached two pieces of grey-coloured wood to the unit to enable temporary installation to the forward left of the Captain's seat.  

A single potentiometer has been used allow calibration of the tiller mechanism.  A metal strip connects the potentiometer with a metal plate that connects to the the central area of the tiller mechanism.  As the steering tiller is turned left or right, the metal plate moves to and fro with a corresponding movement in the metal strip which registers on the potentiometer (see picture).

To create tension when the steering tiller is moved, several heavy duty springs have been used.  Although rudimentary in design, the tension of the springs provides a reasonable and constant pressure.  The springs also allow the handle to center itself easily when released.  Springs are renowned for creaking when they move and to remove this noise, heavy duty lithium grease has been applied to the upper parts of the spring heads where they join the metal. 

Tiller mechanism showing springs and potentiometer.  A linear potentiometer has been used in favour of a rotary potentiometer. Springs provide tension to center the tiller

Interface Card and Calibration

The tiller is connected directly to a Leo Bodnar BU086A interface card, although any joystick card such as a PoKeys card can be used.  A USB cable then runs from the interface card to the main computer.  To allow easy connection to the interface card (Leo Bodnar card) a female JR servo wire security clip has been used.  

The steering tiller requires careful calibration if it's to operate correctly.  Calibration is initially through Windows and then FSUIPC.  Using FSUIPC enables greater accuracy to be achieved.

The steering tiller, when calibrated through FSUIPC does not create an independent tiller axis but piggybacks on the movement of the rudder axis.  The developer has ingeniously written code that enables the tiller to be activated when groundspeed is under 60 kias.  Above this speed the rudder is activated.

How to Calibrate the Steering Tiller

  1. Connect the interface card to the computer via the USB cable.

  2. Using Windows, calibrate the axis of the interface card (if using Windows 7 type into the search bar joystick and select "Joystick Calibration").

  3. Following the on screen instructions, move the steering tiller handle forward and aft.  Then save the setting.

  4. Open Flight Simulator and then open “Settings/Control” in the FSX menu.

  5. Ensure that any joystick commands relating to the interface card are not registered by FSX.  If so, delete them and save.

  6. Open Flight Simulator and then open FSUIPC settings.

  7. Select the FSUIPC “Axis Assignment Tab”.  Then move the tiller handle to activate the calibration software.  (you will observe the numbers moving).

  8. Select from the left side of the screen the tab that says ”Type of Action Required”,  Select "Send Direct to FSUIPC Calibration".  Then open the menu box and scroll down to “Steering Tiller”.

  9. Open the “Joystick Calibration” tab in FSUIPC.  

  10. Scroll through the 11 entries searching for steering tiller (9/11).  When "Steering Tiller" is found, click the SET button which will open three (3) further buttons.  Each button refers to a position on the steering tiller axis.

  11. Turn the steering tiller to the left and click the RIGHT button.  Then turn the tiller to the right and select the LEFT button.  With the tiller in the central position click the MIDDLE button.  Oddly, on some setups the opposite is required.  If calibration fails, try again using the opposite direction.

  12. For more precise and accurate calibration, you may want to use the "Slope" and/or "Null Zone" functionality.

The steering tiller should now be calibrated and ready for use.

Troubleshooting and Suggestions

Some known problems that are easily solveable are:

Leo Bodnar 086A interface card (joystick card)

A:  Only use the steering tiller at very low ground speeds.  If you turn the tiller to the full left or right and the speed is too great, the aircraft may remain stationary or slip; the reason being the nose wheel is locked at a right angle to the direction of travel.  I find the tiller works best turning the handle slowly.

B:  The direction of aircraft travel is opposite that of the tiller handle.  If this occurs, check your FSUIPC settings.  You may have to tick (check) the box that says REV.  REV reverses the direction of the axis (left to right and right to left).

C:  If the tiller exhibits sensitivity issues or if you require a dead zone, open FSUIPC and program the SLOPE function and/or set a NULL ZONE.

D:  If you have issues with the tiller not working correctly, do the calibration again in Windows and FSUIPC.  If calibrated correctly, the tiller will change to rudder control at 60 knots.

OEM is an acronym for Original Equipment Manufacturer.

