Incident
During taxi, the flight crew received the APU fire bell. The captain elected to evacuate the aircraft but did not inform the cabin crew or passengers of the nature of the problem.
Evacuation
Several passengers used the overwing exits located on the same side as the fire and were injured while jumping off the wing. One passenger noted that the wing flaps were not lowered during the evacuation. Four passengers sustained injuries during the emergency evacuation.
Post-Accident Examination
Examination of the APU after the accident revealed that the fire started because of an attempted start of the defective unit. The cockpit voice recorder tape was erased in the cockpit following the accident, preventing review of the flight crew's activities.
Maintenance Actions
Prior to the flight, NWA maintenance personnel attempted to render the APU inoperable from the cockpit by opening circuit breakers. However, the wrong circuit breaker was pulled, leaving the APU operable. The APU had been placarded inoperative before the flight, and the crew had been advised in writing.