Accident Overview
A flight instructor with 2,873 total flight hours, including 550 hours in the same make and model helicopter, was conducting a training flight. The rated student pilot was flying the helicopter while practicing touchdown autorotations.
Sequence of Events
During the climb-out after an autorotation, the helicopter experienced a shudder with a slight vibration through the airframe. The vibration stopped, and the instructor elected to continue the climb, turning toward a downwind leg for the next autorotation. While on the downwind leg, the helicopter shuddered again, this time much more violently. The pilot later stated on the NTSB form that he felt as though the gearbox was locking up. The student pilot then declared "you've got the controls" to the instructor.
The instructor immediately entered autorotation, which caused the shudder to stop. Attempting to regain powered flight, he reapplied throttle, but this created a violent vibration again. He then rolled the throttle completely off and continued with the emergency landing. The altitude at the onset of the incident was approximately 400 feet above ground level.
The instructor initiated a 180-degree turn autorotation to align the helicopter into the prevailing wind. He stated that the transfer of controls, the attempt to regain powered flight, and the subsequent delay in entering the autorotation turn resulted in a low airspeed and low rotor RPM condition, making a successful landing impossible. He waited until the last second and pulled the collective up to the stops. The helicopter landed hard but remained in an upright position. Both the flight instructor and the rated student pilot were not injured.
Post-Accident Inspection
An FAA inspector who traveled to the site noted that the spark plug and helicoil had broken out on the number two cylinder. Further inspection of that cylinder revealed three separate cracks in the cylinder wall. At the time of the accident, the winds were reported from 070 degrees at 10 knots, gusting to 15 knots.