Blog What is an Aircraft Transponder?

What is an Aircraft Transponder?

Communication systems are an essential part of a safely operating aircraft. With over 3,000 flights taking off each day in the U.S. alone, there must be a steady flow of communication between aircraft and control towers. While many components go into an aircraft's communication system

At the simplest level, a transponder is a device that picks up and responds to an incoming signal. These devices can either be passive or active, depending on their need for an external power supply. Passive transducers are found in the magnetic labels on credit cards and similar items, though are not used in aviation. Active transponders are used in navigation and communication systems and can be scaled up to operate over thousands of miles.

Every airplane that operates at an altitude above 10,000 feet is fitted with a transponder that communicates with a signaling device on the ground. Therefore, if an air traffic controller (ATC) needs to know the location of an airplane, they will simply send a radio-frequency signal out and await a reply from the plane's transponder. This communication is nearly instant, which helps air traffic controllers monitor the traffic of the thousands of planes flying at any given time. This information is used as part of a collision-avoidance system and is especially important when airspace is crowded.

The transponder of each airplane is assigned a "squawk code," which is a unique identifier that is provided to the pilots by air traffic controllers before each flight. This four-digit code appears on ATC computers, and it is how they track individual flights. In order for ATC to properly communicate with the transponder, the pilot must input the squawk code appropriately before takeoff. Occasionally, the squawk codes may change in flight due to the aircraft entering restricted airspace or nearing a different tower.

Every squawk code is unique besides the three used for emergencies, which pilots can input to relay information to local ATCs. For example, squawk 7500 relays that an aircraft has been hijacked and needs emergency support. 7600 indicates that the plane has lost communications with the tower and must be directed using aviation light signals. Finally, 7700 is the code used to relay that there is a general emergency. These emergency codes are so ingrained in a pilot's mind that they will skip over the number "7"  when setting the transponder to avoid accidentally sending out an emergency signal.

There have been several significant accidents following the incorrect or omitted use of the aircraft's transponder. For example, Aeromexico Flight 498 was en route from Mexico City to Los Angeles in August of 1986 when it collided with a private Piper aircraft. The Piper was not equipped with a transponder that could indicate its altitude, so the air traffic controllers at LAX had not been alerted that the plane had descended into their airspace without clearance. As a result of the collision, the FAA mandated that all jets in the U.S. be equipped with adequate transponders.


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