Incident Overview
A helicopter pilot was practicing autorotations. During the second autorotation, the helicopter's skid collided with a rock. The pilot reported that the rock was obscured by grass and he did not see it, although he saw other rocks in front of the aircraft as it descended.
Sequence of Events
Following the collision, the helicopter entered ground resonance. The pilot described the rotor blades gyrating out of their normal path, striking the airframe numerous times.
Prescribed Details
No further details about the helicopter's type, registration, operator, occupants, injuries, or damage extent are available in the source.