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It is likely that during the time the engineer was away from the APU, with the oil dolly still connected, the faulty spool valve in the oil delivery line permitted the oil to continue to flow into the APU, resulting in an overfilled condition with excess oil draining into the tailpipe. The APU then surged and automatically shut down. The surge was probably caused by some of the excess oil within the APU escaping through the bearings and entering into the combustion chamber, where it was ignited. This would have resulted in a rapid increase in the exhaust gas temperature. Flame from the combustion would then have "torched" through the turbine stage into the tailpipe, where it ignited the overflow oil that had drained into the tailpipe.

The pilot in command was initially provided inadequate information regarding the APU problem. He was aware that the APU had shut down, and was informed that there was an APU fire. However, this was not confirmed by an ECAM message due to the location of the fire in the tailpipe. This series of events was an unusual situation and did not fit with the pilot in command's expectations of an APU fire. Had he been properly informed of the circumstances of the fire, it is unlikely that he would have considered it necessary to inspect the APU. Consequently, he would have been able to more rapidly respond to ensure the safety of the passengers and crew.

The operator was unable to immediately follow its post-occurrence investigation procedures due to delayed and incomplete reporting of the circumstances of the occurrence.


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