Older transponders used to transmit their reply even as a new code was being selected. Newer transponders delay their transmissions until a set period after digit was selected, giving sufficient time to complete the code selection.
Anyway, I'm in the 'Master OFF' camp. There's bugger all that ATS can do for me while I'm in the air & potentially on fire. On the ground, maybe, *if* I land at an airfield that has RFFS. But in the air? No. They'll get a hint that something is wrong when the transponder stops replying and I don't respond to their calls. Meanwhile I'll be doing what's necessary to deal with the problem.
If things deteriorate then the ELT can do much of the needed MAYDAY notification with the flick of a button. If things go OK then I can inform ATC after restoring power and cancel whatever alert phase they probably invoked when the plane went 'dark'.
Last edited by Tinstaafl; 1st Oct 2012 at 04:24.