Accident Details
The pilot was preparing to take off in furtherance of a contract firefighting flight for the California Department of Forestry & Fire Protection (CAL FIRE). The helicopter was on the ground with its engines at operational power when a mechanical malfunction occurred. The pilot was unable to maintain roll control, and the helicopter initiated an uncommanded slow rollover onto its side.
Investigation Findings
During the accident investigation, components that secured one of the three main rotor blade pitch change servo units to the helicopter were found a few yards away from the main wreckage. The components consisted of a nut, bolt, pin, and plate. They had secured one of the servo units to the helicopter transmission on one end and the helicopter swash plate on the other end. Separation of these components from the helicopter resulted in separation of the servo unit, and consequently a loss of main rotor blade pitch control.
Upon visual examination of the components, evidence of severe wear and corrosion was detected inside the nut holding down the retainer mounting strap. This indicates that this area had not been inspected for some time. Once the main rotor servo cylinder and transmission attaching lug separated, the rotor system would travel to the full pitch position, thereby causing the helicopter to experience an uncommanded lift and roll to the right.
Maintenance Background
The helicopter was operated and maintained by its owner pursuant to a specific maintenance program. A component of this program involved examining each main rotor servo mount pin for rust and corrosion and performing dimensional checks prior to reinstallation of the inspected components. The company mechanic reported that he completed the inspection and found no faults. The records reflect that the components were reassembled and installed on the helicopter, which was then returned to service. The accident occurred about 8 operational hours and 2 days following the reported maintenance inspection.