History of the Flight
The helicopter, operated by Azur Hélicoptère, was conducting a practical examination for a helicopter private pilot licence. The student pilot, seated in the right seat, and the examiner took off from Lyon Bron airport. At around 09:10 local time, while hovering in ground effect at Lyon-Brindas aerodrome, the examiner announced a simulated engine failure in hover and began reducing engine power. The student pilot felt the helicopter sink and yaw right. He applied full left pedal and pulled the collective pitch control to cushion the ground contact. During this maneuver, engine power suddenly increased, causing the helicopter to rotate left and rise. The student asked the examiner to shut down the engine. When engine power was finally reduced, the helicopter sank and struck the ground.
Crew Experience
The examiner held a helicopter commercial pilot licence, a flight instructor rating, and a flight examiner rating. He had logged approximately 13,500 flight hours, including 5,200 hours on piston-engine helicopters. In the previous twelve months, he had logged 120 hours, 17 of which were on the Cabri G2. The student pilot had started helicopter training in September 2018 and held an aeroplane private pilot licence. He had logged 42 helicopter flight hours, including 10 hours of supervised solo flights.
Statements
The examiner stated that after announcing the engine failure, he throttled back but did not reach the mechanical stop. When the helicopter yawed left and engine rpm increased, he took back the controls and applied full right pedal, which he thought had no effect. He was aware of the manufacturer's service letter SL19-001 regarding throttle management during simulated engine failure. The student pilot indicated that the examiner took controls without warning and applied full right pedal. He stated the helicopter fell level from about four meters when engine power was reduced again. Witnesses saw the helicopter hover about one meter above ground, then noted a slight left rotation and sinking. The rotation suddenly accelerated, engine noise increased, the helicopter rose while yawing, then a sharp decrease in engine noise and the helicopter fell.
Engine Power Management and Synchronization
The Cabri G2 has two automatic power management systems: an engine governor that adjusts power to maintain main rotor speed within 515-530 rpm, and a collective pitch/throttle correlator that mechanically adjusts the carburettor throttle valve when the collective pitch is moved. The correlator can be inhibited by turning the twist grip beyond the engine idle position to a mechanical stop called the engine idle mechanical stop. The travel between idle and stop is the dead band. Positioning the twist grip at this stop is essential during engine failure simulations to prevent unwanted engine acceleration. During normal operation, engine and rotor speeds are synchronized. When engine speed is reduced and rotor speed maintained, they desynchronize; increasing engine power re-synchronizes them.
Simulation of Engine Failure in Hover
The flight manual procedure for power failure in hover IGE is: roll off throttle frankly until on its stop, counteract yaw with left pedal, increase collective as ground approaches to smooth landing, push collective down once landed. The manufacturer issued service letter SL19-001 following two accidents from poor throttle management. It states that poor throttle management can cause engine and rotor re-synchronization accompanied by a yawing jerk, and in low rotor speed, a loss of control in yaw. The throttle must be reduced in a single step by turning the twist grip 160°.
Conclusions
The examiner's action on the twist grip had not reached the engine idle mechanical stop when the student pilot started the procedure. The student's collective pitch input caused the correlator to command an increase in throttle, followed by automatic reactivation of the engine governor, initiating engine re-synchronization with the main rotor. This resulted in a significant increase in engine power, causing a left yaw. The left pedal input by the student to counter yaw may have also contributed to accelerating the left yaw while engine power was increasing, leading to loss of control in yaw. When engine power was reduced again, the helicopter, still yawing left, hit the ground hard. Improper management of the twist grip by the examiner combined with the student's application of the engine failure IGE procedure led to the loss of control in yaw.