What happened
On 22 August 2013, a Boeing 747-436, registration G-BNLV, was parked at a stand in London Heathrow Airport. The aircraft was engaged in a commercial passenger operation with 257 passengers and 17 crew members on board. After the Auxiliary Power Unit (APU) had been running for approximately five minutes, the flight crew received a fire warning on the Engine Indication and Crew Alerting System (EICAS).
Following established procedures, the crew executed the APU fire checklist, which included shutting down the unit and discharging the fire extinguisher bottle into the APUS bay. Due to the nature of the alert, the crew initiated a precautionary rapid disembarkation, and all passengers exited the aircraft via the terminal jetty. While the Airport Rescue and Fire Fighting Service attended the scene, no active flames were visible at the rear of the aircraft, though water was sprayed into the APU exhaust as a precaution.
The investigation
An investigation by the AAIB revealed that the APU starter motor had experienced a catastrophic failure, resulting in its gearbox mounting flange shearing off. This mechanical breakdown left the starter motor suspended only by its feeder cables. The failure caused the release of hot oil, which subsequently ignited and led to the fire within the APU bay.
Upon inspecting the APU bay doors, investigators found internal components of the starter motor, such as the sprag clutch, had separated and were scattered near the doors. The APU itself showed signs of damage, including charred instrument lines, blistered paint, and damaged oil filter cluster mounts. While the start contactor was examined, it was found to be fully functional and was not the source of the failure.
Findings
- The primary cause of the fire was the catastrophic failure of the APU starter motor.
- The failure of the starter motor's shear neck was consistent with a seizure of the sprag clutch.
- The release of hot oil during the mechanical failure provided the fuel for the fire.
- The operator determined that the starter motor failed to disengage following the start cycle, leading to the destruction of the clutch housing and flange mount.
Safety action
Although the start contactor was not responsible for this specific incident, the operator noted it had been linked to previous starter motor failures. As a preventative measure, the operator decided to implement Boeing Start Contactor Service Bulletins across its fleet to replace existing contactors with a new design.