...if you weren't squawking C no one is likely to know...
Which reminds me of a topic which came up a while ago, to which I can't remember the answer. I ought to know this really but...
... does Mode C transmit the altitude based upon the current altimeter setting or based upon 1013? I assume that most transponders in simple a/c are not tied in to the altimeters - what would it read given that the C172 I fly has two of them?
In Buzz's example a difference of 450 feet is approx. 15mb. So at a QFE of 994 we assume a QNH of around 1009. If the Mode Charlie is based upon 1013 would the read out on the ATC display have actually been 2770 (i.e. 2800 ft..?).
Cheers!