Until debris from the missing aircraft began to surface on June 6, Air France Flight 447 and its 228 passengers and crew seemed to have vanished into thin air. There were no last-minute distress calls from the cockpit; just 24 automatic satellite messages--some indicating major system failures relayed from the stricken plane to Air France maintenance headquarters. Even now, as recovery teams retrieve flotsam and victims' bodies, the black boxes that recorded the flight's final moments remain as much as 2 miles (3.2 km) deep.
That dearth of data has led to a grab bag of speculation about what doomed the four-year-old Airbus A330. Bloggers and aviation experts flit from theory to theory. A terrorist attack? A lightning strike? Some catastrophic technical failure? The first two explanations have largely been discounted (no terrorist group has claimed responsibility, and planes are built to shrug off lightning strikes). Most aircraft accidents stem from an unfortunate cascade of events rather than from any single system malfunction. It's becoming clearer that some combination of weather, an unknown flight-control failure and perhaps the crew's inability to respond is probably to blame. The pilots' margin for error at the time was small; in addition to encountering bad (but not extreme) thunderstorms, the plane was operating near its "coffin corner" a combination of speed and altitude at which it becomes difficult to maintain stable flight.
So why might Flight 447 have been flying at the wrong speed? The latest theory is that one of its three Pitot tubes external sensors used to measure airspeed iced up, leading to an inaccurate reading. That would have shut down the aircraft's autopilot one of those 24 messages indicated that had occurred and compelled the flight-control computer to shift more responsibility to the pilots. Turbulence would have further whittled away at their safe-speed range. "They might have slowed down inadvertently and flown into a stall," says Hans Weber, an aviation-safety expert at Tecop International Inc. in San Diego. "Or they might have sped up and flown into a dive either of which could have been unrecoverable."
Momentum behind the Pitot theory is growing. Airbus, after all, recommended nearly two years ago that airlines replace Pitot tubes like those aboard 447 with an improved model less prone to icing. While aviation authorities in Europe and the U.S. never made the change mandatory, Air France said it had begun replacing the tubes in May and agreed to speed up the process following the crash at the demand of pilot unions.
Meanwhile, French and U.S. experts continue to listen for the elusive pinging of Flight 447's black boxes, sitting somewhere on the floor of the Atlantic. If they are never found, a theory may be the closest we ever get to the truth.