OEM B737 Landing Gear Mechanism - Installed and Functioning — Flaps 2 Approach

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

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.

landing gear mechanism previously was used by United Airlines N326U, here landing at KLAX (Los Angeles) airport on 26 October, 2008. Photograph courtesy of Airliners.Net

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.

Additional photographs can be viewed in the image gallery.

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