Event Summary
A student pilot was practicing hovering autorotations on an asphalt taxiway under the supervision of a certified flight instructor (CFI). During the practice, several hard landings occurred because the student had difficulty learning the proper application of collective pitch. In response, the CFI instructed the student to move to a sod area adjacent to the taxiway, intending that the softer surface would cushion impacts and reduce forces on the aircraft.
The CFI later stated that he did not inspect the aircraft for damage following the hard landings on the taxiway. The student then performed one hovering autorotation on the sod area. During the subsequent hover, control of the aircraft was lost. The aircraft rolled over and impacted the ground.
Investigation Findings
Examination of the taxiway revealed heavy skid impact marks consistent with the earlier hard landings. The aircraft's engine mount tubular structure at the bottom mount, along with the cross member support tubes, were found to be fractured. The walls of these tubes exhibited irregular thickness, contained extensive corrosion scaling, and could be bent by hand in some places. As a result, the bottom of the engine—which is mounted to the transmission and main rotor mast—was free to move.
Maintenance Records
The last annual inspection of the aircraft was performed five days before the accident. No information was provided regarding any findings or corrective actions from that inspection.