OEM Boeing 737 Control Columns - A Closer Look

/

OEM Captain-side 737-500 series control column.  Previously used by Croatian Airlines

The two control columns have been refurbished and installed into the simulator.  The control columns previously were used in a 737-500 airframe operated by Croatian Airlines. 

I was fortunate to have been able to secure these columns, and although there is some wear on the yokes, the buttons, electric trim switches, chart holders, and trip indicators are operational and in good condition.  Furthermore, a working stick shaker is attached to the captain-side control column.

In this article, I use the words control columns and yoke interchangeably.

Mechanical Set Up

To allow the two columns to be fitted to the 5 inch high platform, the lower cogs have been removed and replaced with bearings.  The bearings support a high strength stainless shaft that connects to a rotating disc beneath each of the columns; movement is synchronised between control columns.

Physical movement of the control column is registered by high-end string potentiometers and any movement converted to an electrical signal that can be read by the interface card.  The interface card used is a Leo Bodnar 836X joystick controller.

The interface card, electrical wiring and potentiometers are installed on a piece of plastic board mounted to a dust proof box and attached to the underside of the platform.  Access to the box is via the front of the platform.

Push and Pull Pressures

In the real Boeing 737 aircraft the control columns are hydraulically driven, and a fail-safe cable mechanism provides redundancy should the hydraulics fail.  The 737 is rather unique in that, although hydraulics control movement of the control column, the pressures needed to move the columns (by hand) are quite stiff.  Therefore, hand flying a 737 can be quite tiring; you must use a little muscle to move and maintain the position of the controls.

The specifications for the real aircraft state that the control column has a 37 pounds push/pull value +- 4 pounds, while the roll pressures are 12 pounds +- 3 pounds.  These pressures can differ from aircraft to aircraft, but fall within the published specifications. To replicate the push, pull and roll forces as accurately as possible, four heavy duty springs have been fitted to the column mechanism. 

Heavy duty pre-tensioned springs provide accurate static control loading

The control column pressure can be adjusted by either replacing the springs with higher or lesser tension springs, or by disengaging the outer springs. 

A pressure test determined that push/pull pressure is 20 pounds and roll pressure 15 pounds.  The push/pull pressure is on the low side, however, will be left as is for the time being.  Springs have been used rather than hydraulic rams due to the simplicity of a spring and ease of replacement.

Although the use of springs is rudimentary, it's acts as an interim measure until control force feedback is installed.  When this is done, the force required to move the control column will alter based on aircraft's speed, flap setting, landing gear position and other environmental variables.

The video at the bottom of this article demonstrates the linkage mechanism and springs in motion.

Configuration - Movement and Buttons

Configuration of the control columns is straightforward. Although there are two control columns, each column is linked to the other.  Therefore, only one interface card is required.  The buttons on the yoke, and the electric trim switch are connected to the outputs on the interface card.

Initial registration of the movement of the yoke and buttons is established in the Windows joystick calibration software.  Further calibration is either done directly in the flight simulation program, FSUIPC, or in ProSim737.  Although it is possible to assign buttons directly via the flight simulator set-up menu, the preferred method is to use FSUIPC or ProSim737.

Backlighting (Trip Indicators)

The actual yoke doesn't have backlighting; any illumination of the yoke is achieved by focusing the map light which is attached to the overhead panel.  However, the numbers on the trip indicators do have backlighting (to illuminate the numbers). 

Trip indicators are an airline specific option and do not come as standard issue.  Pilots use the trip indicator to 'scribe' the flight number of the flight, or to document the Vref speed.  Some crews never use the indicators.  I use the trip indicator as a ready memory pad to document the landing Vref speed (Vref+5).  The backlighting for trip the indicators is powered by 5 Volts.

oem chart holder and cheat sheet

Chart Holders

The chart holder is used to secure the approach plate (paper chart) in an area that it can easily be read during flight operations.  The chart holders have a folding mechanism beneath the plate that allows the holder to be either pushed flush to the yoke, or positioned at a user-selected angle. 

Another function of the chart holder is to provide a ready memory jogger for specific flight modes (checklist).  The adhesive transfer on which this information is printed is specific to each aircraft type and /or airline.  illumination of the chart plate, like the yoke, is achieved using the map light.

