What happened
On 13 September 2019, a Guimlam Cabri G2, registration F-HDTO, was conducting a practical examination for a helicopter private pilot licence at Lyon-Brindas aerodrome. The flight was operated by Azur Hélicoptère and carried a student pilot and an examiner.
While hovering in ground effect, the examiner initiated a simulated engine failure by reducing engine power. As the student pilot attempted to follow the standard procedure for an engine failure in hover, the helicopter began to sink and yaw to the right. In an effort to cushion the descent, the student pilot pulled on the collective pitch control. During this maneuver, the engine power suddenly increased, causing the helicopter to rise and rotate sharply to the left. The examiner attempted to regain control by applying right pedal, but the aircraft continued to yaw. After the engine power was eventually reduced, the helicopter struck the ground, resulting in two injuries and significant damage to the aircraft.
The investigation
The BEA examined the flight controls, the engine management systems, and the actions of the crew. The investigation focused on the interaction between the manual throttle control (twist grip) and the aircraft's automatic systems, specifically the engine governor and the collective pitch/throttle correlator. The investigators also reviewed the manufacturer's service letter SL19-001, which provides guidance on throttle management during simulated engine failures to prevent unwanted engine resynchronization.
Findings
- The examiner had not turned the twist grip far enough to reach the engine idle mechanical stop before the student began the maneuver.
- Because the throttle was not at the mechanical stop, the student pilot's input on the collective pitch control triggered the collective pitch/throttle correlator to increase throttle opening.
- This action, combined with the automatic reactivation of the engine governor, caused the engine and rotor to resynchronize, leading to a sudden surge in engine power.
- Improper management of the twist grip by the examiner, combined with the student pilot's execution of the engine failure procedure, caused the loss of control in yaw.