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 and flight data recorder. ULBs are also sometimes required to be attached directly to an aircraft fuselage. When triggered by water immersion, the ULB emits an ultrasonic pulse of 37.5 kHz at an interval of once per second.

The devices are designed not only to survive accidents, but to function correctly after impact. Research by the BEA showed that they have had an 90% survival rate spanning 27 air accidents over the sea.