Cat. 2 investigation report: simplified-format report, adapted to the circumstances of the occurrence and the investigation stakes.
The pilot took off from the prepared helipad after having refilled the tanks with larvicide and additional fuel. The helicopter was then close to the maximum take-off weight. The F-GBOO pilot made a short, straight, low-level transition from the refuelling site to the next plot to be treated. During this straight-line flight, the pilot lost yaw control and the helicopter rotated several times about the yaw axis before crashing into a field of rushes. The investigation did not bring to light any mechanical failure that could explain a yaw control failure of the helicopter.
The helicopter was roughly flying in a north-easterly direction, with a right-hand tailwind of about 10 knots and in light to moderate turbulence. It was heavy, close to maximum weight, and moving slowly. These conditions were conducive to an unanticipated yaw (commonly referred to as loss of tail rotor effectiveness (LTE)).
Although the scenario of a technical failure cannot be completely ruled out, the scenario of a loss of yaw control due to an unanticipated yaw is also possible.
This phenomenon is described in the DSAC document referenced below (available in French only):