Bell OH-58A N153SA accident near West Branch, Michigan

Probable cause

The pilot's failure to maintain rotor rpm during a precautionary autorotation, which resulted in a hard landing. Contributing to the accident was the airframe vibration for reasons that could not be determined because examination of the helicopter revealed no anomalies.

NTSB analysis

While cruising about 500 feet above ground level, the pilot of the helicopter felt a vibration and entered a precautionary autorotation, though he did not observe any caution or warning lights or horns associated with the vibration. The pilot turned toward an open field for landing and, after clearing some trees, the low rotor rpm horn activated. He reduced collective and made a hard run-on landing, during which the helicopter slid about 20 to 30 ft on its skids before coming to rest upright. The main rotor drooped and one blade struck the tail boom, severing the tail rotor drive shaft and resulting in substantial damage. There was also crush damage to the underside of the tail boom caused by impact forces.

The pilot could not describe the location or frequency of the vibration, and the passenger reported that he felt no vibration. Postaccident and follow-up inspections of the helicopter by revealed no anomalies that would have resulted in the vibration. Although the source of the vibration could not be determined, it is likely that the hard landing was the result of the pilot's failure to maintain rotor rpm during the precautionary autorotation.

Text from the NTSB. Open the NTSB docket for CEN20LA187 for the full record.