What happened
On 7 March 2021, an Airbus AS350 B helicopter, registered F-GIBM, was performing a vertical takeoff from a private helicopter landing site (HLS) in Touques, France. The flight was a private, non-commercial operation intended to transport a passenger to a property in the Oise region.
Shortly after the takeoff sequence began, the helicopter climbed vertically toward a group of trees. At an altitude of approximately 19 meters, the main rotor blades struck several tree branches. The impact of the branches against the blades generated a massive overtorque on the rotor mast. This sudden mechanical stress caused the tail boom to separate from the main airframe. Following the separation, the helicopter lost stability, yawed around its axis, and fell to the ground. The impact resulted in two fatalities, involving both the pilot and the passenger.
The investigation
The BEA investigation focused on the mechanical failure of the airframe and the environmental conditions at the landing site. Investigators examined the wreckage, the flight data from the Brite Saver recorder, and the aircraft's maintenance history.
To understand the structural failure, the BEA developed a dynamic model of the AS350 B main rotor. This model simulated the impulsive forces created when branches strike the blades. The investigation specifically analyzed the shear strength of the 99 rivets securing the tail boom to the airframe. Furthermore, the investigators evaluated the impact of the pilot's vision and the lighting conditions at the time of the accident, noting that the branches were in deep shade, which likely obscured the proximity of the obstacles.
Findings
- The primary cause of the accident was the main rotor blades striking tree branches during a vertical climb.
- The impact created an overtorque on the rotor mast that exceeded the structural limits of the attachment rivets, leading to the immediate separation of the tail boom.
- The choice of the takeoff maneuver was restricted by the specific configuration of the HLS, which forced a vertical climb with a very small safety margin relative to the nearby trees.
- Low light contrast and the presence of shadows on the tree branches likely made it difficult for the pilot to accurately assess the distance to the obstacles.
- The aircraft was not equipped with modern high-energy absorbing seats, which may have influenced the seat separation during the high-acceleration event.