Jan 29, 2020     Squat switch - (2 hours)       Category: Landing Gear

Today I finished cleaning up the RH landing gear and tried to figure out the wiring of the squat switch.

The squat switch is used to identify when the landing gear is supporting the weight of the plane. Most importantly, this switch provides the lockout logic so that one can't retract the landing gear when the plane is resting on the ground. That would be a bad day.

A simple switch is either open or closed requiring two wires. The existing wiring that's coming out of the gear legs is 4 wires. What do all these wires do? What is the logic? I figured this would be a simple and fun process to determine the switch operation and logic. I should mention that the switches are buried into the landing gear and I hadn't been able to figure out how to see them let alone access them. It goes without saying, that the switches are not shown in the drawings. Anywhere. And the drawings of the gear legs are at best vague, and in some cases, different from what was built.

By removing the central rod from the lower leg, I was able to manually feed it through the centre of the upper leg and I could hear it tripping a micro switch. This rod is about 1/8" diameter and is normally secured to the bottom of the lower slider. It slides up into the upper housing when the gear is compressed with the weight of the plane to trip the squat switch.

I stripped the insulation from the four wires and labelled them 1 through 4. The numbers really didn't matter, they are just to identify one wire from another. I tested the continuity between each wire with the rod out and with the rod in. In all cases the circuits were open. That is, it didn't make a difference whether the rod was in or not. Well that won't do. Doing the same thing on the LH gear I was able to determine that there were two pair of circuits. When the rod is inserted (weight on the landing gear) each pair of circuits are closed. When the rod is removed (weight off the landing gear), all circuits are open.

Unfortunately I'm not getting that output from the RH gear.

Going back to the RH gear, I could see that the central hole in the upper leg has threads in threads in the fitting. As luck would have it, the hole was an 8mm thread and I just happened to have some long 8mm bolts that were used to bolt the landing gear assembly to the spar. I drilled a 3/8" hole through the centre of a heavy steel plate and passed the bolt through it, threading it fully into the end of the fitting within the upper leg. Turning it over so that the weight of the plate was hanging on the bolt, I gave the gear leg a couple of vertical jolts. Success! The inertia of the heavy plate pulled the fitting from the gear leg. The fitting was just press fit into the inner cylinder.

With the fitting removed I could see that the squat switch is actually two microswitches. Hence the four wires coming out of the top of the gear legs. These switches are installed in an assembly that's mounted about 6 inches into the gear leg. I'm still not able to access them yet. I believe the inner cylinder is press fit onto a boss at the opposite end of the gear leg. At this point I don't know how to access them or how to remove the inner cylinder. I might have to sleep on it.