If you wish to contribute or participate in the discussions about articles you are invited to join SKYbrary as a registered user


Underwater locating device (ULD)

From SKYbrary Wiki

Article Information
Category: General General
Content source: SKYbrary About SKYbrary
Content control: SKYbrary About SKYbrary


An underwater locating device (ULD) is an aircraft installation which is designed to survive the impact of a crash. The device may be attached to the cockpit voice recorder (CVR), the flight data recorder (FDR) as well as to the aircraft fuselage. The ULD are devices that emit acoustic pulses permitting authorities to locate aircraft wreckage when an accident happens in oceanic areas. ULDs were initially only attached to CVRs and FDRs (37.5 kHz ULDs) and now an additional ULD can be attached to the airframe (8.8 KHz ULDs).

New regulatory requirements

Updated European aviation safety regulations on air operations require that the transmission time of the ULD, 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) ULD. Low-frequency ULDs are required to be compliant with ETSO-C200 or equivalent and they should not be installed in wings or empennage.

Low-frequency ULDs

Low-frequency ULDs 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 ULDs 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 ULD mod itself contains the ULD DK180 Beacon, a mounting kit and an adapter plate.

ULD maintenance

An aircraft maintenance programme is required to ensure that procedures for testing the ULD, conducted concurrently with battery replacement, provide for functionally testing the ULDs prior to replacing the old battery. This ensures that the ULD is still operating properly. The maintenance programme should address the periodic maintenance of the ULD, such as the periodic checking of the device operation in accordance with the manufacturer’s requirements, any life limits on the battery of the ULD and the cleaning of the switch contacts. When installing the ULD 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.

Related Articles

Further Reading