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Underwater locator beacon - Wikipedia, the free encyclopedia

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 pulse of 37.5 kHz at an interval of once per second.

The device is designed not only to survive accidents, but to function correctly after impact. Research 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 30 days. 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).

Maximum detection range
A 37.5 kHz pinger* can be detectable 1–2 kilometres (0.62–1.24 mi) from the surface in normal conditions and 4–5 kilometres (2.5–3.1 mi) in good conditions.



* lol - http://forums.pelicanparts.com/off-topic-discussions/802901-gigs-pings-my-two-pet-peeves.html