Bell OH-58A accident near West Branch, Michigan, May 20, 2020

The NTSB's narrative final · quoted from the NTSB record

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.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.