WPR22LA363

A partial loss of engine power due to a damaged cylinder intake valve and subsequent collision with terrain during an autorotation.

The flight instructor and student had just completed a series of uneventful practice maneuvers and decided to perform off-airport landings at a location that the instructor had used before. After a series of reconnaissance orbits, low passes, and power checks over the landing area and, with the helicopter operating normally, the instructor decided to proceed with a landing. The student was maneuvering at low altitude while attempting to land when they felt a jolt, and the helicopter yawed to the left. The instructor took over the controls, and attempted to maneuver away from trees but the helicopter began to vibrate and yawed aggressively to the left with an accompanying reduction in engine speed. The helicopter began to descend with the low rotor rpm horn sounding, and the instructor performed an autorotation, during which the main rotor blade cut through the tail. Postaccident examination revealed a damaged engine intake valve that would have resulted in a partial loss of engine power. The helicopter’s response during the event was consistent with a loss of engine power, although due to the altitude and environmental conditions, the helicopter was being operated at the upper limits of its performance envelope.

On September 30, 2022, about 1310 mountain daylight time, a Robinson R44 Cadet, N297SU, sustained substantial damage when it was involved in an accident near Cedar City, Utah. The flight instructor and student pilot were not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight.   The flight instructor and student had just completed a series of practice maneuvers and decided to practice off-airport landings at a location that the instructor had used before. They performed a series of reconnaissance orbits, low passes, and power checks over the landing area and, with the helicopter operating normally, the flight instructor decided to proceed with a landing.   The student pilot began to maneuver the helicopter for landing, but he had moved to a sloped area, so the flight instructor asked him to pull to a hover of about 7 ft. They hovered there and noted the manifold pressure gauge was indicating between 20.8 and 21.1 inches of mercury. They initiated a departure when the “full throttle” light illuminated. The manifold pressure gauge indicated 22.5 inches of mercury, and the instructor lowered the collective control and the light extinguished. This resulted in the helicopter descending to about 3 ft. Concerned that they might strike surrounding trees, the instructor asked the student to pull into a 5 ft hover, and taxi forward to clear the tail and then perform a pedal turn to allow the helicopter to face a clearing in the trees so they could climb out.   As the student pushed forward on the cyclic to begin the departure, they felt a jolt, and the helicopter yawed to the left. The instructor was concerned that they might have struck a tree, and he took the controls. Due to the confined area and the helicopter already moving forward and climbing, he continued to apply forward cyclic. The helicopter began to vibrate and yawed aggressively to the left with an accompanying reduction in engine speed. The helicopter began to descend, and the engine speed gauge needle indicated it had now descended below the main rotor speed and was continuing to drop. The instructor lowered the collective and rolled on the throttle, but the engine did not respond, and with the low rotor rpm warning sounding, he initiated an autorotation.   During the landing flare the helicopter contacted the trees, and the pilot applied aft cyclic to stop it from tipping forward down the hill. The main rotor blades cut through the tail as the helicopter came to rest. The instructor reported that the engine was still running after impact, but at sporadic speeds. The instructor reported that due to atmospheric conditions, they were using carburetor heat, with the control pulled out to about 1/4 of its limit. The accident site was surrounded by low-lying cedar trees, within rolling hills located at an elevation of 6,385 ft mean sea level (msl). According to the flight instructor, this was a common location used for stage checks. The helicopter was recovered from the accident site and an examination was performed by a Federal Aviation Administration inspector, with no anomalies noted. The engine had accrued 2,386.7 hours since overhaul and before the accident, both it and the airframe had been scheduled for an overhaul in the weeks to follow. The helicopter was shipped to the overhaul facility for assessment. During the subsequent engine disassembly, the facility reported that the No. 1 cylinder intake valve head appeared discolored, and that a notch was found in both the valve head and its corresponding seat. The overhaul facility was not able to provide further details including photographic evidence of the failure. The pilot provided his performance calculations for the accident flight. Based on the helicopter weight, environmental conditions, the use of carburetor heat, along with the flight school standard operating procedure which included a 500 ft buffer, he calculated the out-of-ground-effect hover ceiling to be 6,400 ft. The calculations appeared to match the data in the performance section of the helicopter’s Pilot’s Operating Handbook.