What happened
At approximately 0829 Pacific daylight time, a Bell 206L helicopter, registration C-FVIX, was performing a cruise flight at roughly 700 feet above sea level near the Sandspit area of the Queen Charlotte Islands. During the flight, the pilot heard an unusual noise followed by a sudden loss of engine power. While the engine remained running, the turbine outlet temperature (TOT) rose rapidly, exceeding the visible range on the instrument gauge.
As the pilot attempted to manage the power loss, the main rotor began to droop. After notifying the two passengers of an impending engine failure, the pilot initiated an autorotation. During the flared landing on a logging road near Tasu Creek, the pilot confirmed the loss of engine power as the low rotor horn sounded. The landing resulted in no injuries and no damage to the airframe.
The investigation
Investigators examined the Rolls-Royce Allison 2-50-C20R engine and found several structural issues. A visual inspection of the engine revealed a four-inch crack in the lower weld joint of the fuel nozzle mount and a crack in the left-hand exhaust duct, which also showed internal impact damage. While fuel and oil samples showed no obvious anomalies, metal particles were found in the upper and freewheel chip detectors.
Technical records revealed that approximately 45 engine hours prior to the event, the fuel-control unit had been adjusted following a hot start. Additionally, the engine was equipped with an incorrect oil pressure and temperature gauge from a different model, and recent replacements of the TOT harness and gauge had been noted due to inaccurate readings.
An engine teardown by the TSB Engineering Branch determined that the failure was triggered by the fracture of a single blade on the second-stage turbine wheel. Examination of the first-stage turbine wheel also revealed multiple cracks in the blade rim.
Findings
- The primary cause of the engine failure was thermally induced fatigue cracking in a second-stage turbine wheel blade. This began as low-cycle fatigue in the blade platform fillet area and progressed into high-cycle fatigue, eventually leading to an overstress rupture.
- The failure was likely driven by temperature transients or hot starts, which can cause the rim to expand against a cooler hub, creating compressive stresses that initiate cracking.
- The engine's technical records indicated a history of hot starts and the use of an incorrect temperature gauge, which may have prevented accurate monitoring of engine thermal limits.