Accident Overview
On September 3, 2018, at about 1537 Pacific daylight time, a Guimbal Cabri G2 helicopter, registration N401SH, was substantially damaged in an accident near Santa Ana, California. The flight instructor and private pilot receiving instruction were not injured. The instructional flight was conducted under 14 CFR Part 91.
Sequence of Events
According to the instructor and pilot, they returned to the departure airport after cross-country flight training and initiated an autorotation to simulate an inflight loss of engine power from about 700 ft msl. They maintained 50 kts during the autorotative descent. At about 150 ft msl, the instructor prepared for a power recovery by slightly opening the throttle. Two seconds later, realizing that the helicopter did not have adequate power to arrest the descent, she advanced the throttle further. As the helicopter reached 40 ft msl, the pilot raised the collective pitch control to stop the descent but did not complete the flare. While descending in a nearly flat attitude, the instructor rolled the throttle to full power, but engine power did not increase as anticipated. The low rotor rpm horn engaged, the nose yawed left, and the helicopter rolled left from about 4 ft above the ground before coming to rest on its left side.
Pilot Information
The instructor held a flight instructor certificate with helicopter ratings and reported 819 total hours of flight experience, all in rotorcraft, with 15 hours in the accident helicopter. The pilot receiving instruction held a private pilot certificate with a helicopter rating and reported 99 total hours, all in rotorcraft, with 13 hours in the accident helicopter.
Helicopter Information and Flight Manual Guidance
The helicopter flight manual, under Section 7-5.1, Autorotation Training, stated that aborting autorotation should not be decided in the flare due to workload and that low rotor speed could prevent adequate yaw control. It outlined power recovery procedures: roll in throttle until governor engages, gradually raise collective pitch to stop descent, and control yaw with pedals, noting to be prepared for yawing to the left when power recovers. The manual advised that the lowest recommended altitude for aborting an autorotation was 300 ft.
Guimbal Helicopters Service Letter SL 19-002, published months after the accident, warned against ending an autorotation with a power recovery during or just after the flare, as the governor re-engaging at low rotor speed could lead to loss of yaw control.
Postaccident Examination
Examination revealed scraping on the lower left fuselage and aft left skid crossbar. All three main rotor blade tips were destroyed with some bending opposite rotation direction. The main rotor mast, hub, pitch change links, elastomeric bearings, and anti-torque pedals were normal. The collective twist grip and governor functioned as designed when tested; the governor could be overridden by the twist grip as intended.
FAA Helicopter Flying Handbook
The FAA Helicopter Flying Handbook (FAA-H-8083-21B) described that during practice autorotation with power recovery, increasing throttle too fast or too much can cause engine overspeed, while too slow or insufficient throttle relative to collective pitch increase can cause loss of rotor rpm.