What happened
On November 11, 2015, a Guimbal G2 Cabri, registration 3A-MIS, was conducting a training flight at the Aix-les-Milles aerodrome. The flight, intended for type rating instruction, included various maneuvers such as hovering engine failure simulations and acceleration-stops. During the session, the instructor took control to demonstrate a simulated engine failure during takeoff.
While performing the maneuver, the instructor accelerated to approximately 20 knots at a height of 5 feet before reducing engine power via the throttle twist grip to simulate a failure. As the pilot applied collective pitch to cushion the touchdown during the running landing, the helicopter experienced a sudden left yaw of approximately 30 degrees. This caused the aircraft to veer off the taxiway and onto the adjacent grass. The uneven terrain caused the helicopter to bounce one or two times, leading to an interaction between the main rotor blades and the tail boom. The blades severed the tail boom, causing the aircraft to pivot 90 degrees to the right before coming to a halt. There were no fatalities, though the helicopter sustained heavy damage.
The investigation
The investigation focused on the mechanics of the power reduction and the pilot's ability to maintain directional control. Investigators examined the aircraft's throttle system, noting that in normal operation, the collective pitch is coupled with engine power. This coupling—where an increase in collective pitch results in increased engine power and a corresponding left yaw due to torque—is only inhibited when the throttle twist grip is turned fully to the idle position.
Witness accounts from the crew indicated that the instructor was relatively inexperienced with this specific maneuver. Furthermore, a subsequent training flight performed by the same instructor days later confirmed that a left yaw movement occurred during the same type of landing. The investigation also looked into the surface conditions of the taxiway and the surrounding grass area.