Originally Posted by Bungfai
...Requirement to turn on transponder from taxi out. Do we select Altitude reporting off or TA only?
I don't think it is possible to provide a 'universal' answer to this question that applies to all aircraft, because the operating characteristics of the transponder will vary depending on the model of transponder installed and the level of integration of the aircraft avionics system.
When I began my career in the 1970s, I was taught that the transponder should be turned on as part of the line-up checklist, and turned off promptly after landing. This was because at that time, the transponder in the aircraft did not have any way of determining the air-ground status of the aircraft, and would emit replies whenever it was turned on. Replies emitted when the aircraft was on the ground were of no value to anyone in the 1970s.
Recently, I have been responsible for specifying avionics fitment for a new production 19 seat aircraft that has a fully integrated avionics system (Honeywell Primus Apex). The transponder in this aircraft complies with Mode S EHS requirements, which means it transmits an extended squitter (containing aircraft lat/long, heading, speed, etc.) that fulfills ADS-B OUT requirements. The transponder is provided with the air-ground status of the aircraft via datalink to other aircraft systems.
In the case of this very modern aircraft avionics fitment, the crew are instructed to ALWAYS leave the transponder turned on, in Mode S TA/RA. If the aircraft is on the ground, the transponder is aware of this and operates in GROUND mode - it will only reply to interrogations that are specifically directed to the aircraft by name (i.e. by aircraft registration, or by flight number). Interrogations of this sort can be generated by airport ground movement monitoring systems. It will not emit unsolicited transmissions.
The transponder will automatically switch to AIR mode when the rest of the aircraft switches to AIR mode (a consequence of weight on wheels, airspeed, and radar altitude). When it switches to AIR mode, it operates in the usual manner for an aircraft fitted with Mode S EHS (ADS-B OUT support) - it emits a signal once per second, unsolicited.
So, the short answer to the original poster's question must be "check the AFM or FCOM", and see what the aircraft manufacturer has to say about how the transponder operates, and how the crew should configure it. The last phrase in the airport instructions, "set to auto if technically AVBL", suggests that leaving it on all the time (if it is a modern transponder installed in an integrated avionics suite) is probably the appropriate action, but this can only be confirmed by reference to the AFM or FCOM.