One tiny technical failure heralded the impending disaster. But the measurement error was so inconspicuous that the pilots in the cockpit of the Airbus A330 probably hardly noticed it.
Air France flight 447 had been in the air for three hours and 40 minutes since taking off from Rio de Janeiro on the evening of May 31, 2009. Strong turbulence had been shaking the plane for half an hour, and all but the hardiest frequent flyers were awake.
Suddenly the gauge indicating the external temperature rose by several degrees, even though the plane was flying at an altitude of 11 kilometers (36,000 feet) and it hadn't got any warmer outside. The false reading was caused by thick ice crystals forming on the sensor on the outside of the plane. These crystals had the effect of insulating the detector. It now appears that this is when things started going disastrously wrong.
Flying through thunderclouds over the Atlantic, more and more ice was hurled at the aircraft. In the process, it knocked out other, far more important, sensors: the pencil-shaped airspeed gauges known as pitot tubes.
One alarm after another lit up the cockpit monitors. One after another, the autopilot, the automatic engine control system, and the flight computers shut themselves off. "It was like the plane was having a stroke," says Gérard Arnoux, the head of the French pilots union SPAF.
The final minutes of flight AF 447 had begun. Four minutes after the airspeed indicator failed, the plane plunged into the ocean, killing all 228 people on board.
Few airline crashes in recent years have subsequently unnerved passengers to quite the same extent. "How was it possible that an Airbus from such an apparently safe airline could simply disappear?" they wondered.
Passengers on the Rio-Paris route are still uneasy as they board their plane. After the accident, the flight number was changed to AF 445. Many frequent flyers have since opted for daytime flights across the Atlantic because pilots can recognize storm fronts more easily during the day.
Another large-scale search for the stricken plane's "black box" flight recorders is due to begin in the coming weeks. Once again some 2,000 square kilometers (800 square miles) of mountainous ocean floor will be swept, some of it by a submarine from from the northern German city of Kiel. "We shouldn't speculate about the causes of the accident until the search has been completed," says Jean-Paul Troadec, the director of the French air crash investigation agency BEA.
Other experts are less guarded in their comments. "We know pretty well why the accident happened," says union boss Arnoux.
'An Accident Like This Could Happen Again'
Over the course of several months of investigation, experts have gathered evidence that allows them to reconstruct with relative accuracy what happened on board during those last four minutes. It has also brought to light a safety flaw that affects all jet airplanes currently in service. "An accident like this could happen again at any time," Arnoux predicts.