OEM B737 Landing Gear Mechanism - Installed and Functioning

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oem 737-800 landing gear mechanism. impossible to upgrade

I have replaced the landing gear lever supplied by Flight Deck Solutions (FDS) with the landing gear mechanism (LGM) from a Boeing 737-500 aircraft.  The reason for the replacement of the landing gear was not so much that I was unhappy with the FDS landing gear, but more in line with wanting to use OEM parts.

Before wiring further, there are a number of differing styles of landing gear mechanisms seen on Boeing aircraft depending upon the aircraft series.  For the most part, the differences are subtle and relate to wiring and connectivity between different aged airframes.  However, there is a difference in the size of the gear knob between the Boeing classics (300 through 500) and the Next Generation; the knob is the opaque knob located at the end of the gear handle.  On the classics this knob is rather large; the Next Generation has a knob roughly 20% smaller in size.  There is also a slight difference in the length of the stem - the Next Generation stem being a little shorter than the classics.

The landing gear mechanism was originally used in a United Airlines B737-300, and had the larger style knob. The knob was removed and replaced with a Next Generation knob. The stem was also shortened to the correct size of the Next Generation.

Anatomy of LGM

The landing gear mechanism is quite large, is made from aluminum and weights roughly 3 kilograms.  Most of the weight is the heavy solenoid that can be seen at the front of the unit.  A long tube-like structure provides protection for the wiring that connects the solenoid to the harness and Canon plug at the side of the unit.  The red-coloured trigger mechanism on the gear stem is spring loaded, and the landing gear lever must be extended outward (toward you) when raising and lowering the gear.

Installation and Mounting

I am using a Main Instrument Panel (MIP) designed by Flight Deck Solutions which incorporates a very handy shelf.  Determining how to mount the gear mechanism was problematic as the position of the shelf would not allow the mechanism to be mounted flush to the MIP.  After looking at several options, it was decided to cut part of the shelf away to accommodate the rear portion of the gear mechanism. 

Once this had been done (rather crudely), it became apparent that, although the mechanism mounted flush to the MIP the landing gear lever was not in the correct position; the lever was too far out from the front surface of the MIP and the trigger, when the lever was in the down position, did not sit inside the half-moon protection shields.

Spacer

The solution to this problem was to design and mount a 0.5 cm thick spacer to the front of the landing gear.  This spacer was made from plastic and cut to the exact measurement of the gap that the landing gear lever moves through.  Attaching the spacer to the lightweight aluminum of the landing gear mechanism was straightforward and was done with four small screws. 

Once the spacer was attached, the trigger of the landing gear sat in the correct position relative to the two half-moon protection shields.

Carefully removing the two ridges from the FDS main backing plate

Cutting the FDS Plate

Another minor hurdle was the aluminum plate located behind the FDS light plate had to be altered.  The FDS landing gear secures to two ridges that are at 90 degrees to the MIP.  These two ridges had to be removed to enable the flat surface of the front of the OEM landing gear mechanism to sit flush.  A Dremel was used to cut through the thin aluminum, and the two ridges were removed.

Custom Bracket

Custom bracket that is used to secure the upper part of the landing gear mechanism to the rear of the MIP

The next issue was how to attach the landing gear mechanism to the MIP.  I made a custom bracket that fitted snugly to the upper part of the gear mechanism. 

To secure the bracket to the gear mechanism, the bracket leg was positioned over two pre-existing holes and secured to the body of the mechanism by two machine screws.  To attach the mechanism to the MIP, the two holes in the bracket were aligned with two existing holes in the MIP and secured by machine screws and nuts. 

To secure the lower part of the landing gear mechanism to the MIP, I replaced the existing bolts used to attach the half-moon protection shields to the MIP, with longer bolts.  I then drilled two small holes in the front plate of the landing gear mechanism and spot welded a nut to the inside of each hole.  The bolts could then be used to secure the gear mechanism to the MIP.  To stop lateral movement of the gear mechanism, I used a standard L bracket to secure the unit to the shelf of the MIP.

The reason for the secure mounting will become obvious later in the post.

Stem Length and Initial Configuration

One aspect to take note is that the Next Generation landing gear lever is one inch shorter than the classics; therefore, one inch of the lever needs to be removed. This involved removing the stem, cutting off one inch, and painting the cut portion with black paint. The stem was cut with an angle grinder

Two buttons were used to enable the three positions of the landing gear (up, center and down) to be calibrated. The center position does not require a button. The two buttons (not pictured) are located inside the unit screwed to the inner side of the housing. The buttons are triggered when the stem of the landing gear passes over them.

landing gear solenoid.  The LGM does have a handy foot beneath the solenoid for attachment to the MIP shelf; however, this foot sits too far forward of the shelf to be of use when the LGM is flush to the MIP. it was designed for an oem mip

Reproduction or OEM

There are three primary reasons for using an OEM landing gear mechanism rather than a reproduction unit.

The mechanism, as mentioned earlier, includes a solenoid.  This solenoid stops the landing gear from being raised or lowered at certain landing gear lever positions.  Reproduction units rely on software to replicate the function of the solenoid.  Using an OEM unit allows the solenoid to be used.

