Accident Overview
A pilot-owner completed an uneventful aerial application flight and began repositioning the helicopter to another site. En route, at 500 feet above ground level, the helicopter's engine began to overspeed. The pilot entered autorotation, but the helicopter struck trees and landed hard.
Examination Findings
Post-accident examination revealed that the elastomeric torsional coupling inside the transmission, as well as the secondary steel drive flanges, were sheared. The manufacturer's service data for the coupling stated that mandatory replacement should occur at 5,000 hours or 5 years from the date embossed on the rubber, or in accordance with maintenance requirements for the coupling, whichever occurred first.
Maintenance History
The raised numbers on the coupling indicated a date of May 1988. At the time of the accident, the coupling had been in service for 9 years, exceeding the 5-year replacement interval specified by the manufacturer.