On Monday, November 25, 2019 at 10:22:16 PM UTC-8, Charles Longley wrote:
OK so I fly a glider which has nothing in it except a radio and basic instruments. I have an Oudie to keep me out of trouble. I just bought a FLARM brick with ADS-B in. Which I’ll install this winter. I’ll probably install a Trig Transponder next winter with ADS-B out. I’ve installed several different ADS-B out transponder systems in power airplanes. I haven’t yet installed a squat switch in any of them. Generally power airplanes are faster than gliders. But there’s obviously Cub types that will fly at glider speeds so why don’t they have squat switches. I am just spit balling here. But it’s a good question.

Why don't you ask the person who did the installation in any "cub types" how they intended it to comply with 14 CFR § 91.227 (d)(1)? And what happens in strong headwinds like those giving near zero groundspeed? Ask them if they plan flying in those conditions.

The simple assumption many power installations will make is to use GPS based air/ground switching... and slow enough GA aircraft will have issues if they get into a strong enough headwind. But they likely do not fly often in those conditions or have never thought about it. We should be thinking about it in gliders as earlier ADS-B Out installations ran into immediate problems in wave.

Heck in strong wave locations towplanes may well run into this issue, so for those operators at places like Minden it might be a good idea in the towplane as well, what the FSDO wants for approval, if anything, if the STC says use GPS determination is up for discussion with them.

I tried to explain what I understand the regulations to say, and point out what folks operating in wave conditions want to avoid (GPS groundspeed switching saying you are on the ground when at altitude in wave). If it was not the apparent need for automatic switching we would likely be better off selecting "none" as a TT22 switch setting and just rely on the pilot switching the transponder manually between ON and ALT.