What happened
During a training flight involving touchdown autorotation practice, the flight instructor noticed the Robinson R22 shuddering with a slight vibration through the airframe during the climb out. The vibration initially ceased, prompting the instructor to continue the climb and turn toward a downwind leg for the next maneuver. However, while on the downwind leg, the helicopter shuddered much more violently. The pilot reported a sensation that the gearbox might be locking up.
Upon the onset of the severe vibration, the student pilot transferred control to the instructor. The instructor immediately entered an autorotation, which stopped the vibration. In an attempt to regain powered flight, the instructor reapplied throttle, but this immediately triggered the violent vibration again. Consequently, the instructor rolled the throttle completely off and proceeded with an emergency landing.
At the onset of the event, the aircraft was approximately 400 feet above ground level. The instructor initiated a 180-degree turn autorotation to align the helicopter with the prevailing winds. Due to the transfer of controls, the attempt to regain power, and a delay in the turning maneuver, the aircraft entered a state of low airspeed and low rotor RPM. The instructor attempted to pull the collective to the stops at the last moment before the helicopter landed hard. Both the flight instructor and the student pilot were not injured.
The investigation
An FAA inspector examined the aircraft at the accident site and observed that the spark plug and helicoil were broken out on the number two cylinder. Further inspection of the number two cylinder revealed three separate cracks in the cylinder wall.