What happened
The incident occurred while the aircraft was being pushed back and the number four engine was in the process of starting. During this phase, the auxiliary power unit (APU), which supplies electrical power to the engine starter, tripped off the line. This loss of electrical power resulted in a hung start condition at approximately 25 percent N2 RPM.
Without electrical power reaching the starter, the engine could not be motored effectively. Consequently, pooled fuel continued to burn within the engine compartment. The crew was advised of the fire condition and subsequently discharged the number four engine fire extinguisher. An evacuation of the aircraft was ordered immediately following this action. The fire terminated shortly after the commencement of the evacuation.
The investigation
The primary mechanical failure identified was the tripping of the auxiliary power unit circuit breaker or relay during the engine start sequence. This event cut off the necessary electrical supply to the engine starter motor, preventing the engine from reaching self-sustaining RPMs and causing fuel accumulation and ignition within the nacelle.