I don’t think a GT 330 knows it is on the ground. You have to feed it the GPS ground speed, or a switch contact. Or select it manually.
There is a funny history to the former. The EU made it illegal some years ago 
Wigglyamp will remember… A UK magazine article said that some large number of the GPS-TXP ground speed connections had to be removed at that point.
I suspect what Garmin have done on the newer units was to remove the manual selection option.
Airports with mode S ground movement radar are often explicit that the transponder should be in ground mode when on the ground.
As I said, I have heard all kinds of things. Because of X you should do Y, where both X and Y are arbitrary reasons/arguments. I have never seen anything written though.
The GTX330 knows if it’s on the ground or not (it automatically switches from Ground to Alt mode), I believe it’s altitude based (maybe from a calculated VS) but can’t find it.
I extremely highly doubt it can do that. See here (search for “differential”).
“The same pressure switch was used to switch both the TAS605 and the GTX330.”
I had been around that block with several UK avionics shops.
If yours switches automatically, you have either the GPS connected to it (which you must have for ADS-B OUT, of course) or you have a differential pressure switch or a landing gear connection.
The GTX330 IM is e.g. here – search for e.g. “GND”.
bookworm wrote:
Mode S transponders have a GND mode and know what to do automatically, as the Garmin guidance that Peter quotes indicates.
No, they don’t! How could they? They depend on a squat switch, dynamic pressure switch or air data to automatically switch between GND and ALT modes. If there is none of that the pilot has to manually switch between GND and ALT modes. My club has aircraft with both kinds of installations.
Manual switching is not permitted by CS ACNS.D.ELS.020. If no automatic switching is available, it has to be treated like a Mode A/C (always airborne mode). In which case my comments on A/C transponders apply.
It used to be set on take-off, and, long ago, to sby while changing squawk. Now transponders don’t change until new code is fully entered, and should never be switched to sby in the air.
And the FAA said, some years ago, to squawk fully once moving on ground. I don’t recall anything from EASA. Or is my recollection wrong?
I did a google on CS ACNS.D.ELS.020 and found this [ local copy ].
At the bottom of page 26 it has:
So a GTX330 is illegal for a new installation in Euro-land?
bookworm wrote:
Manual switching is not permitted by CS ACNS.D.ELS.020. If no automatic switching is available, it has to be treated like a Mode A/C (always airborne mode). In which case my comments on A/C transponders apply.
Then how come the Trig TT31 is approved, while having a manual “GND” setting?
It is also a consequence of CS ACNS.D.AC.020 (c) and CS ACNS.D.ELS.025 (c) that blind altitude encoders are not allowed. Yet lots of aircraft are flying with blind encoders.
There is something odd here.
It all depends when and against which spec they were certified.
CS.ACNS entered into force on 1st Jan 2014.
TT31 ETSO certificate are dated Feb 2011. Its installation manual refers to ETSO.C112 and C166. I haven’t checked the details about the ground mode and I don’t have access to EUROCAE specs anyway.
Original european approval of GTX330 appears to be through the LBA in Oct 2002
