What happened
On October 29, 1996, a Douglas DC-9-87, registration OH-LMC, was preparing for a scheduled commercial flight from Helsinki to Zurich. During engine start-up, the crew noted that the generator for the left engine was not producing electricity. Following standard procedures and consulting with maintenance personnel, the crew determined the flight could proceed using the APU generator to power the left electrical bus.
During the takeoff roll on a wet runway, the aircraft accelerated rapidly. As the aircraft approached V1 speed, the captain noticed a fluctuation in the left engine's EPR reading, followed by the engine instruments dropping to zero. The left engine failed, causing the aircraft to jerk. The captain immediately applied maximum braking and reverse thrust, successfully decelerating the aircraft on the runway. There were no injuries to the 17 passengers or the 5 crew members, and no damage was sustained by the aircraft.
The investigation
The investigation focused on the cause of the engine shutdown and the failure of the shaft within the engine's auxiliary gearbox. Investigators examined the fracture surface of the broken shaft at the State Technical Research Centre, which revealed a fatigue fracture originating from the internal surface of the shaft.
Furthermore, the investigation scrutinized the maintenance history and the flow of technical information regarding previous Service Bulletins and Engineering Orders (EO). Investigators looked into why the specific shaft in the left engine had not undergone the required inspections mandated by previous technical directives.
Findings
- The engine failure was caused by the fracture of the auxiliary gearbox shaft, which led to a loss of power transmission to the engine's fuel pump and power control unit.
- The fracture was a fatigue failure caused by intergranular attack, likely originating from a manufacturing defect during the heat treatment process.
- A significant breakdown in the communication of maintenance instructions occurred. While the manufacturer had issued warnings regarding potentially defective shafts, the specific Engineering Order intended to trigger the inspection was not distributed to the correct planning personnel responsible for scheduling the work.
- The inspection instructions themselves were inconsistent; an early Engineering Order contained an erroneous diagram that pointed to the wrong part of the shaft for inspection.
- The maintenance tracking system for engine shop work was insufficient, lacking a clear deadline for the completion of these specific inspections and failing to ensure that all relevant personnel were notified of the required tasks.