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Underwater locator beacon
An underwater locator beacon (ULB) or underwater acoustic beacon, is a device fitted to aviation flight recorders such as the cockpit voice recorder (CVR) and flight data recorder (FDR). ULBs are also sometimes required to be attached directly to an aircraft fuselage. ULBs are triggered by water immersion; most emit an ultrasonic 10ms pulse once per second at 37.5 kHz (unicode:±) 1kHz.〔http://www.rjeint.com/pdf/DK-100revB.pdf〕
The device is designed not only to survive accidents, but to function correctly after impact. Research by the French ''Bureau d'Enquêtes et d'Analyses pour la Sécurité de l'Aviation Civile'' (BEA) has shown that it has had an 90% survival rate spanning 27 air accidents over the sea. The ULBs fitted in Air France Flight 447, which crashed on 1 June 2009, were certified to transmit on 37.5 kHz for minimum 30 days at 4°C temperature. Investigating the crash, the BEA recommended that FDR ULBs' transmission period be increased to 90 days and that "airplanes performing public transport flights over maritime areas to be equipped with an additional ULB capable of transmitting on a frequency (for example between 8.5 kHz and 9.5 kHz) and for a duration adapted to the pre-localisation of wreckage" (i.e. with increased range).
==Power source and activation==
A beacon is typically supplied with electrical power by a lithium battery, which needs to be replaced after several (DK120/six) years. Once the beacon becomes immersed into water, a built-in "water switch" activates it (water closing an electric circuit), and the beacon starts emitting its "pings"; the battery power should be sufficient for at least 30 days after the activation.
The National Transportation Safety Board has recorded a case when the water switch had accidentally activated during the aircraft's normal operation; as a result, the battery power had been exhausted by the time the accident happened, and the beacon was not emitting the acoustic signal when it needed to do so.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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