What happened
During a commercial flight under instrument flight rules from Ottawa, Ontario, to Syracuse, New York, the crew of a Piper Navajo, registration C-GAFG, experienced a sudden, loud noise while cruising at 10,000 feet. The co-pilot observed the right propeller break away from the engine and move upward and to the right, though it did not strike the aircraft. The event resulted in a significant amount of oil spreading across the right wing. The crew executed emergency procedures and notified the Boston control centre. The aircraft performed a visual descent using only one engine and landed safely at Watertown airport with zero fatalities and no injuries.
The investigation
The investigation focused on the mechanical failure of the engine and the maintenance history of the aircraft. The engine had been in service for 1,247 hours since its last overhaul, which had occurred over 19 years prior. While the engine had relatively low usage in recent years, it had exceeded the 12-year time limit for overhauls recommended by the manufacturer's Service Instruction 1009AJ.
Engine teardown revealed several broken components, including a valve spring, an intake valve, and the propeller shaft-driven gear. While the valve spring failure was attributed to corrosion-related fatigue, the intake valve damage was consistent with an overspeed event caused by the loss of the propeller. Crucially, the propeller shaft-driven gear showed widespread fatigue cracking, ovalization of the rim, and separation of the hub. Investigators found that corrosion pitting was present on the gear teeth, which served as the initiation points for the cracks.
Findings
- The primary cause of the propeller separation was the failure of the propeller shaft-driven gear, driven by a combination of fatigue and increased play caused by corrosion pitting.
- The corrosion pitting resulted from a lack of effective corrosion prevention measures in the maintenance records.
- The engine had exceeded the manufacturer's recommended 12-year interval between overhauls.
- Neither the maintenance manager nor Transport Canada had accounted for the manufacturer's specific service instructions regarding time limits for engine overhauls when approving the on-condition maintenance program.