On Saturday, January 5, 2019 at 10:33:58 AM UTC-8, Darryl Ramm wrote:
Using GPS groundspeed to determine airborne status with these Trig installs is a frogging mess. I like Trig products but this is a cluster**** that should have been documented long ago. It’s not just wave flying, a strong headwind can have the ADS-B Out transmitting it is on the ground.

The “correct” thing to do...

If a 2020 compliant install with a TN72 (in an experimental glider) use the Pereguine STC kit for the TN70. Which is really just a pitot pressure “squat” switch and some t-adapters and pneumatic house pieces. The TT22 documentatin describes it connected between pin 19/ground. Test this with the transponder set to “ON”. As you (cough, carefully) apply pitot pressure you should see the transponder display switch from GND to ATL as the ASI passes about 27 knots. The pressure switch is easy to install, make sure it is connected with correct orientation (marked on the switch) between the pitot and static lines, easiest place is often just behind the ASI.

Any TT22/TN70 installs should be following the STC and using the pitot squat switch anyhow.

2020 Compliant installs are required by 14 CFR 91.227 to automatically switch between air and ground modes, it’s not clearly written there but it is an implied requirement.

If you are doing a TABS/SIL=1 install you are not required to have that automatic air/ground determination and can just set the squat switch to none or not connected (I can’t remember the exact value name) in the TT21/22 setup. That will transmit airborne messages at all times including when in the ground... as specifically allowed by TSO-C199.

If you are doing a SIL=0 install (to what save a few hundred dollars on a TN72)... ah your an idiot, and I am being polite.

I have explained to all the glider market Trig dealers in the past the importance of them selling a TN70 STC kit squat switch with any TN72 intended for every SIL=3/2020 compliant install. And I have pointed out to them how they order the kit through Edmo. If your dealer is not doing this, not asking or explaining this to you when you buy a TN72 then find a better dealer.

If folks want to use/source a different pressure switch that’s entirely possible, but I cannot reconnect any specific options there.

And to make clear I appreciate that flow chart from Trig, it lays things out well. And I'd seen an early version of that/had discussions with the Trig marketing folks. It's the other main documentation that does not make clear that a pitot pneumatic switch is a good idea is what frustrates me.

Having your ADS-B Out transmitting ground not air mode messages while airborne is a potential safety hazard and will certainly eventually get you a polite please explain letter from the FAA ADS-B surveillance folks.

The FAA ADS-B flight reports (https://adsbperformance.faa.gov/paprrequest.aspx) will flag mistakes like the ADS-B Out transmitting in ground mode during flight very clearly, so always get one or more of those after doing an ADS-B Out install. And not just for 2020 Compliance, do that checking for TABS or SIL=0 installs as well (TABS and SIL=0 reports will be flagged with multiple red items as not 2020 Compliant... that's what you *want* to see for them). Anybody can email me the PDF report for their flight to help check it over. But tell me what you are trying to achieve (2020 Compliance, TABS etc.).

All this GPS groundspeed squat switch stuff was really noticed in the very first TN72 install I helped with way back whenever in Ramy Yanetz's ASG29, it was quickly obvious using the GPS groundspeed is broken for use in gliders. That experience largely triggered that Trig flowchart recommendation. (Thanks Ramy for being a guinea pig).