On 1 June 2009, Air France Flight 447, an Airbus A330-200, crashed into the Atlantic Ocean, killing all 216 passengers and 12 crew members. No one knows why the plane fell out of the sky, because no one has ever found its black box.
The plane plunged so deep that the black box's sonar beacon could not be heard, and by the time the French navy had dispatched a submarine to the area, the beacon's battery had evidently died. Crash analysts were thus reduced to poring over information the airliner had transmitted before going silent, information too sparse to determine what had happened, let alone how to prevent it from happening on some other airliner.
For half a century, every commercial airplane in the world has been equipped with one of these rugged, reinforced, waterproof boxes, which each house a flight data recorder and a cockpit voice recorder. For hundreds of crashes, they have given investigators the often heartbreaking details of the plane's demise: the pilot's frantic last words, his second-by-second struggles to keep the plane airborne, and the readings of the gauges and sensors that reveal such key parameters as the airspeed, altitude and the state of the plane's engines and flight-control surfaces. Such information has enabled analysts to infer the causes of most crashes and, often, to come up with preventive measures that have saved thousands of lives.