Thread: Emergency Transponder Codes on Aircraft

I have heard that passenger aircraft transponders are able to signal to Air Traffic Control that they are being hijacked if the pilot or co-pilot keys 7500 into the transponder. Sometimes the failure of any of the flightcrew on any of the planes to do this on 9/11 is seen as evidence of a conspiracy. Does anyone know when emergency codes were introduced to commercial airliners?

I've tried looking for this information on the Internet but nothing satisfactory has shown up yet.

Dunno.

I went into a big British CAA ATC radio site in the about 1990 and was quite shocked. The equipment looked like it dated back to the 50s.