Incident Overview
During cruise flight, the helicopter experienced a partial loss of engine power. The pilot perceived severe vibration and a decrease in rotor rpm. In response, he manipulated the flight controls to maintain a controlled descent and initiated an autorotation.
Precautionary Landing
While searching for a suitable landing area at approximately 200 feet above ground level, the pilot observed power lines in the helicopter's path. He maneuvered in an unsuccessful attempt to avoid the wires, and the helicopter subsequently landed hard in a dirt field.
Post-Accident Examination
A mechanic with inspection authorization (IA), who regularly serviced the helicopter, conducted an examination after the accident. He reported that a fan belt used in the integral engine cooling system had either broken or come loose. This belt wrapped around the pulley, causing the transmission to drag. The mechanic noted that the engine operated normally following the accident and that the fuel system was free of particulates and contaminants. He stated that he had performed the annual inspection approximately 14 flight hours prior to the accident and believed he had examined the belts at that time. He further noted that the helicopter was stored outside in a corrosive environment.