Robinson Helicopter R44 N8361X accident near Torrance, California

Probable cause

A main rotor overspeed due to the pilot’s failure to maintain rotor RPM within allowable limits during a practice autorotation.

NTSB analysis

The pilot reported that he initiated a practice autorotation to a paved helipad. As the helicopter descended through about 100 feet agl, the pilot initiated a recovery by “slowly rolling on throttle.” The pilot stated that as the helicopter descended, he noticed the rotor RPM “increase quickly.” Subsequently,the helicopter landed on the helipad uneventfully. Post accident examination of the helicopter revealed that the tail boom of the helicopter was structurally damaged, and the tail rotor drive shaft was bent and separated in multiple locations. The drive shaft exhibited scoring around the circumference at regularly spaced intervals on the shaft. The damper assembly was heavily damaged, and the intermediate flex coupling assembly was damaged. According to the helicopter’s manufacturer, this damage to the tail rotor drive shaft is indicative of a rotor over speed to at least 134%. No pre-accident mechanical anomalies were noted with the engine or airframe that would have precluded normal operation.

Text from the NTSB. Open the NTSB docket for WPR09LA436 for the full record.