What happened
At 0900 Atlantic daylight time, a Bell 407 helicopter, registration C-GOFL, was performing a flight from the Jacques Lake drilling site toward Postville, Newfoundland and Labrador. The aircraft was carrying an empty fuel tank on a 75-foot long line. While flying at 500 feet, the pilot observed an engine chip light and heard indications of engine failure. This was accompanied by FADEC failure and degradation alerts, followed by an audible engine-failure horn. As engine power output dropped, the pilot initiated an autorotation. At approximately 200 feet, the pilot released the long line and landed the aircraft in a bog. The pilot exited the helicopter without injury, and the aircraft sustained no damage.
The investigation
Maintenance personnel from the operator removed the engine, the power and accessory gearbox, and related hardware for examination at the operator's base in Goose Bay. Upon removing the gearbox cover, investigators discovered a fractured gear. Subsequent disassembly at the manufacturer's facility revealed that a section of the helical torquemeter gearshaft had separated and was loose inside the gearbox.
Technical analysis showed that the torquemeter gear exhibited significant tooth damage. Microscopic examination identified a fatigue crack originating on the driven surface of a tooth. Measurements indicated the gear had 0.004 inch of wear, which was at least twice the established in-service wear limit of 0.001 to 0.002 inch. The gearbox had accumulated 6076.9 total hours since new, and the specific gear had operated for 4576 hours since its last inspection, exceeding the 3500-hour MPI requirement by 1076 hours.
Findings
- The helical torquemeter gear failed due to an undetected fatigue crack.
- This failure caused the loss of engine power to the helicopter's transmission.
- The gear's in-service wear reached 0.004 inch, significantly exceeding the recommended limits.
- There is no requirement to track the specific time-in-service for individual gears within the gearbox, making it possible for components to exceed inspection intervals.
- Current visual and radius scribe inspection methods may be insufficient to detect cracks in the gear teeth.
- While Rolls-Royce has introduced redesigned gears to improve reliability, replacing the older parts remains a customer option rather than a mandate.
Safety action
- Rolls-Royce issued Commercial Engine Bulletin 72-6061 to advise customers of redesigned power gears to improve reliability.
- Rolls-Royce announced the development of a new visual inspection for the 2000-hour inspection section of the Operation and Maintenance Manual.
- Transport Canada approved a Service Difficulty Alert regarding this issue for publication.