Accident Description
While approaching a remote mountaintop site for landing, a helicopter was at an altitude of about 100 feet above ground level and traveling at approximately 40 miles per hour when the engine lost all power due to a contained engine failure. The helicopter subsequently executed a hard landing.
Investigation Findings
A teardown inspection of the engine revealed that the first-stage turbine wheel had failed due to thermal fatigue. The fatigue originated on the trailing edge outer diameter rim surface of the wheel. Additionally, both the first-stage and second-stage turbine wheel airfoils exhibited evidence of overtemperature operation during the flight.
Examination of the fuel control unit confirmed it was properly adjusted. No evidence of an internal engine oil fire was found. The investigation noted that fast engine shutdowns can contribute to overtemperature conditions if sufficient cool-down time at idle power is not allowed before the engine is shut down. Furthermore, the turbine outlet temperature gauge installed on the helicopter did not include an overtemperature warning light feature.
Conclusion
The engine failure resulted from a first-stage turbine wheel failure caused by thermal fatigue, with accompanying evidence of overtemperature operation. The lack of an overtemperature warning light on the turbine outlet temperature gauge may have reduced awareness of thermal stresses.