What happened
On November 9, 1997, a private flight involving a Hughes Schweizer 269 C, registration F-OIAZ, was attempting to transition into a hover near Signal Islet. During the final approach, the pilot attempted to establish a hover while the aircraft was still above the ground effect. To stabilize the helicopter, the pilot applied right pedal input; however, the aircraft began a right yaw that could not be countered. The helicopter entered a series of rotations around its vertical axis. In an attempt to maintain a level attitude, the pilot reduced the collective pitch, leading to a hard landing. The aircraft bounced upon impact before coming to a rest, resulting in heavy damage to the airframe.
The investigation
The investigation focused on the aerodynamic performance of the aircraft relative to the environmental conditions at the time of the accident. Investigators analyzed the power requirements for hovering out of ground effect versus the maximum power available. It was noted that the aircraft's main rotor rotates counter-clockwise, which induces a rightward yaw torque. To counteract this, left pedal input is required, which increases power consumption. The investigation examined the relationship between engine torque, rotor RPM, and the lateral force generated by the tail rotor.
Findings
- The aircraft was operating at the limit of its available power due to the specific weight and temperature conditions (24 °C) present during the flight.
- As the pilot increased collective pitch and applied the necessary pedal inputs to stabilize the aircraft, the power demand reached the maximum available threshold.
- This high power demand caused a decrease in the rotational speed of both the main rotor and the tail rotor.
- The reduction in tail rotor RPM diminished the lateral force necessary to counteract the engine torque, leading to the unrecoverable right yaw.
- The pilot's only viable recovery method was to reduce collective pitch and descend into the ground effect to regain sufficient power and maneuverability.