Originally Posted by
DXjr
The only data sent from a transponder to an ATC radar site is an encoded altitude and (if the plane has mode S, which all commericial aircraft should) a hexidecimal aircraft idenifier. Nothing else.
And, of course, the squawk code, for example:
1200 = VFR
1201 = Special VFR transit corridor above LAX
7700 = Distress
7500 = Hijack
7600 = NORDO (Radio inoperative)
7777 = Military interceptor
More on
Wikipedia, varies by coiuntry
The transponder also transmits a momentary signal to ATC when the pilot pushes the "Ident" button.
I found an interesting but unconfirmed comment saying that commercial pilots are to switch back and forth quickly between 7700 and 7500 to indicate the hijacker intends to use the aircraft as a missile, 9/11 style, and the pilot is suggesting that he should be shot down.