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On April 3, investigators announced the discovery of the wreckage of Air France Flight 447, which crashed into the Atlantic Ocean on June 1, 2009 amid mysterious circumstances after taking off from Rio de Janeiro bound for Paris. The long-awaited find brings new hope that the causes of the Airbus 300 330 crash will be illuminated—if the black boxes can be found.

But just what does a black box do? While the National Transportation and Safety Board wouldn't comment for this story because it is not involved in the AF447 investigation, Popular Mechanics got to the bottom of the issue in a piece for our Hollywood Fact vs. Fiction series about the television show Lost, in 2008, when the show featured a recovered black box. James Cash, chief of the vehicle recorders division at the NTSB, told us that there are two major components to the black box: The cockpit voice recorder (CVR) that records dialogue and ambient noise in the cockpit, and the flight data recorder (FDR) that records parametric data such as altitude, airspeed and heading. Data from the virtually crash-proof boxes is recorded to solid-state drives—these allow the data recorders to track thousands of parameters and have increased how long the voice recorders can record audio.

The circumstances of a crash determine which part of the recorder is going to be more helpful. "If it was an airplane problem, the flight recorder would give you a good idea of how it crashed," Cash said. "If it's a crew mistake or some kind of procedural mistake, the flight recorder would look perfectly normal." In a situation where a plane runs into a mountain, the FDR would show that the aircraft was operating normally and then just stopped—at which point, the data from the CVR becomes incredibly important.

Honeywell manufactured Air France 447's two black boxes, which were both bolted in the tail section of the airplane, according to Bill Reavis, the director of media relations for Honeywell's Air Transport & Regional, Business and General Aviation divisions. (This isn't the only black-box setup. Some planes combine both recorders into a single unit, while others have two black boxes—one at the front of the plane and one in the back.) The plane's FDR could record several thousand parameters for up to 25 hours, while the CVR could record 2 hours of standard-quality and 30 minutes of high-quality audio. Both units were equipped with underwater locator beacons that transmitted at 37.5 kHz for over 30 days before the signal faded. The bright orange steel-armored boxes are designed to withstand fire and explosions and impacts up to 1500 g's (as a comparison, astronauts withstand a mere 3 g's on liftoff).

Immersion in seawater shouldn't hurt the units, either. Cash told us that one recorder (the manufacturer is not known) survived nine years at the bottom of the Mediterranean. "The water, in general, doesn't hurt them at all," Cash told us. "It's the air that hurts them once they've been wet. It starts the corrosion and rust process." What might be a problem, though, is pressure. The wreckage of AF447 has lain at 13,000 feet below the surface for nearly two years; the pressure at that depth is about 400 times our atmospheric pressure. Honeywell's FDRs are designed to withstand depths of 20,000 feet for 30 days. "We won't know [if pressure will be a problem] until we see the units," Reavis says. "Until then, we can't knowledgeably tell you anything. We'd only be guessing, and I really don't want to do that."

If the black boxes are found, they will be transferred to a lab in a water-filled cooler so the data can be retrieved and copied. The boxes will likely go back to France's Bureau d'Enquêtes et d'Analyses (BEA) for analysis. "The French investigatory authorities will be in charge of that," Reavis says. "We stand ready to help if needed, but the French will have the box."

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