What happened
On Friday, June 25, 1999, at approximately 12:00, a Hughes Schweizer 269 C helicopter, registered F-GHKF, was performing an aerial photography mission near La Burbanche, France. While the pilot was approaching a clearing at an altitude of only two to three meters above the ground, the aircraft suddenly entered an uncontrolled right-hand rotation. The pilot was unable to regain control using the pedals, and the helicopter struck the ground on its right skid before overturning.
The investigation
The investigation focused on the mechanical failure of the tail rotor assembly. Upon examining the aircraft, investigators discovered that the interior of the rear tail boom extremity and the pitch change control bellcrank were covered in a significant amount of grease. This grease had originated from the tail rotor mechanism.
Technical analysis revealed that a failure in a pressure relief valve allowed grease to escape the mechanism. This leakage led to mechanical wear that eventually caused the tail rotor drive to decouple from the system. The volume of grease found—which was several times the capacity of the mechanism's housing—indicated that the leak had been active for several days prior to the accident.
Findings
- The primary cause of the accident was the loss of tail rotor drive due to mechanical decoupling.
- A faulty pressure relief valve allowed grease to escape the tail rotor mechanism, leading to excessive wear.
- The pilot, who was also responsible for routine maintenance and pre-flight inspections, failed to detect the abnormal accumulation of grease during pre-flight checks.