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Looking for M20J gear safety switch


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Hi folks,

I had an interesting flight in my 1983 M20J a week ago, where the gear did not come down, and this time it wasn't a CFI messing with the breaker ;)

The good: the manual gear extension procedure is very easy and works great ;) 

The bad: my mechanic told me that it's the gear safety switch (the one that prevents the gear from coming up while on the ground) that's gone bad, and that the factory lead time for this part is 6 weeks.

They're trying to source it elsewhere but I thought I'd try my luck here. Anyone know where I could source one?

I couldn't get the exact P/N from my mechanic before they went home for the weekend, but I can follow up with that on Monday.

Adi

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Do you mean the airspeed safety switch?  I’m pretty sure (somebody correct me if I’m wrong) that switch is only in the retract circuit and shouldn’t affect lowering the gear. 

But if that is the problem and it is the microswitch that has gone bad (rather than the diaphragm) then check to see if there is a second unused microswitch available. 

https://www.knr-inc.com/2014-07-13-01-32-38/instrument-panels/20-manufacturing/55-1ip-mooney-pre1969

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They could be talking about the squat switch on the landing gear, but it shouldn’t stop the gear from extending either. The only electrical components that should stop it from extending would be the down limit switch, the down contractor or the gear switch. Then there is always the wiring.

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5 hours ago, N201MKTurbo said:

They could be talking about the squat switch on the landing gear, but it shouldn’t stop the gear from extending either. The only electrical components that should stop it from extending would be the down limit switch, the down contractor or the gear switch.

A couple of notes: It's a contactor, no R, an electrical contractor is person. ;-)

And if you ask a mechanic, Lasar or look in the parts manual they are called relays. Contactors are used for heavy loads, our gear motors are small by comparison.

The landing relays are right next to the gear actuator.

This is what a down limit switch looks like in my 78:

 

LandingGearSwitch4.jpg

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Thanks folks, I actually wondered about that myself and asked if the safety switch would prevent extension, they said: "This switch just opens or closes the circuit path if one pathway is closed when it should be open it will cause failures." 

Tom, where is the switch in this picture? :) 

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Switch

relay

motor

Down limit switch

wires

circuit breaker

For retraction... Airspeed safety switch or squat switch needs to be working...


All things that need to work together to get the gear to go down... and stop...

If any one of these is not working properly, the gear will misbehave...

Could be as simple as the limit switch is saying the gear is down already...

Or the gear switch not sending the signal that you want the gear to be down...

You can always ask your mechanic for additional details... to find out what he knows...

Best regards,

-a-

 

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I've never traced out the schematic, but my recollection of the gear swing during annual is that I only have to press the red button to override the airspeed safety switch to raise the gear and it goes back down when I just move the gear switch handle. Be sure it is a airspeed safety switch before ordering a replacement -- these things are pretty pricey. According to the Maintenance Manual, only S/N 24-0001 through 24-0237 (excluding  S/N24-0084) had squat switches; all the later models went to airspeed switches.

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Thanks folks, I actually wondered about that myself and asked if the safety switch would prevent extension, they said: "This switch just opens or closes the circuit path if one pathway is closed when it should be open it will cause failures." 
Tom, where is the switch in this picture?  

Belly, aft IIRC.

When you (or mechanic) remove the belly panel(s), take a minute and crawl under with your phone and take some pictures or video. It helps when looking at a parts manual to know what it looks like in real life.
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They said they need any of the following: 

DLB2141W01-L3C

SB1DDX492-2

940085-501 (Gear Safety By-Pass Retrofit)

Will call back later today to get more details. It sounds like it's the button itself?

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I was told that when I had the inflight failure the switch had failed closed (which disables the gear retraction / extension mechanism altogether), but after they pressed it a bunch of times it has now failed open (which means the gear retracts / extends in all conditions).

This is unfortunate..

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4 hours ago, Parttime_Pilot_Blake said:

Illustrated parts manual is showing PN 800335-501 for the squat switch. Google'ing for the part number didn't return a ton of results so it may require the lead time quoted. 

