Pilot Report
The pilot reported that during the descent for landing, he reduced the collective from 23 inches of manifold pressure to 21 inches, at which point the engine began shuddering. Increasing the manifold pressure back to 23 inches appeared to smooth the engine. After entering a left base and turning final, he again reduced manifold pressure. On final approach, the helicopter was slowed to approximately 10 miles per hour. When the pilot added collective, the rotor RPM dropped to the bottom of the green arc. The pilot stated he lowered the collective and added throttle. The helicopter was unable to maintain altitude, and the pilot initiated a run-on landing in grass. During the landing, the left skid contacted the ground and collapsed, causing the helicopter to roll onto its left side.
Examination Findings
Examination of the helicopter revealed that the forward and aft left skid struts were bent inboard approximately 135 degrees. Two main rotor blades were bent downward with delamination near the root and on the trailing edge. The third main rotor blade was bent upward near the tip with trailing edge and upper skin deformation. Control continuity was established for the tail rotor, cyclic, collective, over-running clutch, throttle, and mixture controls. The drive belt exhibited a shaved area near the clutch assembly, and the over-running clutch operated normally. The fan shroud displayed circumferential scoring.
Engine Test
An attempt to start the engine using cockpit controls resulted in the engine starting and idling smoothly. Higher RPM settings were not attempted.