A fire aboard a Boeing 787 Dreamliner at London’s Heathrow airport in July 2013 was caused by a short-circuit inside the battery of the aircraft’s emergency locator transmitter, an investigation report revealed.
According to the UK Air Accidents Investigation Branch (AAIB), the wires of the emergency beacon's lithium-metal battery were crossed and trapped under the battery cover – a latent fault also identified by the device’s manufacturers as the most likely.
The ensuing short-circuit led to an ‘uncontrolled release of stored energy from the lithium-metal battery’ causing ‘extensive heat damage in the upper portion of the aircraft’s rear fuselage’, the report said. As there are no other aircraft systems located in this area, the investigators were able to conclude the cause of the fire must have been within the ELT.
The aircraft, operated by Ethiopian Airlines, caught fire while parked on a remote stand with nobody aboard.
The AAIB further stated that ‘neither the cell-level nor battery-level safety features prevented this single-cell failure, which propagated to adjacent cells, resulting in a cascading thermal runaway, rupture of the cells and consequent release of smoke, fire and flammable electrolyte’.
The trapped battery wires subsequently provided a path for the flames and fumes to escape from the ELT into the surrounding structures, damaging the thermo-acoustic insulation in the immediate vicinity of the ELT.
The fire was further fuelled by the decomposing resin in the composite material. As a result, the fire continued to propagate even after the energy from the battery itself was exhausted.
The AAIB made 14 safety recommendations based on the investigation results including redesign of the ELT reflecting the findings.