Transponder Locates Plane for Air Traffic Controller on Radar Screen

The collision of Aeromexico Flight 498 and a single-engine Piper Cherokee Archer has focused public attention on transponders--small electronic devices that most people had never heard of before.

The transponder, which sends an answering code every time a radar beam sweeps past the plane, is the device that shows a controller on his radar screen where the plane is and--in some cases--how high the plane is flying.

Typically painted black and measuring about 1 1/2 inches high by about 6 inches wide in an aircraft instrument panel, a transponder is a critical component in the Federal Aviation Administration’s air traffic control system.

It costs from $1,000 to $2,000 and has a four-position control knob, four dials that each can select a number from 0 to 9 and a rectangular green button.

Pilots who are taking off on a trip under visual flight rules, such as the pilot of the Piper in Sunday’s accident, use the four dials to tune the number 1200 into their transponder.

That causes a special image to appear on the controller’s screen indicating the airplane’s lateral location and tells the controller that the pilot is flying under visual flight rules, or VFR, meaning that the pilot is not under radar control, is not flying in clouds, has visibility of at least three miles and is totally responsible for seeing and avoiding other aircraft, according to officials of the National Transportation Safety Board.

Airliners and other aircraft flying under instrument flight rules--meaning they are under positive guidance by air controllers--or planes flying under VFR conditions receiving radar services, will send a different four-digit number with their transponders--a number assigned individually to their plane for that flight, or even just that leg of a flight, by an air traffic controller, according to NTSB officials.

If the plane sending the discreet code is an airliner like Aeromexico 498, a computer-enhanced radar image displays for the controller the airline and flight number, altitude, speed and course, officials explained.

Even small private planes, when under radar control or guidance, are indicated by their “N” number, the six-digit number that is like a license number for private planes, and their altitude, if the plane is equipped to broadcast its altitude.

Altitude has become a major issue in the investigation of Sunday’s crash. If the controller at Los Angeles Terminal Radar Control (TRACON) who was handling the Aeromexico flight had known that the Cherokee was flying at the same altitude as the DC-9, he would have been better able to warn the DC-9 or even to divert it.

But there were only two ways for the controller to learn how high the Piper was flying: The Piper would have had to be equipped with an “encoding altimeter” to allow its transponder to send the altitude data or the pilot would have had to call the controller on his two-way radio and tell him how high he was flying.

According to investigators, the Cherokee’s pilot had not contacted the controller, nor was his transponder tuned to transmit altitude data. They have not determined whether the plane had an encoding altimeter.