Accident Sequence
The helicopter was maneuvering at a low altitude while the pilot was searching for animals. The pilot experienced a severe vibration through the flight controls. The helicopter then began an uncontrolled descent. The pilot attempted to stop the descent by using the collective control, but these attempts were unsuccessful. The helicopter subsequently contacted the ground and was substantially damaged.
Examination Findings
After the accident, an examination of the helicopter revealed that an AN-320-5 nut had come off of a pin on the rotating swashplate ring. The purpose of this nut was to secure the link and scissors to the swashplate pin. With the nut off, a loss of continuity occurred in the rotating controls of one main rotor blade. A cotter pin was required to secure the nut. The absence of a cotter pin would have allowed the nut to eventually work its way off during operation.
Maintenance History
The helicopter had been operated for about 26 hours since a 1200-hour inspection had been performed. During that inspection, the above-mentioned parts and components had been removed, inspected, and reinstalled. The missing cotter pin allowed the nut to detach, leading to the loss of control.