Transponder Turn Off

Q. In the 9/11 Report and elsewhere, much has been made of the skyjackers having turned off the cockpit transponder — the device that relays a plane’s position and height to air traffic control. Why can a transponder be turned off? Shouldn’t there be an emergency function that assures this equipment is always on?

Transponders will occasionally malfunction and transmit erroneous or incomplete data, at which point a crew will recycle the device — switching it off, then on — or swap to another unit. Typically at least two transponders are on board, and you can’t run both simultaneously. Bear in mind too that switching the unit “off” might refer to only one of the various subfunctions, or “modes” — i.e. mode C, mode S, — responsible for different data.

Very few of a plane’s components are hotwired to be, as you say, “always on.” In the interest of safety — namely fire and electrical system protection — it’s important to have the ability to isolate a piece of equipment, either by a standard switch or, if need be, through a circuit breaker.

In any case, killing the transponder will not necessarily make a flight invisible. Coverage will lapse and vary, but ATC radar can often portray a transponder-less plane as what’s called a “primary target” — a blip that provides the aircraft’s position and speed, though not its altitude. The September 11th aircraft were tracked this way during portions of their wayward journeys.


This article is part of a collection that originally appeared on Salon.com. Patrick Smith, 38, is an erstwhile airline pilot, retired punk rocker and air travel columnist. His book, Ask the Pilot (Riverhead) was voted “Best Travel Book of 2004″ by Amazon.com. Patrick has traveled to more than 55 countries and always asks for a window seat. He lives near Boston.