Accident Sequence
The helicopter was on final approach to landing when, at an altitude of approximately 45 to 50 feet above ground level, the pilot heard a loud snapping sound. This was immediately followed by a loss of tail rotor control. The pilot initiated an autorotation, and the helicopter subsequently landed hard on a grassy staging field before coming to a stop upright.
Examination Findings
Post-accident examination of the helicopter revealed that two segments of the tail rotor drive shaft had fractured. Additionally, one of the two bolts connecting the aft end of one of the fractured segments to a disc coupling (Thomas Coupling) was missing. The missing bolt was not recovered; however, a nut and two beveled washers were found on top of the tail boom beneath the driveshaft. Examination of the remaining bolt showed evidence of fretting, wear, and deformation limited to the last two and a half threads.
Maintenance Records
Review of the helicopter's maintenance records indicated no entries documenting removal of any tail rotor drive shaft segments in the two years preceding the accident.