History of Flight
On 23 September 2021 at 1400Z, a pilot flying a Schweizer 269C single-engine light helicopter, registration ZS-HEW, was engaged in a game capture operation at Camdeboo Game Reserve in Graaf-Reinet, Eastern Cape Province, South Africa. The pilot had performed a pre-flight inspection with no abnormalities noted. The aircraft had 12 gallons of Avgas LL100 fuel, and the flight was planned for about 45 minutes. After takeoff, the pilot climbed to approximately 200 feet above ground level heading north. At that time, the pilot felt a severe vibration in the tail rotor pedals. The pilot immediately initiated an autorotation and landed on an open field. The helicopter's tail rotor assembly and stabiliser sustained substantial damage. The pilot reported no injuries.
Damage and Injuries
The tail rotor assembly and stabiliser were substantially damaged. There were no injuries to the pilot or any persons on the ground.
Technical Findings
A metallurgical test report determined that the fork hinge bolt failed due to a fatigue fracture that initiated in the shank section of the bolt. Evidence of rotation in the assembly during operation resulted in localized surface damage (stress raisers), which contributed to the initiation of the fatigue fracture. The investigation noted that several factors, such as under-torque of the bolt, wear or collapse of one or more washers, damage to the fork itself, or incorrect assembly during maintenance, might have contributed to the incident. However, due to insufficient evidence submitted for testing, the exact cause could not be determined.
Probable Cause
The helicopter’s fork hinge bolt failed on the bolt head due to a fatigue fracture which initiated in the shank section of the hinge bolt over time, causing an imbalance in the tail rotor. This led the pilot to execute an autorotation landing. It is likely that several causes, such as under-torque of the bolt, wear/collapse of one or more washers, damages to the fork itself or incorrect assembly during maintenance, might have contributed to this incident. Due to insufficient evidence submitted for testing, the cause could not be determined.