Underwater locator beacon

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An underwater locator beacon (length: 4 inches (10 cm)), with a ballpoint pen to provide scale
A ULB attached to a bracket on a cockpit voice recorder

An underwater locator beacon is a device that guides the search and rescue teams to a submerged aircraft by emitting a repeated electronic pulse.


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 ± 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. 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]

Power source and activation[edit]

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 via the water's presence completing an electrical circuit, and the beacon starts emitting its "pings"; the battery power should be sufficient for at least 30 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]

Maximum detection range[edit]

A 37.5 kHz (160.5 dB re 1 μPa) 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) under ideal conditions.[9][10]

See also[edit]


  1. ^ Knight, Matt (24 March 2014). "Navy prepares black box locator to search for missing Malaysia Airlines flight". WTKR. Retrieved 25 March 2014.
  2. ^ ELP-362D Emergency Locator Beacon User's Manual (PDF). Teledyne Benthos. June 2011.
  3. ^ http://www.rjeint.com/pdf/DK-100revB.pdf
  4. ^ Stone, Lawrence D.; Keller, Colleen; Kratzke, Thomas L.; Strumpfer, Johan (20 January 2011). Search Analysis for the Location of the AF447 Underwater Wreckage (PDF) (Report). Metron Scientific Solutions. p. 39.
  5. ^ Interim Report n°2 (PDF) (Report). Bureau d'Enquêtes et d'Analyses pour la Sécurité de l'Aviation Civile. December 2009. pp. 11, 71, 77.
  6. ^ Resolution A.886 (21). PT9 NINETY Beacon MSC.333(90) Novega July2014
  7. ^ "DK180 Low Frequency Acoustic Beacon" (PDF). Archived from the original on April 6, 2014.CS1 maint: Unfit url (link)
  8. ^ Safety recommendation A-91-49 (PDF) (Report). National Transportation Safety Board. 12 July 1991.
  9. ^ Kelland, Nigel C. (November 2009). "Deep-water Black Box Retrieval - November 2009, Volume 13, Number 09 - Archive". Hydro International. Retrieved 19 March 2014.
  10. ^ Barmak, R.; et al. (2017). "Underwater Locator Beacon signal propagation on tropical waters". Retrieved 27 June 2018.

External links[edit]