NCYankee’s comments explain what I have recently seen. My GTX330 always switched to GND on landing but since it has been
upgraded to ES with the latest software it stays in ALT mode after landing and during taxi. If I switch it to Standby it will switch automatically to ALT at take-off so from now on I will leave it in ALT all the time. Garmin confirm there is no GND mode for the ES version.
In Zurich, you have to have the transponder on even on the ground. Check the AIP for specific procedures. On the club’s Seneca, the GND mode has been removed by a firmware update a few months ago (it has a GTX 345 I think).
Silvaire wrote:
As a result, since almost all my flying is done squawking VFR in the US I cannot clearly remember the last time I touched any button or knob on the transponder. It gets turned on and off with the avionics master switch.
That is what I do as well. Most transponders power up to the last mode. I never set ALT mode, except after the unit comes out from maintenance where an avionics tech or mechanic has either powered off the transponder using the transponder button/switch rather than the avionics master or set it to STBY Then it either does not turn on with the avionics switch or ends up in STBY. That will mess me up as it won’t discover it until I am out at the runway as part of my pre-takeoff check list.
Peter said:
“The above installation was bodged (by an EASA145 company)”
No, they weren’t really trying with that…..how about when they connect the vac line instead of the static to the switch. It passed the pitot/static test stuff of course….but switched to airborne when you started the engine!!
NCYankee wrote:
At the major airports in the US with ASDEX, transponders are required to be on so they can be tracked on the movement area and runways. This applies to mode A/C or mode S transponders. Mode A/C transponders don’t broadcast anything, but do reply to either mode A or mode C queries. Mode S transponders include data and the reply content is different for ground and airborne state
In other words if I were to presumptuously paraphrase, and as per the original posts in this thread, in the US regardless of transponder or aircraft type we now generally leave transponders in ALT all the time. Worth understanding when dealing with US built avionics anywhere.
As a result, since almost all my flying is done squawking VFR in the US I cannot clearly remember the last time I touched any button or knob on the transponder. It gets turned on and off with the avionics master switch.
In US regulations, ADS-B Out is required to be in the transmit mode at all times.That is interpreted to be during taxi on the movement area or runways on the ground and when airborne. That differs from the transponder on requirement, which is when the aircraft is in controlled airspace. At the major airports in the US with ASDEX, transponders are required to be on so they can be tracked on the movement area and runways. This applies to mode A/C or mode S transponders. Mode A/C transponders don’t broadcast anything, but do reply to either mode A or mode C queries. Mode S transponders include data and the reply content is different for ground and airborne state. The ground/airborne state is included in the reply and when in the airborne state, the reply includes the pressure altitude, while in the ground state, the aircraft length and wingspan categories are included instead of the pressure altitude. ADS-B Out via 1090 MHz also uses a ground and airborne state to control the data in the broadcasts. The modes on the GTX 345 are ON, ALT, and STBY. ON and ALT will reply to queries and broadcast ADS-B Out, the difference being that ON does not include pressure altitude. STBY does not broadcast ADS-B Out or reply to queries. Ground/Airborne state is managed by the GTX 345 and is either controlled by Weight on Wheels or GPS input. In effect this just changes the content of the ADS-B Out and mode S replies. The normal state that the GTX 345 should be left in is ALT on the ground or airborne.
From here the differential pressure switch looks like this
The above installation was bodged (by an EASA145 company); I recommend using a better quality tube and some proper jubilee clips 
If you get a static leak, you are in for a whole load of grief, expense, hassle (your plane will be grounded in the hangar of the company doing the altimeter test, which now has got you over a barrel) and all sorts of subtle but weird symptoms.
AFAIK this solution works with all Garmin transponders, GTX330 onwards.
It is a lot easier to install than sorting out a signal from the landing gear squat switch circuit, which requires an above average amount of “electrical intelligence” (every maintenance person I have ever spoken to about this was completely stumped on how to do it), it probably requires some kind of approval which will rule out any “clever” solution using electronic components, needs to be done with care because you don’t want to disable the squat switches, and anyway works only with a retractable.
The GPS GS method tended to be used with King transponders, IIRC.
I think the newer transponders are quite smart about it. The GTX3x5 you just turn on and leave it on ALT all the time. If you use something like the PilotAware’s receiver page, you’ll see the transponder output change based on what it thinks it’s doing. The GTX3x5 has an aircraft stall speed setting in its configuration, and I suspect it uses that to decide whether you’re on the ground or flying (if the transponder has a GPS position source of course).
I’ve been moved! Thanks Peter. Now I’m still somewhat confused, but on a much more informed level.
It is very easy to make this automatic, with a landing gear switch, or a differential air pressure switch (across the pitot and static). I have the latter.
