What happened
On 8 June 2018, a Guimbal Cabri G2, registration G-PERH, was conducting a PPL(H) skills test at Goodwood Aerodrome, Sussex. The flight crew, consisting of a commander and a student pilot, had completed several exercises successfully. Following a satisfactory initial simulated engine failure from the hover (SEFH), the student requested a second attempt to improve their performance.
During this second attempt, the student pilot began to apply left pedal and raise the collective to cushion the landing before the throttle had been fully closed by the commander. This sequence caused the engine speed to increase, leading to a rapid leftward yaw and a climb. The helicopter entered a spin, reaching an altitude of approximately 40 ft agust before descending rapidly. The aircraft struck the ground, causing severe damage to the airframe, including the detachment of the Fenestron tail rotor and damage to the main rotor blades. Both occupants sustained serious injuries.
The investigation
The AAIB examined the wreckage and performed a 3D digital scan of the airframe. The investigation found that the landing gear had penetrated the fuselage and the tail boom was misaligned. Analysis of the seat systems showed that while the left seat's energy-absorbing strut extended significantly, the right seat occupant was subjected to intense lateral loads due to the high rate of yaw during impact. The investigation also reviewed the mechanics of the engine governor and the correlation cam, which links throttle and collective inputs.
Findings
- The accident was likely triggered by the premature application of left pedal and the raising of the collective before the throttle was fully closed.
- The student pilot likely anticipated the yaw and reacted before the commander's command to close the throttle was completed.
- Raising the collective while the throttle was not fully closed caused the engine speed to increase via the correlation cam, resulting in the loss of directional control.
- The reduction in rotor speed eventually rendered the Fenestron tail rotor ineffective at maintaining yaw control.
- The survivability of the occupants was aided by the intact cabin volume, the absence of a post-crash fire, and the energy-absorbing properties of the composite structure and seat systems.