Gear is delayed or not coming up - intermittent problem M20J

[201Steve]

Thought I would post this to the group for anyone diagnosing a gear won't come up after takeoff problem. This was an intermittent problem, gear not coming up on an early M20J when gear up was selected, about half of the time. Sometimes not coming up for quite a long time but usually after about 20 seconds of selection.

I read through alot of the "gear won't come up" (or down) threads and I hadn't seen anyone describe this, so for the next victim who might be searching, here's something to think about.

The gear up limit switch is a DT-2R-A7 micro switch, encased in a separate actuator. The actuator has a double action spring loaded plunger that makes contact with the switch, as well as the paddle on the pushrod tube. When the paddle hits the actuator, it stops the system (gear is up).

What was found is, the plunger in the actuator assembly is prone to getting stiff. 50 years of dirt and grease, will partially seize the actuator plunger and prevent the microswitch from disengaging the circuit. It basically holds the switch down telling the system not to come up any further (at all).  Below is a picture of what the actuator looks like. It is easily disassembled and cleaned, making it slide like butter once again.

 

 

[EricJ]

Good find!

[Ragsf15e]

14 minutes ago, 201Steve said:

Thought I would post this to the group for anyone diagnosing a gear won't come up after takeoff problem. This was an intermittent problem, gear not coming up on an early M20J when gear up was selected, about half of the time. Sometimes not coming up for quite a long time but usually after about 20 seconds of selection.

I read through alot of the "gear won't come up" (or down) threads and I hadn't seen anyone describe this, so for the next victim who might be searching, here's something to think about.

The gear up limit switch is a DT-2R-A7 micro switch, encased in a separate actuator. The actuator has a double action spring loaded plunger that makes contact with the switch, as well as the paddle on the pushrod tube. When the paddle hits the actuator, it stops the system (gear is up).

What was found is, the plunger in the actuator assembly is prone to getting stiff. 50 years of dirt and grease, will partially seize the actuator plunger and prevent the microswitch from disengaging the circuit. It basically holds the switch down telling the system not to come up any further (at all).  Below is a picture of what the actuator looks like. It is easily disassembled and cleaned, making it slide like butter once again.

 

 

I had that problem with mine a few years ago.  Very frustrating to troubleshoot since it was only happening once in a while.  Once was in the weather on a SID with a climb gradient which was fun.  I actually called DMax and talked to him about it and he recommended cleaning that plunger with contact cleaner or similar (it’s not technically an electrical contact) each annual.  I did that and haven’t had that problem recur at all!

[A64Pilot]

On things like that plunger I’ve had good luck by not using any kind of oil on reassembly, not even silicone spray. Glad that was all it was because I’ve heard some of those switches are very expensive IF you can find one.

I use teflon spray, just search for it, everybody sells it even WD-40 brand, I like Dupont because I think they were the original manufacturer of PTFE?

Anyway it’s perfectly dry, any oil etc will accumulate dirt eventually even if it takes 50 years, but dry teflon powder won’t. We even use it on sails, there it’s sold as “sail kote” it won’t stain the sail cloth, great for zippers etc.

https://www.mclubemarine.com/sailkote/  It’s just over priced teflon spray though

Oh, and I think Teflon is completely non reactive if I use the term correctly, it won’t “melt” any kind of plastic etc like some solvent carriers can.

Edited November 22 by A64Pilot

[Rotorhead]

Would the same issue possibly happen in the reverse direction, ie. putting the gear down? Apologies for my lack of knowledge on the part.

-KC

[Ragsf15e]

3 minutes ago, Rotorhead said:

Would the same issue possibly happen in the reverse direction, ie. putting the gear down? Apologies for my lack of knowledge on the part.

-KC

I have had that happen once as well.  Gear switch down, nothing… maybe 30” later, it came down normal.  I exercise/clean both plungers now.  It hasn’t happened since, but then im not sure exactly what caused that issue.

[Echo]

Where are these located?  Is this in Dukes and later systems?

[PT20J]

I think if gear operation either up or down is delayed, it is most likely the plunger. I’ve has two cases where the gear never retracted. One was a bad relay (identified by tapping on it while on jacks which caused the gear to operate) and the other was the up limit switch or maybe the plunger. I replaced both relays the first time and cleaned the plunger and replaced the switch the second time. The down limit switch differs from the up limit switch in that the plunger is integral with the switch. It is probably less likely to get gummed up compared to the open design of the up limit plunger. Just don’t lube these things - it will attract dirt. A shot of contact cleaner is fine if you find them sticking. 

[EricJ]

49 minutes ago, Echo said:

Where are these located?  Is this in Dukes and later systems?

Any of the electric gear systems have up and down limit switches.   They're in the belly.   One of the pushrods has paddles on it that actuate the switches at the travel limits.

[201Steve]

51 minutes ago, Echo said:

here are these located?  Is this in Dukes and later systems?

This is a dukes setup. I don’t know how different the other systems are but this one, the up limit is a micro switch encased in a mechanical actuator. I don’t know why they did it. Every other limit switch in the system (down limit, and squat switch) are fully self contained switches with no seperate actuator. 

[OneSaltyItalian]

Mostly responding to save this for myself for future use, but generally where is this located on the belly?  I did a very preliminary scan through the maintenance manual with no luck on actual location (I read through the "electric gear safety devices" section).

[Ragsf15e]

28 minutes ago, OneSaltyItalian said:

Mostly responding to save this for myself for future use, but generally where is this located on the belly?  I did a very preliminary scan through the maintenance manual with no luck on actual location (I read through the "electric gear safety devices" section).

We have pictures somewhere around here, but the rod that has the paddles which hit the switches is basically right in the middle (left /right) and directly between the gear (fore/aft).  

[201Steve]

Right near the gear actuator. Almost directly under the front seats. The main gear pushrod tube. Paddles on it. Switches on each side. One for up, one for down. It was the up switch in question here. 

[PT20J]