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Trig Biannual Transponder check after ADSB install

Feetwet

Active Member
Installed a Trig TT31 with a Garmin GPS20A feed for ADSB and initially had problems with the airborne performance check getting the Trig to switch from Airborne to Ground on landing. Eventually installed an airspeed switch which did the trick, but only after setting the Trig system setup selection "Squat Switch Source" to "Low when airborne" instead of "not connected".
When I eventually took the aircraft to the avionics shop for the Biannual Transponder check, the only way we could check the transponder with the test box was to disconnect the GPS20A, and then to again select "not connected" in the Trig setup Squat Switch Source selection to force the unit into the airborne mode. I then got a pass on the Biannual Transponder check.

The TT31 manual while generally good did not mention the options available for the setup Squat Switch selections, at least where I could see. They are, at least for my unit:
"not connected"
"Low when airborne"
"Low when GND"
"Auto on GPS"
It is entirely possible that I could have avoided using an airspeed switch if I had tried the "Auto on GPS" selection before the airborne performance check. I am not sure if the TT22 has the same setup or options.
 
The install manual talks about it:

7.1 Automatic Air/Ground Considerations
If you have configured the transponder with an automatic air/ground determination method based on GPS data with or without an Air Data Computer the transponder should be in ?Ground? mode when parked on the ramp and will not reply to Mode A/C interrogations or Mode S all-call interrogations when the system has switched to GND mode. To carry out a transponder test on the ground one of the easiest ways of making the transponder airborne is to turn off the attached GPS receiver. Without GPS data the transponder will always switch to airborne mode. This allows you to complete all the conventional transponder tests. ADS-B status messages will also be transmitted and can be verified, but position and velocity reports will be missing. When the GPS is switched on and a GPS position fix is first obtained, the transponder waits for a few seconds before switching to GND mode. It will transmit airborne position and velocity squitters for a short period before returning to GND mode, and this will allow you to complete the ramp test.
 
Trig TT31 New Manual

Interesting: My unit was initially installed in Feb,2017, my manual is dated 18 June 2014 and does not have that paragraph. The new manual is dated 28 August 2017 and has the stated paragraph. I will download the later manual.
 
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