What happened
On 12 July 2013, a parked and unoccupied B787-8, registration ET-AOP, sustained extensive heat damage to the upper portion of its rear fuselage while at London Heathrow Airport. The fire was localized to the area containing the aircraft's Emergency Locator Transmitter (ELT). There were no fatalities and no injuries as the aircraft was not in operation at the time of the event.
The investigation
Investigators examined the ELT, a Honeywell RESCU 4/06AFN unit, and found that the internal battery pack had undergone a high-energy thermal event. Detailed examination of the battery cells revealed that all five cells had been breached, with burnt material ejected from the unit.
Technical analysis identified that the battery wires had been improperly installed, resulting in them being crossed and pinched between the battery cover-plate and the ELT case. This physical arrangement caused the cover-plate to bulge and prevented a proper seal. Metallurgical testing confirmed that the positive wire had made metal-to-metal contact with the aluminium cover-plate. Because the ELT case is grounded to the aircraft's electrical network, this contact created a short-circuit condition.
Findings
- The fire originated from a thermal runaway within the lithium-metal battery pack.
- Improperly routed wires caused a short-circuit between the positive conductor and the grounded cover-plate.
- The pinched wires compromised the seal of the battery compartment, allowing flames and debris to escape into the aircraft structure.
- The investigation suggests the failure was likely triggered by the short-circuit in combination with the rapid depletion of a single cell, which then caused a cascading failure across the remaining cells.
- Existing certification standards (DO-227) were found to be insufficient, as they focus on cell-level testing rather than the thermal performance of a battery when installed within its parent equipment and aircraft structure.