What happened
On 12 July 2013, a parked B787-8, registration ET-AOP, sustained extensive heat damage to the upper portion of its rear fuselage while at London Heathrow Airport. The fire, which occurred while the aircraft was unoccupied, resulted in significant damage to the aircraft structure near the location of the Emergency Locator Transmitter (ELT).
The investigation
Investigators examined the area of the damage and found that the fire originated within the ELT battery. A detailed teardown of the Honeywell RESCU 4/06AFN ELT revealed that the five-cell lithium-metal battery pack had undergone a severe thermal event. The internal battery cells had experienced thermal runaway, leading to the breach of all five cell cases and the ejection of burnt material.
Technical analysis of the unit uncovered that the battery wires had been improperly installed during the production assembly of the ELT. The wires were found to be crossed and pinched between the battery cover-plate and the ELT case. This physical obstruction prevented the cover-plate from sitting flush and caused the metal-to-metal contact of the positive wire against the aluminium cover-plate. Because the ELT case is grounded to the aircraft, this contact created a short-circuit condition.
Findings
- The primary cause of the fire was a short-circuit caused by improperly routed battery wires.
- The pinched wires created a path for current to flow to the aircraft ground, triggering an uncontrolled discharge.
- The failure of the battery's safety features allowed the initial cell failure to cascade into a full thermal runaway of all five cells.
- The pinched wires also compromised the seal of the cover-plate, providing a path for flames and hot debris to escape the ELT and ignite the surrounding aircraft insulation.
- Maintenance records indicated no recent work on the ELT, suggesting the wiring error occurred during the original manufacturing of the component.
Safety action
Following the incident, the FAA issued an Airworthiness Directive requiring the inspection or removal of similar ELTs on B787-8 aircraft. Additionally, the manufacturer modified the assembly instructions for new production units to ensure wires are routed safely to prevent future trapping under the cover-plate.