What happened
While taxiing for takeoff, the flight crew received notification from the tower regarding a fire on the right side of the aircraft. The incident occurred following an attempt to start a main engine using the auxiliary power unit (APU). The engine start sequence was aborted after the APU failed to provide sufficient bleed air pressure.
The investigation
A post-incident inspection of the APU revealed that a turbine exducer blade had become liberated. This failure resulted in an imbalance of the turbine wheel, which allowed smoke and flames to escape from the APU exhaust located at the right wing root.