Accident Overview
While in cruise flight at approximately 1,700 feet above ground level (agl), the flight instructor heard a loud "bang" from the back of the helicopter. The alternator warning light illuminated, and he initiated the alternator emergency procedure. He noticed that the engine's rpm was fluctuating, with the main rotor rpm indicating high and the engine rpm low. The instructor entered an autorotation and landed on uneven terrain. During the landing, the main rotor struck the tail boom, and the helicopter rolled over onto its left side.
Examination Findings
A Safety Board investigator examined the helicopter. The two main drive V-belts were located in the drive compartment: one was positioned on the drive pulleys in the middle slot (between the normal operating positions for the forward and aft belts), and the other was located completely off of the drive pulleys. Both belts exhibited damage on the inside, including scratch marks, material and color transfers, and gouges consistent with damage by a rubber-based foreign object. Rubber and color transfer in a transverse orientation inconsistent with the drive belts was noted on the upper and lower pulleys. The alternator belt was missing and not recovered. The clutch assembly was located in the up position with the actuator fully extended, and the safety stop activated. The oil cooler sustained damage to the outer fins. Rubber transfer was located on the oil cooler, starter, oil lines, and exhaust tube on a plane with the alternator belt's rotation. The alternator belt is located directly below the lower main drive pulley. No other preimpact anomalies with either the airframe or engine were discovered.
Investigation Outcome
As a result of this investigation, Robinson is changing the specification for the alternator drive belt.