Accident Sequence
During a low-altitude maneuvering operation to herd cattle, the helicopter experienced a loss of tail rotor drive continuity. The pilot reported a loss of directional control and executed an autorotation into a confined area. During the descent and landing, the main rotor blade struck a tree, causing the helicopter to roll onto its right side.
Investigation Findings
An investigation revealed excessive wear on the tail rotor gear and drive assembly, specifically the transmission output quill gear. Metallurgical examination determined that the gear's shaft, which served as the inner raceway of an associated cylindrical bearing, had not been carburized as required by the design manufacturer. Rotation of the bearing rollers over the relatively soft surface of the shaft had worn a groove in the shaft. Subsequently, the associated tail rotor drive assembly bearing failed.
Further investigation established that the gear and bearing were unapproved parts and did not conform to the manufacturer's design specifications. The source of the unapproved parts was not determined. The packaging of the parts was reported to resemble that of the manufacturer. Part numbers were found on the gear and bearing, similar to authorized parts, although a different method of marking (etching) was used to mark the gear.