Accident Overview
During a night cruise flight, the helicopter experienced a complete loss of engine power. The pilot reported that the engine failed, emergency annunciator lights illuminated, and a yaw was felt due to loss of full torque. An autorotation was initiated to a bean field. The pilot stated that, because of very dark conditions, depth perception was difficult, resulting in a premature flare. The aircraft ran out of lift at approximately 10 to 12 feet above the ground and impacted the terrain.
Damage and Examination
The helicopter was substantially damaged. The main rotor blades sheared off the front right upper portion of the cabin and the tailboom. At the accident site, the engine's N1 rotor system would not rotate. Disassembly revealed that the first stage turbine wheel had all blades broken off 0.2 to 0.4 inch above the rim at the leading edge and 0.1 to 0.15 inch above the rim at the trailing edge.
Metallurgical Findings
Scanning electron microscope examination of the fractures showed no fatigue progression. The fracture surface was oxidized, and macroscopic features were interdendritic, indicating overload separation. Metallographic sectioning of one blade revealed loss of metal due to hot corrosion. Distinct spherical particles were observed ahead of the corrosion front, consistent with sulfidation, and the particles' chemistry was rich in sulfur. The second stage turbine wheel also exhibited features consistent with hot corrosion damage. Further examination confirmed that the microstructure, hardness, and chemistry met engineering drawing requirements.
Conclusion
The forced landing resulted from a complete loss of engine power. The pilot's flare timing was affected by night conditions, but the underlying engine failure was associated with hot corrosion and overload separation of turbine blades.