Two years ago, an Air France Airbus 330 crashed in the Atlantic while en route from Brazil to Paris.
The reason for the crash was initially a mystery. Eventually, after piecing together the few tidbits of information they had, investigators concluded that the cause was a combination of mechanical failure, weather, and pilot error.
Now, however, after the recovery of the plane's "black boxes" from the bottom of the ocean this spring, the cause has become clear:
One of the plane's pilots, a 32-year old junior officer named Pierre-Cedric Bonin, badly misinterpreted what was happening during a brief failure of the plane's airspeed indicators and did exactly the wrong thing.
With the plane under control and flying normally, Bonin pulled back on the stick and began ascending when there was no apparent reason for him to ascend. This eventually put the plane into an aerodynamic "stall" that the pilots never realized that they were in--despite the plane's stall warning going off 75 times in the next few minutes as the plane plunged toward the ocean.
(An "aerodynamic stall" is not an engine stall. The plane's engines were functioning perfectly, as were the rest of the instruments and systems. An aerodynamic stall occurs when the plane's "angle of attack" relative to the air passing over the wings becomes too severe and the plane's wings suddenly lose lift. For more than two minutes, Air France Flight 447 plunged toward the ocean with its engines at full power and the plane's nose pointing up at about a 40-degree angle. The engines helped to slow the fall, but they could not keep the plane in the air.)
The pilots' confusion appears to have been aided by a couple of quirks in the Airbus design.
First, the two control sticks in the cockpit operate independently rather than in tandem, so it was not obvious to one pilot what the other pilot was doing. (On Boeings and most small planes, the control sticks move together, so if I'm pulling back on mine, yours will pull back also).
Second, when it temporarily lost its airspeed indicators, the plane switched to a different "mode" of operation in which, ironically, the pilots had much more control over the plane's control surfaces than they normally do. Normally, the Airbus's computers prevent the pilots from doing mystifying things like putting the plane into a stall. Under the "alternate" mode, however, the pilots have more manual control. Thus, the pilots may simply never have thought it possible that they could put the plane into a stall and thus merely concluded that the stall warning was faulty.
At almost any time during the plane's descent, had the Air France pilots realized what was happening, they could have pushed the stick forward and recovered from the stall. Alas, they did not begin to figure out what was happening until they were about 2,000 feet above the water, and by then it was too late.
Unfortunately, the obvious conclusion about what happened is that it could happen again. Not in exactly the same way, perhaps--pilots will now be on alert for this mistake during similar situations--but in another challenging situation under slightly different circumstances.
Popular Mechanics has published the full transcript of the pilots' conversation and confusion during the last few minutes of the flight. They have also interspersed the comments with details about what was happening at the time.
The account is riveting. And, obviously, terrifying.