As the search continues for the missing Malaysian Airlines flight, a key unanswered question is what happened to the Boeing 777's transponder.
Transponders emit electronic signals containing information that shows up on air traffic controllers' screens. The information includes the plane's unique identifying code and its direction, speed and altitude.
Transponder signals are used by air traffic controllers to keep track of flights and are also employed in collision-avoidance systems.
In the case of the Malaysian jetliner, the transponder stopped about an hour after takeoff, when the plane was above the Gulf of Thailand between Malaysia and southern Vietnam.
Transponders can be shut off simply by turning a knob, said Capt. Ross Aimer, CEO of Aero Consulting Experts and a retired United pilot who also was a 777 instructor for Boeing.
Under normal conditions, Aimer said, a pilot would have no reason to turn off the transponder. But he said there are rare occasions when a pilot might have to - say, if the crew noticed smoke coming from the device and feared an electrical fire would break out.
Aimer said the 777 has two transponders, one of them a backup. If one goes out, the other comes on automatically. He said both can be shut off by turning a single knob on a small box, about 2 inches by 5 inches in size, in the cockpit panel.
Shutting off the transponder does not mean radar contact is lost, but the unique identifier and other information would no longer show up on a controller's screen.
"The misnomer is if you turn off the transponder you turn off everything. That's not true. You still have a blip on the radar screen that comes from ground-based radar. You can never turn that off," Aimer said.
The jet would continue to show up on a radar screen if the aircraft were within range of a radar station, a distance that can extend several hundred miles, depending on the terrain and the plane's altitude, said Brent Spencer, air traffic control program director at Embry-Riddle Aeronautical University's Prescott, Ariz., campus.
"It's possible to track an aircraft without a transponder with just raw radar, but it's much more difficult," Spencer said.
And if an airplane flew too low to be picked up by radar, controllers wouldn't have any information about the flight, Spencer added.