No one knows the cause of the latest airline disaster, the Germanwings crash yesterday in Southern France. As is usually the case after crashes, most first-day speculation is wrong or implausible. Also as is usually the case, Patrick Smith of AskThePilot has debunked many of the most fanciful cable-news theories, for instance that the plane might somehow have been remotely controlled, like a drone, or victim of "hacking" of its flight software. Without getting into all the details, this is vanishingly unlikely to have been the cause, and is so far-fetched as to merit no on-air discussion time.
The main fact that is now established is that the airplane flew steadily along its course, descending at a faster-than-normal but not-necessarily-emergency rate of 4,000 feet per minute, until it flew right into a mountainside. This is the scenario known in aviation as Controlled Flight Into Terrain, or CFIT, and it usually occurs at night or in the clouds when a flight crew does not realize what it is about to hit. It is different from what you would expect if the plane had broken apart or suffered some other major structural or control failure while aloft.
The long, controlled flight path to disaster, combined with the reported absence of any radio transmission from the crew, would be consistent with the flight crew somehow being incapacitated and unable to control the plane. This scenario would involve: