Weather and Flight Conditions
Prior to the flight, the pilot obtained a weather forecast that indicated winds from the southwest at 30 mph with gusts up to 40 mph. The helicopter departed and was in cruise flight at an altitude of 9,000 feet (700 feet above ground level). At that time, it experienced a 20-25 knot headwind and light turbulence.
Severe Turbulence Encounter
While in cruise, the flight encountered severe turbulence. The pilot stated that the helicopter went up and down violently for a period of 2-3 seconds. During this event, the tailrotor drive shaft was severed by the main rotor blades. Subsequently, the helicopter experienced a right yaw that could not be corrected by full application of left pedal. The engine and rotor rpm increased beyond their upper limits. However, the pilot was able to bring both the engine and rotor rpm back within limits and regain control of the helicopter.
Autorotation Landing
After regaining control, the pilot performed an autorotation to sloping terrain, which was completed without further incident.
Witness Observation
A witness who was fishing on a lake approximately 4 miles from the accident site reported observing water spouts between 2 and 8 feet tall and high swirling winds near the time of the accident.