What happened
On 12 July 2013, a Boeing 787-8, registration ET-AOP, was at London Heathrow Airport when a fire broke out involving its Emergency Locator Transmitter (ELT). The incident was classified as a serious incident. The fire originated within the ELT battery unit and subsequently caused damage to the aircraft structure. The crew and cabin crew engaged in firefighting efforts to manage the situation.
The investigation
AAIB investigators conducted an extensive examination of the aircraft and the ELT components. The investigation included a detailed structural analysis of the aircraft's airframe, specifically looking at the impact of heat on the skins, stringers, and frames. The ELT battery underwent rigorous testing, including computed tomography (CT) scans and a full forensic teardown.
To understand the failure mechanism, investigators performed various laboratory tests, including single-cell tests, battery-level external short-circuit tests, and battery drain tests. Calorimeter tests were also utilized to measure the specific heat capacity and thermal energy release during battery discharge. The investigation also examined the functionality of the Positive Temperature Coefficient (PTC) protective device within the battery pack.
Findings
- The fire was triggered by an external short circuit within the ELT battery.
- The investigation identified the presence of pinched wires as a contributing factor to the electrical fault.
- The discharge current during the fault was insufficient to trip the PTC protective device, preventing the circuit from being interrupted.
- The investigation explored various failure scenarios, including the possibility of the PTC being bypassed or shorted, and the impact of cell voltage reversal.