So it's been more than a week since Malaysia Airlines flight 370 went missing. The entire world is speculating on what happened to the airplane, so I'll go ahead and add my scenario. It's not as exotic as the current hijacking scenarios, but I think it's a lot more realistic.
The aircraft disappeared at FL350 (35,000') while in cruise flight as it was being handed off from one air traffic control agancy to another. The radio communications were normal - there was no distress call and no terrorists accidentally transmitting over the radio thinking they were talking on the PA (as happened on 9/11). Therefore, whatever happened aboard the aircraft happened quickly. If someone had been trying to break into the cockpit, you can bet there would've been a transmission over the radio. The possibility that one or both of the pilots simply stopped transmitting over the radio, turned off the transponder, disabled ACARS and flew the airplane to an unknown destination - or just crashed it - is very unlikely. As of the most recent news I've seen, neither pilot had any sort of motive to do something like that.
The event aboard the aircraft happened quickly, and I believe it was a pressurization issue. As pilots we are trained to don our oxygen masks, establish communication with the other pilot, and perform an emergency descent to 10,000' if we lose pressurization. Unfortunately, there have been cases where the pressurization warning has gone off and the crew believed it was a different warning, so they began troubleshooting before donning their oxygen masks and simply passed out before they figured out what was really happening. On the 737 (I can't speak for the 777) the cabin altitude warning horn is the EXACT same as the takeoff configuration warning (the horn that sounds if we don't have the flaps out for takeoff). You can see how that would be confusing in flight and could lead to the crew's incapacitation as it did on Helios 552.
I believe it's most likely that the crew of Malaysia 370 had a rapid or explosive depressurization. Following the event the pilots became hypoxic while attempting to place the aircraft on a course toward land, keeping lateral navigation engaged with the autopilot. The vertical navigation was probably controlled by "Level Change" as we would do in an emergency descent, which explains why the plane descened and climbed indiscriminantly. In their state of reduced mental and physical capacity they mistakenly switched off the transponder, and in an attempt to establish communication they moved the third VHF radio from "Data" to a communication frequency, thus disabling ACARS.
My theory has been hinted at, as in this article, but I'll get even more specific in my speculation. The emergency oxygen supply for the pilots is contained in cylinders similar to scuba tanks. These tanks have been known to fail (read explode) as happened with this Qantas 747. Even if the pilots had donned their masks, the oxygen supply was nonexistent, and their incapacitation was assured. IF that's the case in this event, the pilots did a remarkable job attempting to begin a descent and turn back toward home before oxygen starvation took their lives.
Regardless of what we find out, I hope the aircraft will soon be discovered. The families deserve to know what happened to their loved ones, and those of us who live and breathe aviation will probably learn something as well. I've never been more anxious for answers.
I welcome your comments and questions!