Can you elaborate a bit, Silvaire1?

1090ES, the datalink by which a Mode S transponder is also capable of broadcasting its position and other information (ADS-B), operates in the same vicinity (1090 MHZ) as UAT (978 MHz). To send the same data over the same range at the same rate requires similar power. And what's more, ADS-B is broadcast, so you send the data whether it's needed or not. Why do you describe the transponder as "inefficient".

Silvaire, How do you see other non-transponding aircraft being detected without the provision of radar?

Gliders, some light aircraft / microlight aircraft, aircraft with u/s transponders etc. also need to be considered if the ATC "known environment" is to be maintained.

You're fortunate to be able to do that. In UK we are now seeing the introduction of transponder mandatory zones (TMZ) around busy airfields (e.g. London Stansted)

Silvaire, what kind of "GPS data" would you want to download while airborne? If you mean a GPS receiver supplying raw GPS data to whatever more complex device to interpret and display it, the receiver will draw a max of 200 mA, amounting to +/- 2,5 Watts on a 12V system (which I think hefty already). A transponder will draw a lot more, though. Generally, transmitting takes much more electricity than receiving.

I'm currently on a weeks glider course. The gliders we fly all have a single lead-acid battery, weighing about one kilogram. This powers the mode-S transponder (which is not properly used here :ugh: and left in ACS mode - the equivalent of ALT - all day long, even when on the ground), the radio (which also doesn't see much use :ugh:), Flarm (the glider variant of TCAS, purely based on GPS position broadcasts) and the vario. Battery capacity is somewhere around 12 hours at least.

So we're not talking about excessive weights to carry transponders and the like, even in an unpowered aircraft.

I think Silvaire is onto something here, this sounds like something the good people at
Trig would be well able to come up with.

Do you know the context in which this is alleged to happen? If not, you are making quite a jump to conclusions.

There are circumstances where mode A is mandated, even if mode C equipped. Usually it will be in the AIP, or by ATC request, or because you have a duff encoder which is not all that unusual.

Clearly what you state above is true but it doesn't fit with...

... which implies that the selection of ALT is down to the personal whim of the instructor!

Also, the fact that the original poster had not even had Mode A/C explained to them seems lax?

When I was training, one of the aircraft briefly had a faulty altitude encoder and we were told - for obvious reasons - not to select ALT in that aircraft. But the reason was explained and understood.

To just say 'ON is sufficient' without any kind of further explanation is deficient regardless of the other circumstances isn't it?

I've seen it written in AIP, but I think most people interpret it wrong. Usually, AIP states something like this "Transponder shall be set to mode A, code 7700." This is actually the requirement to operate the transponder in a way that it will reply to mode A interrogations. But there is no prohibition (at least I've yet to see it) to simultaneously reply to mode C and S interrogations. The same thing applies for example, if controller orders you to "squawk charlie" - you will set transponder to ALT, but on most S-mode transponders you will at the same time reply to mode S interrogations, in fact, on most of them it's not possible to select only mode A and C replies without disabling the mode S (e.g. deleting the ICAO24 code in the maintenance mode of the transponder). I agree on the part if you have a faulty encoder, usually the ATC request would be to switch to ON (mode A only or A-S with S-mode transponders) and usually this would be aircraft's last flight (except for ferry flight to an airport with maintenance facilities) in controlled airspace, at least in many parts of Europe, since controllers don't trust "odd VFR pilots" to maintain accurate altitude.

Yes, it is deficient. Sadly some instructors, at least in UK, have little "operating" experience other than gaining the qualifications to instruct. They are barely off the bottom rung of the ladder.

The reasons why some instructors want it turn to mode A only is so that they can getaway with airspace infringments and also if they bust a not above clearance. There is also issues with not having any radar evidence if some one accuses you of a rule 5 bust. If you have carlie on they will find some radar that had you on. With it off its your word against whoever reported you that you were below 600ft agl.

There is no other reason for it.

It doesn't help either that some towers tell you to turn it either off or to mode A in the circuit thus removing any safe guards for commercial traffic.

Airline pilots reporting TCAS TA/RA and submitting proximity reports where traffic was in normal traffic pattern on a CAVOK day with 100 km visibility don't help either.

You have to report RA's if you like it or not. Its a Manditory reporting item.

And if they feel that the aircraft is in danger they should also put a report in.

Whats a few reports compared to a collision in the ATZ which has happend quite a few times already this year.

Haven't read the whole thread but a transponder should be always ON.

If it is Mode C it should always be in ALT mode, unless otherwise requested by ATC.

To do otherwise is just nuts, because it makes a Traffic Service practically worthless so all you are doing is screwing others, and preventing TCAS systems seeing you, so you are not doing yourself any favours.

Yet loads of pilots continue to do it.

BTW transponders used a travelling wave tube, not a magnetron which is used for shorter wavelengths (radar). All modern transponders are solid state.