What happened
Following landing, the crew taxied the aircraft to the gate and started the auxiliary power unit (APU) under normal conditions. While maneuvering near the gate, a warning light for a number 3 pneumatic duct failure illuminated on the flight deck. Shortly thereafter, a warning light for the center accessory compartment (CAC) door activated. Ground personnel observed smoke emanating from the fuselage outflow valves.
After the engines were shut down, smoke entered the cabin and the APU automatically tripped off. Fire department personnel arrived and found the exterior of the CAC door hot. Upon opening the cabin floor hatch, they observed flames approximately one to two feet high inside the compartment. The fire was extinguished with damage limited to the CAC.
The investigation
The investigation revealed that the primary mechanical issue was an improper or loose battery ground connection. Normal cable movement during the routine disconnection of the battery would loosen this connection further. Although the burned insulation blankets met specifications, they were coated with synthetic hydraulic fluid from a slat actuator leak and jet fuel.
The fire pattern suggested that fuel had seeped along a bulkhead fastener line. The battery ground connection had been last installed in November 1987, and the battery was last charged two weeks before the incident. A relevant service bulletin regarding the battery ground connection had not been complied with. Additionally, inoperative CAC drain valves allowed fuel to pool in the CAC bilge. Notably, no CAC fire warning or extinguisher system was installed on this aircraft.
Findings
The ignition source was identified as a loose battery ground connection. Contributing factors included fuel pooling in the CAC bilge due to inoperative drain valves and the presence of combustible fluids. The lack of a dedicated fire detection and suppression system for the compartment meant that the fire was not detected or extinguished by aircraft systems until it became visible externally.
Safety message
Compliance with service bulletins regarding electrical connections is critical to preventing loose grounds that can generate sparks. Regular maintenance of drain valves in accessory compartments prevents the accumulation of flammable fluids.