The UK Air Accidents Investigation Branch (AAIB) investigation into the fire on board a Boeing 787 operated by Ethiopian Airlines at London Heathrow Airport on July 12, 2013, discovered improper wiring of the lithium metal battery that powered the aircraft’s Honeywell 406AFN fixed emergency locator transmitter (ELT). According to an AAIB special bulletin published last week, the investigation concluded that the battery had been incorrectly wired to the ELT during the manufacturing process.
To date, the AAIB said, 28 of the same model ELTs have been found with the same faulty wiring, prompting the agency to develop five safety recommendations for the FAA. The AAIB wants the U.S. regulator to develop enhanced certification requirements for the use of lithium metal batteries in aviation. It also advocates a new requirement that electrical performance and design-abuse certification tests for lithium metal batteries are conducted while these units are installed in parent equipment (such as an ELT) to view the battery’s thermal performance more safely.
Additional recommendations include testing to create the worst possible thermal-runaway cases before the units are installed anywhere and that the agency also require equipment manufacturers using lithium metal batteries to demonstrate they can mitigate all the hazardous effects of a thermal runaway or explosion.
The AAIB asked the FAA to investigate whether the technical standard order is the most effective means to certify batteries when their unknown characteristics can be duplicated only once they are installed inside another device.
Honeywell told the AAIB investigators that it is not aware of any previous in-service thermal events involving the ELT battery. Following a February 2013 incident in which a faulty ELT had been returned to the manufacturer with a discharged battery, Honeywell discovered wires trapped under the cover-plate, cuts in the gasket and insulation damage exposing the positive conductor. This prompted it to revise the assembly process for new ELTs but no inspections or modifications were recommended to operators of existing equipment or to the manufacturers of aircraft using the ELT.