Accident Sequence
During a traffic reporting flight for a radio station, the pilot noted the onset of a severe vibration felt through the anti-torque pedals. Within seconds, the helicopter lost directional control and began a rapid, uncontrolled spin to the right. The aircraft reportedly tumbled out of control for a time, but the pilot was able to regain partial control as it rolled through a relatively upright attitude. Before crashing, the rate of descent was slowed with full collective application, and a rotating impact occurred in a horizontal attitude.
Investigation Findings
Examination of the wreckage revealed that the tail rotor drive shaft spline coupling, part number 28-13609, had become worn. The teeth of the coupling were worn away, and iron oxide and carbonized lubricant were found within the coupling. The splined male end of the coupling was observed to have rotated within the splined female housing. The coupling had accumulated 149.1 hours of operation since its last lubrication, while the lubrication chart and check sheet (pages MM-3-5 and MM-3-6 of the maintenance manual) indicate that the coupling should be lubricated at 100-hour intervals. No other causal factors were identified.