When mass murderers took over the cockpits of four American airliners on September 11, 2001, one of the first things they did was turn off the transponders, so the planes would not register properly on civilian radar. A few months later, the Council on Foreign Relations published a book, How Did This Happen?, about the mistakes leading to that awful day. I wrote the aviation security chapter, which highlighted vulnerabilities in the way airliner transponders operate.
If the transponders had not gone silent on 9/11, air traffic controllers would have quickly realised that two jetliners en route to Los Angeles had made dramatic course changes and were bound straight for Manhattan. Instead, controllers lost precious time trying to figure out where the aircraft were.
At the time, I would have bet my life’s savings that the transponder, which broadcasts an aircraft’s location and identity, would be re-engineered to prevent hijackers from turning such units off. But nothing was done. Almost 13 years later, Malaysia Airlines Flight 370 sparked a lengthy worldwide search when, it appears, another transponder was turned off. The issue today is exactly as it was on 9/11. Pilots like their locations to be known — for ground assistance, and because the transponder warns other nearby planes of their course and altitude.
Only a hijacker at the controls of an aircraft would want the transponder silent. Flight 370 was not unique: Most of the world’s jetliners have transponders that can be turned off. On the 777-200, the type of plane used on the flight, there’s a simple rotary switch near the first officer’s left hand. All someone has to do to turn the transponder off is rotate the dial. Of course, transponders aren’t the only way to detect a plane: There’s always radar. In the movies, radar screens show incredible detail about everything. In real life, radar is easily confused, doesn’t see small planes, and may have trouble determining altitudes. Transponders solve this by reporting an aircraft’s altitude, speed, directional heading and identification code to air traffic controllers and nearby aircraft, using an electronic format that syncs with radar. And the identification codes tell controllers which blip is which flight, something radar has no way to detect. Some military radars can provide Hollywood-style detail, but military radar is not usually watching civilian flights, and when it is, it needs the transponder code to know what it’s looking at.
Why is there a transponder switch in the first place? Until recently, transponders had to be off when a plane was on the ground, to avoid sending signals that disrupted airport radar. The designs for some private aircraft — but not yet the large continued…