Why Aircraft Identification Matters
A transponder's aircraft ID signal is crucial for the flight's safety.
In January of this year, a United 777 had to divert to Toronto due to a malfunctioning transponder. It seems that coffee was inadvertently spilled onto the center console, which contained the control panel for the aircraft’s transponder. Because the coffee caused an electronic short, a hijack code was transmitted to air traffic control.
Even after that miscommunication had been cleared up, the aircraft could not continue safely to Frankfurt. (In fact, without a functioning transponder, obtaining a clearance to cross the Atlantic would have been an impossibility. More on that later.) One of ATC’s most basic requirements is to identify every aircraft “on the radar,” and today that happens via transponders.
I first learned about the idea of aircraft identification as a little boy growing up on military bases all over the world. My father would take my brothers and me to watch airplanes take off and land while he planned his flights. I was intrigued by the different types of planes, and we would have aircraft recognition contests to see who could identify each one most quickly. This early exposure gave me a head start when I entered the Marines and later went to Pensacola for Naval Flight Training. But it wasn’t until I got into flight school that I really grasped the importance of identifying aircraft—not only visually but also electronically.
Electronic flight identification with radar (RAdio Detecting And Ranging) began in the early 1940s, during World War II. Initially it was used for military purposes, but over time, the expanding civilian aviation market found it necessary to assign its aircraft these unique electronic identifiers.
The IFF (Identification Friend or Foe) device, or transponder, is a box that electronically transmits a four-digit code identifying a specific aircraft and transmits its altitude to ATC radar. In its early days, the transponder was given the code name of “parrot,” and the numeric code entered by the pilot into its database was called a “squawk.” Today, before every airliner takes to the skies, it is issued a departure clearance and an exclusive squawk that will identify it to ATC controllers.
Two types of radar are used for identifying aircraft: primary and secondary. The basic primary radar alerts controllers that an aircraft is nearby. The secondary radar interrogates the onboard transponder to transmit data, such as its unique code or its altitude. With this information, ATC can track aircraft and keep them separated as they cross a particular piece of airspace. A transponder also has many more modes, but most of them serve military purposes and can be encrypted for secrecy and stealth.
For aircraft flying internationally, the transponder becomes a critical instrument for transiting foreign airspace. Each nation is encircled by its own ADIZ (Air Defense Identification Zone) and, in many cases, is defended by military aircraft. For smaller nations without air forces or ATC radar, the ADIZ or FIR (Flight Information Region) acts more like an economic zone.
Each time an aircraft overflies a country’s airspace, the carrier must pay overflight fees to the host nation based on the number of air miles flown over its territory. In the U.S., our ADIZ is defended by NORAD (North American Aerospace Defense Command), which will scramble fighter jets to intercept any aircraft that enters our ADIZ without authorization and a proper squawk. All our advanced fighter planes can electronically identify an unknown aircraft by interrogating its squawk, and every fighter pilot is an expert in visual identification as well.
