(CNN) -- As investigators search for clues to what happened to Malaysia Airlines Flight 370, the answer to one question may prove key: Why did the transponder in the Boeing 777-200ER stop transmitting information?
The fact that it happened at all is astonishing to John Nance, a broadcast aviation analyst and veteran pilot. "It is hard to conceive of a situation in which a triple seven would lose all ability to have its transponder on and the crew would not find some way to communicate," he told CNN.
A senior Malaysian air force official said Tuesday that the plane traveled hundreds of miles in the opposite direction from its original destination, and had stopped sending identifying transponder codes before it disappeared from radar screens.
Suggestions that the plane veered off course and that its transponder was not working raise questions about a hijacking, but a catastrophic power failure or other problem might also explain the anomalies, analysts said.
Turned off intentionally
Kit Darby, a longtime pilot, said Tuesday it was not clear whether the transponder was turned off intentionally. A power failure would have turned off the main transponder and its backup, and the plane could have flown for more than an hour with such a power failure, the president of Aviation Information Resources told CNN's Wolf Blitzer.
But Nance expressed doubt that that could have been the case. The electrical system aboard the plane is so robust and the transponder draws so little power that it would be one of the last pieces of equipment to go dark, even after a catastrophic event like an engine explosion or a breach of the cabin and rapid decompression, he said.
"I'm in a head-scratching mode," Nance said. "The most likely probability is that a human hand turned that off. Then you get into the logic tree of who and why and there aren't that many channels in that tree."
He added, "This is beginning to look very, very much like a hijacking."
A former Federal Aviation Administration safety inspector agreed. David Soucie, author of "Why Planes Crash," cited the redundant electrical, charging, battery and communications systems on Boeing 777s. Much had to go wrong for the aircraft to lose its transponder and then to veer off course, he said, adding that it stands to reason "that someone forced those pilots to take control of the aircraft and take it off course."
Turning off a transponder requires a deliberative process, said Peter Goelz, former managing director of the National Transportation Safety Board. "If someone did that in the cockpit, they were doing it to disguise the route of the plane," he told CNN. "There might still be mechanical explanations on what was going on, but those mechanical explanations are narrowing quickly."
'Patchy' radar
Alastair Rosenschein suggested that a pressurization problem may have been to blame. If the plane lost pressure and the pilots failed to don their masks within a few seconds, "they would become unconscious," the aviation consultant and former British Airways Boeing 747 pilot told CNN. "The aircraft would continue on the last heading.If that happened, the plane may have crashed in the middle of the Indian Ocean, nowhere near where the search is going on, he noted.
How does he explain the loss of the transponder signal?
"It is possible that, even with the transponder putting out a signal, the radar controllers didn't notice it, pay attention to it or receive it," he speculated. That would be rare, he acknowledged, but so too is the disappearance of a jetliner.
And radar is "pretty patchy" around there, he said.
The transponder broke
The mechanics of the device may have been at fault, said Kirk Fryar, president of Sarasota Avionics, which sells the devices. "They're not supposed to break, but they do break," he told CNN. "Sometimes the transponder itself could be off frequency, not sending the right pulse."
The boxes are located in the cockpit, within reach of the pilot, copilot or both, he said. Each is equipped with an on, standby or off mode.
And there is often more than one. "Boeing would have at least two transponders," he said. "What happens is sometimes you're flying along and, say, your transponder breaks and reports the wrong code or wrong altitude, air traffic control will go, 'You need to turn it off because we're getting erroneous readings,'" he said.
And, if one were to break, the pilot or copilot would have to flip a switch to replace it, something that a pilot stressed during an emergency might not do.
An antenna malfunction may also have been to blame, but that would be unlikely on a Boeing, which undergoes routine maintenance checks, Fryar said.
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