by Jonathan M. Stern
The transponder is a device used solely to assist ATC in tracking airplanes on radar. The transponder signal enhances a "raw radar" return, which is often hard to see and follow on a radar scope. The transponder also permits the radar controller to identify an aircraft by a discrete four-digit code that the pilot enters in the aircraft. In certain types of emergency situations, the selected code is used to provide ATC with information about the emergency. For example, the code 7-5-0-0 indicates a hijacking situation, whereas 7-6-0-0 is used to indicate communications radio failure.
The Flight Simulator Cessna is also equipped with an altitude encoder that encodes the aircraft's altitude into the transponder signal so that the controller has an altitude readout for the aircraft.
In most situations, a pilot files a flight plan with the Federal Aviation Administration to initiate an IFR flight. The flight plan is inputted into a computer, which considers the acceptability of the proposed routing and altitude and decides whether or not to give an alternative routing to the destination. The computer then prints out a set of flight progress strips, one in the control tower at the departure airport and one at the departure control facility.
The clearance delivery controller uses the flight progress strip to give the aircraft its IFR clearance. The strip is then handed to the ground controller, who is thereby aware of the aircraft's intended route of flight when it first calls. When the airplane taxis to the departure runway, the ground controller passes the flight progress strip to the local controller. After takeoff, the local controller or cab coordinator advises the departure controller over an interphone line that the airplane is in the air and about to be handed off to departure control.
When the airplane is detected by the departure controller's radar, the computer recognizes the transponder code (assuming it was correctly entered by the pilot) and displays the aircraft identification, groundspeed, and altitude alongside the target on the controller's radar display. The sequence follows with new flight progress strips automatically being printed in each new facility with which the aircraft will communicate during the flight. Likewise, each new radar control facility will have a data block display of aircraft information that automatically appears on the radar scope.