What happened
During the approach descent phase of a scheduled air carrier flight, the crew experienced an APU fire warning when attempting to start the auxiliary power unit. The event occurred while the aircraft was in the process of descending toward its destination airport.
The investigation
Subsequent examination of the aircraft revealed that the EGT (exhaust gas temperature) probe wiring harness had become pinched between the fire seal and the lower APU shroud. This physical interference compromised the integrity of the fire seal, which is designed to prevent fuel from entering sensitive areas.
Findings
The compromise of the fire seal allowed pooled fuel to enter the lower APU area. Once in this location, the fuel caused a localized fire that resulted in a delayed ignition event. The pinched wiring harness was identified as the primary mechanical failure that initiated this chain of events. The incident highlights the critical importance of proper installation and clearance of wiring harnesses near fire seals and shrouds to prevent fuel ingress and subsequent fire hazards.
Safety message
This incident underscores the need for rigorous inspection and maintenance procedures regarding EGT probe wiring harness installations. Ensuring that these components are not pinched or improperly seated against fire seals is essential to maintaining the integrity of the APU fire containment system and preventing fuel-related fire hazards.