Gear won't retract after transponder install

[Mike A]

I had a Garmin GTX345 transponder installed in my M20K last week and after getting it back the gear would not retract on the first attempt or without pushing the override button

Initially when the gear handle was put in the "up" position the "Gear Not Safe" light would illuminate and the "Gear Locked" (or whatever it says) would go out.  Recycling the handle would put the gear back down in the locked position then moving it back up would retract the gear, however, it has progressed to when the gear handle is put "up" both the "gear locked" and "gear not safe" lights would illuminate but the gear would not have moved.  Pushing the override button would make the gear retract without incident. 

 

I went to my MSC to have them try and do a diagnosis during an oil change and they could not figure it out either.  They are going to look deeper during my annual in July but I wanted to try and get some ideas from those who may have seen this before.  Any thoughts?  I think that something happened during the transponder install because the gear worked perfectly before.  Swinging the gear in the shop took between 60 and 100 kts on the airspeed indicator to get them to move (sometimes).  Is this an up limit switch issue or a airspeed switch issue or something else.  The actuator is in good shape, retracts smoothly and has been ruled out as the issue (so far).

[irishpilot]

I'd say if the gear are swinging at the airspeed you stated, it sounds more like an upswitch problem. However, is it coincidence that you started having gear issues after they were in the panel replacing the transponder? I would take a voltmeter to the gear handle as well (behind the panel) and see if it is consistently making contact. The plane will need to be on jacks.

I had two gear issues on mine and once it was the gear handle and the other was the upswitch. r
Good luck figuring it out.

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[DanM20C]

2 hours ago, Mike A said:

Pushing the override button would make the gear retract without incident

I believe the override button is just a momentary switch across the airspeed switch.  I would bet you have a failing airspeed switch. 

Did they perform a Pitot Static check (IFR Cert) when they installed the transponder?  Often shops unknowingly damage our 40 year old airspeed switches.  I always ask that they bypass the switch when doing a leak check.  I learned this the hard way once, unfortunately they are not cheep. 

Cheers,

Dan

[Mike A]

No IFR cert during the transponder install (not due till 4/2020)

We seemed to get a wide range of speeds at which the gear started retracting (up to 100 kts), but that was also very unscientific and involved someone blowing in the pitot tube while someone else looked at the airspeed indicator and estimating when the gear movement started.

The dual "gear safe" and "gear unsafe" indications at the same time were throwing everyone off

2 hours ago, irishpilot said:

However, is it coincidence that you started having gear issues after they were in the panel replacing the transponder? I would take a voltmeter to the gear handle as well (behind the panel) and see if it is consistently making contact. The plane will need to be on jacks.

 

I don't believe in coincidences and think the install has something to do with it.  The new transponder coax cable runs right in the same area as the up limit switch.  

I am less inclined to believe its the handle as every time you move it it does something, even if its just changing the annunciation light from gear locked to gear unsafe and back, but will keep that up my sleeve.

Edited May 23, 2019 by Mike A

[Oldguy]

Just a side-bar, but they did do a transponder test, right?

[irishpilot]

I don't believe in coincidences and think the install has something to do with it.  The new transponder coax cable runs right in the same area as the up limit switch.  
I am less inclined to believe its the handle as every time you move it it does something, even if its just changing the annunciation light from gear locked to gear unsafe and back, but will keep that up my sleeve.

I think you are on the right path. Let us know what you find out.

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[Mike A]

46 minutes ago, Oldguy said:

Just a side-bar, but they did do a transponder test, right?

Yes.  They did the necessary ADS-B certification tests, just not a full IFR recert.

[EricJ]

I think some transponders have an automatic ground mode switch that uses the squat switch, which for many of us is really the airspeed switch.   Maybe look at whether the transponder installer did something with the airspeed switch signal so that the gear relay doesn't get enabled any more.    That would explain why it works with the bypass button.

Anyway, if you're looking for a correlation with the transponder install that's all I've got.

 

[PT20J]

If the override button lights and holding the button in allows normal retraction, it’s the airspeed switch. 

Not a good idea to blow into the pitot. Use a rubber hose with rubber bulb to gently pressurize pitot system. 

Basic troubleshooting would be to jack up plane, jumper across airspeed switch and see if gear operates normally. 

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[mike20papa]

Me and my butt simple A model have no idea what this subject "'tis about."   

[PT20J]

2 hours ago, EricJ said:

I think some transponders have an automatic ground mode switch that uses the squat switch, which for many of us is really the airspeed switch. 

In case some of you missed this (really, who can keep up with all this stuff?), the FAA changed position on transponder use a while back. It used to be that FAA didn't want you squawking on the ground and the standard procedure was to put the transponder in standby mode after landing and until just before takeoff. Garmin added a feature to its transponders called Ground Mode  that would do this automatically. Then the FAA decided that with the coming of ADS-B, it really wanted transponders in ALT mode at all times. Recent installations of Garmin transponders will have Ground Mode disabled. See attached Safety Alert.

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SAFO15006.pdf