OEM Verses Reproduction

Several companies manufacture reproduction control columns: Precision Flight Controls (PFC), CH Products, Revolution-Sim and Ace Engineering to name a few.  Over the years I have used products from ACE, CH Products and PFC.  Without transgressing into a 'tit for tat' argument, you get what you pay for.  

A CH yoke retailing at $100.00 cannot be compared with an ACE yoke retailing around $1300.00, however, both products have been manufactured to cater towards differing segments of the market.  This said, the difference between ACE and PFC is marginal.  I cannot comment on Revolution-Sim having not used their products. 

So what is the different between a high-end reproduction yoke and a OEM yoke and column?

The main difference is the feel and finesse of the genuine item.  Boeing has spent a lot of money (more than PFC, ACE or Revolution-Sim combined) in the development and engineering of the control column, and this is very difficult to replicate in a reproduction.

The OEM yoke and column is engineered to provide faithful service for many years.  It's also built to suffer use and abuse from real-world pilots, and I am certain anything a virtual pilot can throw at it, will not cause any damage.  The buttons and electric trim switches are solid, feel good to manipulate and are very reliable.

Yoke Performance

The yoke moves left and right across its range of motion with a smooth and silky feel without staggering, binding or rough patches.  Likewise, the columns move forward and aft very smoothly.

The electric trim switches are far more responsive than the reproduction switches I have used.  A slight application of pressure to the switch engages the electric trim.  The electric trim switches response is a akin to a hair trigger on a firearm - it only needs a light touch to engage. 

The control column is very responsive, and if calibration has been done correctly, very accurate.  If the yoke is turned 15 degrees to the left, the measurement on the aileron tape is exactly 15 degrees.

Synchronization

I was concerned that synchronisation between the two control columns would not be perfect, however, my concern was short-lived.  The use of high-end bearings at the end of the control linkages removes any chance of slop (loose movement) between the two control columns. 

Yoke Switches

  • OEM 737 yokes have several switches and buttons.

  • Momentary press push button - auto pilot deselect.

  • Momentary rocker switch - electric trim up/down. This switch is interesting as it incorporates redundancy.

  • Momentary rocker - push to open channel (push to talk PTT).

  • Rocker switch - Intercom.

  • Trip Indicator - used as memory aid for flight number.

oem 737 flight controls in simulator

Appearance of Yoke - Used Look

If you carefully study the pictures of the yokes, you will observe that the yokes are not pristine condition, but show solid use (and probably abuse when it was striped from the aircraft).

The baked-plastic covering of the yoke shows scratches and some of the metal has been rubbed clean of paint.  Some enthusiasts dislike this look and prefer a brand new 'out of the showroom' appearance.  If this is you, then I suggest an OEM yoke may not be for you, unless you wish to completely overhaul the yoke and pay the large amount of money required to re-bake the plastic coating.

I like the 'used' look and feel it adds to the simulator.  I have been in many cockpits, and very rarely do you find a flightdeck in brand new condition, other than in the first few months of service.  More often than not, gauges, yokes and panels are scratched, dented and stained from many hours of sustained use from individuals that are more interested in flying, and going home after the flight, than maintaining the desk!

Below is a short video showing the under floor mechanism, springs and linkage rods.  If you listen carefully you will hear the springs creaking.  This is not an issue when the simulator is running as any noise is cancelled out by the noise of the engines and flight deck ambient noise (electrics, 400 hertz noise and wind).

 
 

Glossary

Control Wheel - Yoke.

FSUIPC - Flight Simulator Universal Inter-Process Communication (interface software that provides a bridge between flight simulator and outside programs).

OEM - Original Equipment Manufacturer (aka real aircraft part).

  • Updated 20 June 2020.

Construction Commenced - New Platform to Install OEM Control Columns

/

I thought it time to post what’s happening with regard to the construction of the simulator.  Additions and improvements are in the pipeline and it’s hoped that OEM control columns and a new platform will be installed very shortly.

Currently the simulator is mounted on a fiber-board and wood platform, which I constructed when I received my Main Instrument Panel (MIP) just before Christmas 2010.  The platform has served me very well and was perfect for the installation of the ACE yoke and Precision Flight Controls (PFC) rudder pedals.  

