An underwater locator beacon (ULB), underwater locating device (ULD), or underwater acoustic beacon is a device that emits an acoustic pulse intended to guide searchers equipped with a suitable receiver to the location of the beacon underwater.
An underwater locator beacon is 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. These ULBs are triggered by water immersion; most emit an ultrasonic 10ms pulse once per second at 37.5 kHz ± 1 kHz. [1] [2] [3]
Research by the French Bureau d'Enquêtes et d'Analyses pour la Sécurité de l'Aviation Civile (BEA) has shown that it has had a 90% survival rate spanning 27 air accidents over the sea. [4] The ULBs fitted in Air France Flight 447, which crashed on 1 June 2009, were certified to transmit at 37.5 kHz for a minimum of 30 days at a temperature of 4 °C. After investigating the crash, the BEA recommended that FDR ULBs' transmission period be increased to 90 days and for "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). [5]
On ships built on or after 1 July 2014, underwater locating devices must ensure a regulatory transmission time of at least 90 days. [6]
A beacon is typically supplied with electrical power by a lithium battery, which needs to be replaced after several years. Once the beacon becomes immersed into water, a built-in "water switch" activates it via the water's presence completing an electrical circuit, and the beacon starts emitting its "pings"; the battery power should be sufficient for 30 to 40 days after the activation. The minimum battery voltage is 2.97 volts and the maximum is 3.5 volts. [7]
The National Transportation Safety Board reported a case in 1988, involving an Aerospatiale AS-355F-1 helicopter, 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. [8]
A 37.5 kHz (160.5 dB re 1 μPa) pinger can be detectable from the surface from a distance of 1–2 kilometres (0.62–1.24 mi) in normal conditions and 4–5 kilometres (2.5–3.1 mi) under ideal conditions. [9] [10]
Sonar is a technique that uses sound propagation to navigate, measure distances (ranging), communicate with or detect objects on or under the surface of the water, such as other vessels.
Echo sounding or depth sounding is the use of sonar for ranging, normally to determine the depth of water (bathymetry). It involves transmitting acoustic waves into water and recording the time interval between emission and return of a pulse; the resulting time of flight, along with knowledge of the speed of sound in water, allows determining the distance between sonar and target. This information is then typically used for navigation purposes or in order to obtain depths for charting purposes.
The Bureau of Enquiry and Analysis for Civil Aviation Safety is an agency of the French government, responsible for investigating aviation accidents and incidents and making safety recommendations based on what is learned from those investigations.
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Le Bourget is a commune in the northeastern suburbs of Paris, France. It is located 10.6 km from the center of Paris.
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The disappearance of Malaysia Airlines Flight 370, a scheduled international passenger flight from Kuala Lumpur International Airport to Beijing Capital International Airport on 8 March 2014, prompted a large, multinational search in Asia and the southern Indian Ocean that became the most expensive search in aviation history. Analysis of communications between the aircraft and Inmarsat by multiple agencies has concluded that the flight ended in the southern Indian Ocean.
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