Santa Ana, CA, USA
N401SH
Guimbal CABRI
The flight instructor and private pilot receiving instruction (pilot) initiated a practice autorotation from about 700 ft in the airport traffic pattern. As the helicopter reached about 150 ft, the instructor slightly opened the twist grip throttle to initiate a power recovery but realized shortly thereafter that the applied power was insufficient to arrest the helicopter’s autorotative descent. The instructor applied additional power, and as the helicopter reached 40 ft, the pilot receiving instruction raised the collective, but the helicopter continued to descend in a flat attitude. The instructor then advanced the twist grip to full power but reported that the engine power did not increase as expected. The low rotor rpm horn activated, the nose yawed left, the helicopter rolled to the left, and the main rotor blades impacted the ground, resulting in substantial damage. A detailed examination of the collective control, twist-grip throttle, and governor control system revealed no pre-impact mechanical anomalies. It is likely that the flight instructor could not apply sufficient power to abort the practice autorotation following the pilot’s collective input, which resulted in an unrecoverable low rotor rpm condition and subsequent loss of control. The helicopter’s flight manual stated that the lowest recommended height for aborting an autorotation was 300 ft; however, the instructor initiated the recovery at 150 ft, which resulted in reduced time and altitude available to recognize that the engine was not responding as expected and continue with a power-off, touchdown autorotation. In a service letter published after the accident, the manufacturer advised against initiating a power recovery from an autorotation in the flare, as it could result in a loss of yaw control upon application of maximum engine torque.
On September 3, 2018, about 1537 Pacific daylight time, a Guimbal Cabri G2 helicopter, N401SH, was substantially damaged when it was involved in an accident near Santa Ana, California. The flight instructor and private pilot receiving instruction were not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. According to the instructor and pilot receiving instruction, they returned to the departure airport after conducting cross-country flight training and initiated an autorotation to simulate an inflight loss of engine power from about 700 ft mean sea level (msl). They maintained 50 kts throughout the autorotative descent, and upon reaching 150 ft msl, the instructor prepared for a power recovery by slightly opening the throttle. Two seconds later, the instructor realized that the helicopter did not have adequate power to arrest the descent and advanced the throttle further. The instructor added that 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 its full-power position to complete the power recovery, but the engine power did not increase as anticipated. The low rotor rpm horn engaged, and the nose yawed to the left. The helicopter rolled to the left from about 4 ft above the ground before coming to rest on its left side. Postaccident photos furnished by the helicopter operator showed substantial damage to the main rotor blades and that the fuselage had separated from the skid attachment points. The instructor held a flight instructor certificate with ratings for helicopter and instrument helicopter. She reported 819 total hours of flight experience, all of which were in rotorcraft. She reported 15 total hours of flight experience in the accident helicopter at the time of the accident. The pilot receiving instruction held a private pilot certificate with a helicopter rating. He reported 99 total hours of flight experience, all of which were in rotorcraft. He reported 13 total flight hours in the accident helicopter at the time of the accident. The helicopter flight manual, under section 7-5.1, Autorotation training, stated: …the instructor should keep in mind that aborting autorotation should not be decided in the flare, when there is too much workload and that low rotor speed could prevent adequate yaw control… If power recovery is decided during autorotation: Roll-in throttle until governor engages, Gradually raise collective pitch to stop autorotation and descent, Control yaw during power recovery with pedals Note: …Be prepared to yawing to the left when power recovers. The flight manual also advised that the lowest recommended altitude for aborting an autorotation was 300 ft. Guimbal Helicopters Service Letter SL 19-002, Controllability in Yaw at Low Rotor Speed, published several months after the accident, warned against ending an autorotation with a power recovery during or just after the flare. When performing a power recovery, the engine governor will re-engage as soon as the engine speed reaches 2,000 rpm and will fully open the throttle. With the engine torque at maximum, this could lead to a loss of yaw control. Airframe Examination Examination of the helicopter revealed that the lower left side fuselage and aft left skid crossbar exhibited some scraping. Each of the three main rotor blades tips were destroyed and the blades exhibited some bending opposite the direction of rotation. The main rotor mast and hub did not show any abnormalities, as the pitch change links were connected to each of their respective main rotor hub brackets. The elastomeric bearings were normal in appearance and did not exhibit any extrusion. The anti-torque pedals moved normally and were continuous to the tail rotor blades at the fenestron. The collective twist grip was advanced to the full power position multiple times from the mechanical idle position. The twist grip and governor performed as designed with no anomalies noted. The governor was also functionally tested in accordance with the maintenance manual. Testing showed that the governor functioned normally and could be overridden successfully by the twist grip as designed. Helicopter Flying Handbook The FAA Helicopter Flying Handbook (FAA-H-8083-21B), when describing a practice autorotation with power recovery procedure, stated: If the throttle is increased too fast or too much, an engine overspeed can occur; if throttle is increased to slowly or too little in proportion to the increase in collective pitch, a loss of rotor rpm results.
The flight instructor's delayed application of power during a power recovery following an autorotation, which resulted in an unrecoverable low rotor rpm during the landing flare and subsequent loss of control.
Source: NTSB Aviation Accident Database
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