On Wednesday, May 30, 2018 at 10:54:02 AM UTC-7, Dirk_PW wrote:
Question. So if you are doing a SIL=1 install, and you set the squat switch to "none", is the ground-vs-air transmit modes controlled manually? (i.e. via the GND and ALT switches on the head unit? And a secondary question, if you set it to "GND", then will the unit transmit "ADS-B ground" messages?

There are some deeper questions hidden there, which I wish I had my head fully around. But this is touching on stuff I'm curious about so give me some time and I'll get an email off to Trig and others. But this really does not affect these scenarios in practice. I can't imagine any practical situation where a glider transmitting ADS-B surface messages was actually helpful. If anybody found themselves taxiing a glider at a major airport with say ADSE-X surface surveillance then that is still going to detect your glider location (with transponder in ALT) via SSR interrogations.

My main concern is you never want to transmit ADS-B surface messages when in the air. Which is what the GPS ground speed based determination may do. I would just never use that, it is potentially causing more problems than it's solving.

The RTCA standards for ADS-B Out actually allow light aircraft (and gliders) to never transmit ADS-B surface messages (i.e. OK to transmit airborne only messages when on the surface). That seems a smart RTCA decision because of that potential down side of erroneously transmitting surface messages when actually airborne . But the FAA seems to disagree, It's subtle wording in CFR 14 91.227 (i.e. for 2020 Complaint installs) about needing to transmit aircraft width and length (that are only broadcast in ADS-B surface messages), and clarifications in things like AC 20-165B that says this should be a automatic determination, that vendors and others are interpreting as requiring 2020 Complaint ADS-B Out installs even in light aircraft to transmit ADS-B surface messages if on the ground, and to have that automatically determined.... which is why I'm suggesting using that pitot pressure switch for all 2020 Complaint (SIL=3) installs with the TT22/TN72... even if in practice you would likely be just as safe with no switch and the transponder always in ALT mode. TABS TSO-C199 makes it clear that for TABS/SIL=1 installs surface messages are entirely optional (and therefore keep it simple: just don't ever transmit surface messages).

A general comment on transponder modes: Things changed from the old days where transponders were supposed to be in STBY mode until say entering a runway. Now transponder should almost always be in ALT mode, including when on the ground. The only exceptions I can think of is STBY if you think the encoder really needs to warm up (~minute or so) or ON mode if you know the encoder or ADS-B GPS is producing garbage or ATC requests you put the transponder into ON or STBY if they see it is not working correctly. Being in any mode except ALT when flying is really bad, ATC can't see your altitude, and it prevents TCAS from working and airliners etc. being able to avoid running into you. Since most glider pilots rarely talk to ATC, they may never work out their transponder is in the wrong mode. My way of thinking is if I never put that transponder in anything except OFF or ALT (full clockwise or counterclockwise turns of the mode knob on a TT21/22) I am unlikely to make a mistake.