EC 225 rescue hampered by wristwatch locators

 - July 28, 2009, 6:22 AM

The 27-minute search for the life-rafts occupied by the survivors of a Bond-operated Eurocopter EC 225 helicopter that ditched into the North Sea on February 18, less than 1,500 feet from an offshore oil platform, prompted the UK Air Accidents Investigation Branch (AAIB) to recommend improved training in the use of personal locator beacons (PLBs) and emergency locator transmitters (ELTs). The search was hampered by darkness and fog, the AAIB noted in its report of the accident, which was released in June.

But also in the report, the investigative body said it was probable that the operation of PLBs/ELTs on board the helicopter were inhibited by the use of wristwatch-type PLBs that were worn by passengers but which were not part of the helicopter’s certified equipment. All 16 passengers and two crewmembers were rescued; three of the passengers sustained minor injuries.

Four hand-portable locators, which were of the same design (TechTest 500-12Y), were carried on the helicopter–one PLB in each crew- member’s life jacket and one ELT in each liferaft. Once switched on, these units transmit on 406 MHz to the COSPAS/SARSAT satellite distress alerting system. In addition, the TechTest 500-12Y ELT/PLBs can broadcast continuous homing signals on 121.5 MHz and 243 MHz. The signal-detection range is 40 nm. For maximum effectiveness, the antenna of the ELT must be vertical and the upper section must be extended. However, it appeared that the flight crew was not aware that the upper section was telescopic.

These ELTs work in a coordinated fashion. When several are in close proximity, all but one stop emitting. This eliminates the difficulty rescuers experience when homing to multiple ELTs that are not far apart. If the “master” ELT ceases to transmit, another one takes over. This feature also preserves the battery life in the ELTs that are in standby mode.

The ELTs also have a voice broadcast feature (on 121.5 MHz and 243.0 Mhz), operated by a “press-to-transmit” button, which is meant to be used during the final rescue phase. This function overrides the suppression of the signal by another ELT. After they had been rescued, the EC 225’s pilots reported that they attempted voice transmission when they heard a rescue helicopter. However, no rescue vessel or aircraft received any voice signals.

The AAIB reported that the flight’s passengers wore wristwatch personal locator beacons, which they were given before boarding the helicopter and are worn when they work on the platform. These devices activate when they come in contact with salt water. Tests conducted after the accident showed that just one such wristwatch PLB can suppress both the 121.5 MHz and 243 MHz signals from an operating TechTest 500-12Y ELT/PLB when within 48 meters (157 feet), such as when wristwatch-PLB-wearing survivors are in liferaft with an ELT or a pilot with a PLB. Moreover, the signal from a wristwatch beacon is, in practice, much weaker than that of a certified ELT/PLB.