Youneverseethem，butthey'rewithyoueverytimeyoufly．Theyrecordwhereyou'regoing，howfastyou'retravelingandwhethereverythingonyourairplaneisfunctioningnormally．Theirabilitytowithstand（经受）almostanydisastermak
You never see them，but they're with you every time you fly．They record where you're going，how fast you're traveling and whether everything on your airplane is functioning normally．Their ability to withstand （经受）almost any disaster makes them seem like something out of a comic book．They're known as the black box．
When planes fall from the sky，as a Yemeni airliner did on its way to the Comoros Islands in the Indian Ocean June 30，2009，the black box is the best bet for identifying what went wrong．So when a Frenchsubmarine（潜水艇）detected the device's homingsignal five days later，the discovery marked a huge step toward detennining the cause of a tragedy in which 152passengers 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 model for a black box，which became a requirement on all US commercial flights by 1960．Early models often failed to withstand crashes，however，so in 1965the device was completely redesigned and moved to the rear of the plane一the area least subject to impact--from its original position in the landing wells（起落架舱）．That same year，the Federal Aviation Authority required that the boxes，which were never actually black，be painted orange or yellow to aidvisibility．
Modem 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 （隔绝的）case and surrounded by quarter-inch-thick panels of stainless steel，the boxes can withstand massive force and temperatures up to 2000°F．When submerged，they're alsoable to emit signals from depths of 20，000ft．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 tu