OEM B737 Landing Gear Mechanism - Installed and Functioning
September 11, 2013 at 12:00
FLAPS 2 APPROACH in B737 Flight Simulator, B737-800 Boeing Flight Simulator, Boeing, Hydraulic Pressure, Landing Gear Mechanism, Landing Gear Mechanism, Landing Lever, OEM, OEM Conversion, Original Equipment Manufacturer (OEM) / Genuine 737 Parts, Real Aviation Part, Solenoid

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 gear was not so much that I was unhappy with the FDS landing gear, but more in line with wanting to use OEM parts.

LEFT:  OEM landing gear mechanism - large and hefty, but impossible to upgrade.  Note that spacer is attached (click image to view larger).

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 NG.  The knob is the opaque knob located at the end of the gear handle.  On the classics this knob is rather large while on the NG the knob is roughly 20% smaller in size.  There is also a difference in the length of the stem - the NG being a little shorter than the classics.

The landing gear mechanism I obtained came from a United Airlines B737-300; therefore, had the larger classic style knob.  I replaced this knob with a NG style knob.

Anatomy of LGM

The landing gear mechanism is quite large, is made from aluminum and weights roughly 2 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 is spring loaded and the landing gear lever can 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 gear mechanism. 

Once this had been done (rather crudely as I didn’t want to use a dremel on the actual MIP), 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 was not within the half-moon plates when in the down position.


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 position of the trigger in relation to the half-moon plate was more accurate.

Cutting the FDS Plate

Another minor hurdle needing climbing was to alter the aluminum plate located behind the FDS light plate.  The FDS landing gear secures to two ridges that are at 90 degrees to the MIP.  These two ridges need to be removed to incorporate the flat surface of the front of the OEM landing gear mechanism.  The trusty Dremel was used to cut through the thin aluminum to remove the two ridges

LEFT:  Carefully removing the two ridges from the FDS main backing plate)

Custom Bracket

The next issue was how to attach the landing gear mechanism to the MIP.  I made a bracket that fitted snugly to the upper part of the gear mechanism.  To secure the bracket to the gear mechanism, it was a matter of ensuring that the bracket leg covered two preexisting holes.  A bolt was then used to secure the bracket.  Two further holes at the front of the bracket secure the upper part of the gear mechanism to the MIP.  Once again, I picked up on existing holes in the MIP. 

LEFT:  Custom bracket that is used to secure the upper part of the LGM to the rear of the MIP.

To secure the lower part of the landing gear mechanism to the MIP, I replaced the existing bolts used to attach the half-moon to the MIP, with longer bolts.  I then drilled a small hole to the front plate of the landing gear mechanism and spot welded a nut to the inside of the 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 Buttons for FS configuration

One aspect to take note is that the NG landing gear lever is one inch shorter than the classics; therefore, one inch of the lever needs to be removed.

To connect the actual up and down functionality to Flight Simulator requires the use of two configurable buttons; one for gear up and the other for gear down.  The two buttons (not pictured) are located inside the unit screwed to the inner side of the housing.

Reproduction or OEM

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

LEFT:  LGM 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.

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.  Reproduction units usually do not incorporate a spring-loaded trigger as a solenoid is not connected.  The OEM unit 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.  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.


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

LEFT:  Canon plug on ABS plastic mounting plate.  The use of the Canon plug enables a cleaner wiring configuration.

Mounting the interface cards in the SIM provides a central point for card mounting with the benefit of also being able to use this card for other functions for other OEM parts with minimal wiring.

The solenoid requires 28 volts to enable activation.

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.

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, air loads 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.

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.

Obviously, hydraulic pressure is not simulated...


OEM parts have been used; there are rarely new.  Therefore, it's expected that any part removed from an operational airliner may have slight aesthetic damage. 

LEFT:  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.

Often the damage occurs when someone removes the part from the aircraft at the scrap yard.  In the case of this part, the landing gear lever was scratched.  Although it has been repainted, the scratches are still apparent if you look very closely. 

Historical Perspective

It's easy to forget that a separated part actually did one belong to an operational airliner. In this case the landing gear mechanism belonged to United Airlines 737-300 registration number N326UA. 

Although arbitrary, I like the idea that the part I'm using was actually used on a real aircraft, has flown the miles and served the airline.  It makes the simulator "feel" a little more real.

LEFT:  United Airlines N326U landing at KLAX (Los Angeles) airport on 26 October, 2008.  Photograph courtesy of Airliners.Net.

Is the effort of installing an OEM 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, while the tightness of the movement of the gear handle between a reproduction and OEM unit is obviously different.


OEM - Original Equipment Manufacture
FDS - Flight Deck SolutionsMIP - Main Instrument Panel
LGM - Landing Gear Mechanism
NG - Next Generation (B737-800NG)
Half-moons - the two plates that are positioned either side of the trigger of the landing gear when in the DOWN position

Article originally appeared on Flaps 2 Approach (http://www.flaps2approach.com/).
See website for complete article licensing information.