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You never see him, but they're with you every time you fly．They record where you are going,how fast you're traveling and whether everything on your airplane is functioning normally．Their ability to withstand(承受)almost any disaster makes them seem like something out of a magic book．They're known as the black box．
When planes fall from the sky, as a Yemeni airliner did on its way to Comoros Islands in the India ocean June 30, 2009, the black box is the best bet for identifying what went wrong．So when a French submarine(潛水艇)detected the device's(設備)signal five days later, the discovery marked a huge step toward determining the cause of a tragedy in which 152 passengers were killed．
In 1958, Australian scientist David Warren developed a flight-memory recorder that would track basic information like altitude and direction．That was the first mode for a black box, which became a requirement on all U．S．commercial flights by 1960．Early models often failed to withstand crashes, however, so in 1965 the box was completely redesigned and moved to the back of the plane － the area least affected by impact － from its original position in the landing wells(起落架艙)．The same year, the Federal Aviation Authority required that the boxes, which were never actually black, be painted orange or yellow to be discovered more easily．
Modern airplanes have two black boxes：a voice recorder, which tracks pilots' conversations, and a flight-data recorder, which monitors fuel levels, engine noises and other operating functions that help investigators reconstruct the aircraft's final moments．Placed in an insulated(隔絕的)case and surrounded by a quarter-inch-thick panels of stainless steel, the boxes can withstand huge force and temperatures up to 2,000℉．When in deep water, they're also able to send signals from depths of 20,000 ft．Experts believe the boxes from Air France Flight 447, which crashed near Brazil on June 1,2009, are in water nearly that deep, but statistics say they're still likely to turn up．In the approximately 20 deep-sea crashes over the past 30 years, only one plane's black boxes were never recovered．
