Introduction
A helicopter being ferried to a new owner/operator experienced a loss of main rotor drive during cruise flight. The pilot, who had accumulated 1,351 flight hours, executed an autorotation landing in rolling rough and uneven terrain. The subsequent examination of the wreckage revealed damage to the main rotor blades, tailboom separation, and evidence of engine overspeed. The drive belt system was found damaged with one belt missing and the other displaced.
Accident Narrative
According to the pilot, the helicopter was in level cruise flight at an altitude between 700 and 900 feet above ground level, with an indicated airspeed of 70 to 75 knots. The pilot heard a loud "thump" from behind the cabin, followed immediately by the activation of the clutch light and a change in engine and rotor noise. Within a few seconds, a second "thump" occurred, after which engine RPM began to increase while rotor RPM decreased. The pilot initiated a 180-degree turn to align the helicopter into the wind while maintaining 70 knots, then entered an autorotation. The helicopter touched down in rolling rough and uneven terrain and came to rest upright.
Post-Accident Examination
FAA inspectors examined the wreckage at the accident site. Both main rotor blades were damaged, and the tailboom was severed from the helicopter and broken into several pieces, consistent with the main rotor contacting the tailboom during landing. The engine showed evidence of an overspeed greater than approximately 110%. One of the two drive belts was missing; the remaining belt was off the upper sheave and exhibited considerable damage. The wreckage was transported to the Robinson Helicopter Company facility in Torrance, California, for a detailed examination. No discrepancies were found in the assembly, installation, or alignment of the engine, clutch assembly, or actuator assembly, apart from impact-related damage. No evidence of pre-impact damage to the flight and powerplant controls was discovered. The failed belt was found separated between the vees, with only a 5-inch section still intact.
Conclusion
The investigation determined that the drive belt failure and subsequent engine overspeed occurred, but the source provides no stated probable cause.