Another difference is the trigger.  Because reproduction units do not use a solenoid, a spring-loaded trigger is not required. An OEM LGM requires a spring-loaded trigger to engage or disengage the solenoid.

Furthermore, reproduction units often do not provide correct positioning of the trigger in relation to the half-moon protection shields.  The half-moon and trigger are safety features, and the trigger should be partially hidden between each of the two half-moons when the landing gear is in the DOWN position.

Canon plug on ABS plastic mounting plate.  The use of the Canon plug enables a cleaner wiring configuration. it also facilitates easier removal of the mechanism if necessary

Interfacing

To enable the solenoid to be used, a Phidget 0/0/8 relay card was used.   The card interfaces the actions of the solenoid (on/off) and is then read by the avionics suite (ProSim737). 

The Phidget card is mounted in the System Interface Module (SIM) and connection from the card to the landing gear mechanism is via the Canon plug. 

To enable the Canon plug to be used, the pin-outs were determined using a multimeter in continuity mode. The solenoid requires 28 volts to enable activation, and the power connects directly to the Canon plug from a Meanwell 28 volt power supply.

Muscle Required!

To use OEM landing gear requires muscle!  Pulling the gear lever from its recess position is not a slight pull.  Likewise, moving the gear lever between down, off and up requires a bit of strength.  This is why mounting the mechanism securely is very important.

Operation and Safety Features

Boeing has incorporated several devices in the aircraft, such as squat switches, computerized probes and mechanical locks (down and up-locks) to ensure that the landing gear cannot be raised when there is weight on the main landing gear.  If weight is registered, then the landing gear lever lock is activated inhibiting the gear lever from being able to be placed in the UP position.  This lock is controlled by the solenoid.   

An override trigger in the lever may be used to bypass the landing gear lever lock.  Depressing the trigger will disengage the lock and allow the gear lever to be moved to the UP position.  The reason for the half-moons should now be obvious.  By partially covering the trigger, the half-moons act as a physical barrier to stop a pilot from easily accessing the trigger mechanism to disengage the landing gear lever lock.

After rotation, the air/ground system energizes the solenoid which opens the landing gear lever lock allowing the gear lever to be raised from the DOWN to the UP position.

Scratching to the gear lever shaft.  Note the access pin on the shaft that allows removal of the retractable trigger.  Also note the smaller NG style knob which replaced the larger knob used on the classics

How it Works in the Real Aircraft (Hydraulic Pressure)

In the real Boeing aircraft, hydraulic pressure is used to raise the landing gear.  This pressure is supplied through the landing gear transfer unit.  

Hydraulic system B supplies the volume of hydraulic fluid required to raise the gear.  Conversely, hydraulic system A, by supplying pressure to release the up-locks, is used to lower the landing gear.  Once the up-locks have been disengaged, the gear will extend by gravity, the air load, and to a limited extend hydraulic pressure.  

Moving the landing gear lever to OFF (following take off) will remove all hydraulic pressure from the system.

Lineage

Originally the landing gear mechanism was used in United Airlines N326U. Unfortunately, due to copyright, an image cannot be posted.

In-Flight Testing

The solenoid and trigger mechanism operate in the simulator as it does in the real aircraft.  When you start flight simulator and ProSim737 there is an audible clunk as the solenoid receives power.   Immediately after rotation, you hear another audible clunk as the solenoid is energized (to open the landing gear lock).

If you want to raise the gear lever to UP whilst on the ground, the only way to do so if by depressing the trigger to override the landing gear lock.

Hydraulic pressure is not simulated.

Final Call

Is the effort of installing an OEM landing gear mechanism to the simulator worthwhile?  I believe the answer is yes. The use of the solenoid provides added realism as does the use of a spring-activated trigger. Furthermore, the effort that is required to extend and move the landing gear lever in stark contrast to the effort required when using a reproduction unit.

Acronyms

OEM - Original Equipment Manufacture

FDS - Flight Deck Solutions

MIP - Main Instrument Panel

LGM - Landing Gear Mechanism

NG - Next Generation (B737-800NG)

Half-moons - the two protection plates that are positioned either side of the trigger of the landing gear when in the landing gear is in the DOWN position

OEM Boeing 737 Control Columns - A Closer Look

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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.

Control Force Push and Pull Pressure

The 737‑800 uses a mechanically‑linked, hydraulically‑powered elevator control system with artificial feel provided by the Elevator Feel and Centering Unit. Typical control column forces in normal operation are light to moderate because the hydraulics carry the aerodynamic load, but force increases with airspeed due to the feel system’s airspeed‑sensed pressure. There is a fail-safe cable mechanism that 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.

Based on the system design:

  • At low speed: Forces are light; the feel unit provides minimal resistance.

  • At climb/cruise speeds: Forces increase noticeably; the feel unit stiffens the column to prevent excessive pitch inputs.

  • At very high speed (near Vmo/Mmo): Column forces become significantly heavier to protect the aircraft from overstress.

  • In hydraulic failures: With one hydraulic system lost, forces increase. With both systems lost (extremely rare), the aircraft reverts to manual reversion, and forces become very heavy, requiring two‑handed operation.

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. This will be a software option.

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 Holder

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.

Synchronisation

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 flight deck 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, wind etc).

 
 

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.