~Blake

 

Landing Gear Illustrated.jpg

A 1983 would not have a squat switch

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The part numbers are for the gear override pushbutton. The DLB part is an NKK part number and the SB1 is a Cuttler-Hammer part number. I would check the electronic parts houses such as  Mouser or Digi-Key or Onlinecomponents.

1477025704_Screenshot2021-03-16at8_46_49PM.thumb.png.d7eefe84c520450ec049a32410eca83e.png

2139962457_Screenshot2021-03-16at8_47_30PM.thumb.png.ee71fc67a4238ace97eca3897e017141.png

I am skeptical that the bypass switch has anything to do with lowering the gear. It only appears to be in the UP circuit.

1314659138_Screenshot2021-03-16at8_54_30PM.thumb.png.588d59c8a7afd42eccc5c400e2682cde.png

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I found the gear safety switch / bypass switch / gear switch area on my circuit diagram too. I have no idea how the switches themselves are wired, but I agree it seems suspicious that the override switch (or even the safety switch, which seems to be wired in parallel) would have any effect on gear retraction.

> I was told that when I had the inflight failure the switch had failed closed (which disables the gear retraction / extension mechanism altogether), but after they pressed it a bunch of times it has now failed open (which means the gear retracts / extends in all conditions).

I got my "open" and "closed" wrong in this statement, at least with respect to a circuit being "closed" when the pushbutton is pushed. I think it is now failed *closed*. Seems reasonable the gear would swing freely in that case. But it also seems to me that it would be safe to completely remove this button from the circuit and fly the airplane until a replacement can be found. The safety would still be active on the ground, I just wouldn't be able to force the gear to retract if that switch fails - not a big deal.

image.png

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So...

You have identified the faulty switch...

Getting a proper replacement for it...

Installed properly is key...

 

 

What does you plane use for a safety switch?

An airspeed switch or a squat switch?

 

We kind of get two choices...

Right parts, proper installation.... nets predictable results...

Wrong parts, or improper install... can net crazy results... :)

 

When it comes to crazy results and landing gear...

Avoid uncommanded Gear up... or stuck in the up position...

Following how the gear safety and back up works is pretty easy...

You can set up a logic table to see what has to work to get the gear to come down or go up...

The gear are easily tested on jacks in the hangar...

Testing it in flight, would lose the opportunity to work on it if they get stuck in the up position...

There is a test to make sure the air speed switch is working too...

PP thoughts only, not a mechanic...

Best regards,

-a-

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13 hours ago, Adi said:

Thanks Skip, would you mind sharing the source for that image? I found a wiring diagram online (http://mooney.free.fr/Manuels M20J/M20J/Mooney Service Manuel M20J Vol. 2 of 2.pdf, page 10 for my 24-1381) but it's really hard to read.

Here are a couple of scans. The first one Mooney sent me. The second one came from someone on MS.

Mooney Service Manuel M20J (low res) Vol. 2 of 2.pdf

Mooney Service Manuel M20J Vol. 2 of 2.pdf

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Thank you all again.

Thought I'd update this thread: I asked my mechanic to disable the bypass switch altogether. As expected, the airspeed safety continued to work but with no possibility of bypassing it. We placarded the switch INOP and still looking for a replacement, but with less urgency. The shop double-checked the rest of the gear-related equipment as well.

My mechanic said that "someone had crossed the wires on the airspeed safety switch" and he redid them correctly per the Mooney diagram. No idea how that ended up happening, how the gear system still worked for a while with crossed wires, or if this explains my in flight failure to extend the gear which seems like it shouldn't happen due to a bypass switch failure. They couldn't tell me explicitly how the switch was miswired so I can't do that logical exercise either.

In case it helps or is of interest to someone, I followed the diagram and annotated where each of the relevant wires go, here: https://drive.google.com/drive/folders/1zgGbb877mLz4R1QMP-nNIEX-hJ_EaWlM?usp=sharing. If someone from the future visits this thread, remember this is for a 1983 J, your schematic may be different :)

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@Adi there is the possibility of getting that expensive switch OH’d...

The resource is probably discoverable around here somewhere...

I have never used the by-pass switch in anger... :)

It may get used during the annual inspection...

in lieu of the switch.... maintainers use air pressure generated by squeezing a hose on the front of the pitot tube... a standard practice ...

PP thoughts only, 

-a-

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