Soon after constructing the platform and purchasing the ACE yoke, I was able to secure two OEM B737-500 control columns. I was surprised to find these units so quickly and I was fortunate that my timing coincided with the dismantling of a late model B737-500.

Fitting the OEM control columns to the wooden platform appeared to be problematic, as the platform was a tad low in height and it was awkward to retrofit the linking rod that connects the control columns for duel operation.  Therefore, I decided that a new platform was required; custom designed  to fit the control columns.

Aluminium Modular Design

Rather than use wood and fiber-board, I selected aluminium tubing cut appropriately and TIG welded together.  To facilitate future transport, the platform has been constructed in modular form.  The forward portion comprises three modules bolted together in strategic places, while the rear part of the platform (not shown), where the seats and center pedestal reside, abuts snugly to the forward section.  It’s intended to use high density ¼ inch plastic/vinyl as the upper cover on the platform  as this material is easier to work than aluminium sheeting, is light in weight, very strong and comes from the factory in Boeing grey.

In the photographs (click to enlarge) you can see the control columns (striped completely) fitted to the forward modular section of the platform.  The control columns are connected to each other by a ¾ inch heavy duty shaft and heavy-duty double bearings.  Forward and aft movement of the control column is controlled by a heavy duty spring and left and right roll movement is controlled by another spring. 

Control Column Pull Pressures

The pull pressure on the control column is set to 24 pound which is slightly less that the standard pull in the B737 which is 34 pound.  The pull can be easily altered by moving the spring forward or backward on the spring retainer.  The pressure on the roll component is presently 12 pounds.  I've been told the roll pressure as per the Boeing maintenance manual is +_15 pound; therefore, I'm well within the ball park.

Installing the ACE Yoke & 737-300 Throttle Quadrant

/

ace engineering 737 yoke

ACE Yoke & Column

Now that the seats are attached, it’s time to secure the ACE yoke to the Captain side of the flight deck and then secure the throttle quadrant and center pedestal to the floor. 

Attaching the ACE yoke is straightforward; measure correctly against the MIP the spacing as per the Boeing specifications and attach with four screws – presto!

Throttle Quadrant

Ace engineering 737 yoke and column

I am hesitate to secure the throttle quadrant to the floor until I am very sure that the wiring is correct and everything functions.

Throttle Quadrant Does Not Sit Flush

The throttle quadrant does not sit flush with the MIP, the later having an angled front while the quadrant is a straight 90 degree angle.  I want to fabricate two angled side walls to cover this open space so you cannot see the wiring at the front of the quadrant.  I'll fabricate these panels probably from Perspex or MDF wood and paint in Boeing grey or stark white.  They will be screwed in place and be easily removed for wire maintenance (if necessary)

oem weber seats, 737-300 throttle quadrant and two bay center pedestal

I also want to determine how much the throttle moves when the trim wheels rotate; this will determine how and where I secure the throttle quadrant to the floor structure. 

Maintenance

Everything may be functioning on the throttle quadrant now, but in 12 months time it may be different. Maintenance is an ongoing task with anything that moves; therefore, it is important to enable easy access to wiring, etc. At some point the throttle quadrant may have to be removed from the platform, and the method used to secure the quadrant must facilitate easy removal.

Slowly Taking Shape

It's has taken some time, but the simulator is now beginning to look like a simulator rather than a room full of aviation junk.

Boeng 737 Trip Reminder Indicator

/

OEM 737-800 Trip Indicator Reminder

Trip reminder indicators are an airline option that can be inserted to the upper portion of the yoke on both the Captain and First Officer side.  The indicator is a manual three digit memory device.

Initially used to remember the flight number of the flight, they were rarely used and eventually phased out of service. 

The pilot uses his thumb to move the three segmented dials to indicate the flight number.  I am told that when flight numbers began moving into 4 digits the devices were replaced with 4 digit indicators or their use discontinued.

I find the trip indicator very helpful and use it as a prompt to remember the landing speed (Vref).  After Vref has been calculated I scribe the speed into the trip indicator. 

The indicator is backlit from three 5 Volt incandescent bulbs, and it's a basic task to run a wire from the rear of the indicator through the yoke and column to a 5 volt power supply.  Other than a power wire, no other configuration needs to be done.

Although the trip reminder indicator is a very small addition, it's pieces of equipment such as this that provide increased immersion.