Originally Posted by
Lost in Saigon
Of course you can change your aircraft ID (call sign) in the cockpit. The call sign is your flight number as in "MH 370". This changes for each flight and is not specific to any one aircraft. I think this only applies to ADS B or CPDLC where the crew enters the flight number for the particular flight in the Flight Management Computer. Non ADS B flights would have the Call Sign entered by ATC when they assign the transponder code for the flight. (please correct me if this information is not correct)
The FlightID can be selected/changed in the cockpit via the radio management unit, FMS or by a dedicated control head - depending on the aircraft model. Part 121 operators typically set this to the flight number of the specific scheduled flight.
The mode S ID however can NOT be changed by the flight crew. This is a unique 24-bit number derived from the aircraft's registration number, and is hard-wired into the transponder mounting rack. Changing it (typically) involves moving wire jumpers on a programming plug, and can only be done on the ground by maintenance engineers.
The mode S ID is hard-wired in this way so that if the transponder has to be changed, the replacement unit will automatically assume the correct ID, merely by being slid into its mounting tray.