Originally Posted by LH2
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.
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.