An underwater locator beacon (ULB), also called an underwater locating device (ULD) or underwater acoustic beacon is a device fitted to aviation flight recorders such as the cockpit voice recorder (CVR), the flight data recorder (FDR) as well as to the aircraft fuselage. Once activated by immersion in water, a ULB is designed to emit an ultrasonic pulse at 37.5 kHz every second for a duration of at least 30 days. ULBs attached to the airframe transmit at 8.8kHz and are called low frequency ULBs.
Note: The abbreviation ULD is also used for Unit Load Devices (ULD), which are containers for baggage and cargo carried in the holds of aircraft.
The devices are designed not only to survive accidents, but to function correctly after impact. Research carried out for the Bureau d'Enquêtes et d'Analyses (BEA) and published in 2011 showed that they have had a 90% survival rate spanning 27 air accidents over the sea.
Figure 1: Underwater Locator Beacon
New regulatory requirements
Updated European aviation safety regulations on air operations require that the transmission time of the ULB, attached to the flight recorders, be extended from 30 days to 90 days by 1 January 2020 at the latest. The same rules also require that by 1 January 2019, most large aeroplanes operated over routes that go farther than 180 NM from a shore are equipped with an additional airframe low frequency (8.8 KHz) ULB. Low-frequency ULBs are required to be compliant with ETSO-C200 or equivalent and they should not be installed in wings or empennage.
Low-frequency ULBs have a very-long detection range and thereby provide an effective help to reduce the time and cost of locating the wreckage. They transmit an 8.8 kHz acoustic signal (pinger) for a minimum of 90 days and the low frequency ensures an increased detection range (four times greater) of 13-22 km (7-12 NM) over the standard ULBs as installed on the FDRs and CVRs. The maximal operational depth is 6,000 m (20,000 ft) and they are activated upon immersion in both salt and fresh water. The battery is a single-cell type with a six-year life. The ULB mod itself contains the ULB DK180 Beacon, a mounting kit and an adapter plate.
An aircraft maintenance programme is required to ensure that procedures for testing the ULB, conducted concurrently with battery replacement, provide for functionally testing the ULDs prior to replacing the old battery. This ensures that the ULB is still operating properly. The maintenance programme should address the periodic maintenance of the ULB, such as the periodic checking of the device operation in accordance with the manufacturer’s requirements, any life limits on the battery of the ULB and the cleaning of the switch contacts. When installing the ULB on the flight recorder it is important to ensure that the switch contacts are located in a manner that is not likely to encourage the build-up of debris that will cause the contacts to short inadvertently. The contacts should either be vertical or facing down.