Accident Overview
A helicopter encountered a total loss of drive to the rotor system a few seconds after takeoff, when the aircraft was approximately 20 feet above the ground. The helicopter subsequently touched down hard and sustained substantial damage.
Maintenance History
Company mechanics had performed maintenance on the helicopter that involved handling of the rotor drive shaft and a flex coupling. The flex coupling is normally secured with three bolts and self-locking nuts. The manufacturer's maintenance procedure requires that during installation, the nuts are torqued and then cotter pins are inserted.
Wreckage Examination
During the examination of the wreckage, the bolts were found sheared. No evidence of cotter pins was found in the wreckage.
Manufacturer's Statement
According to the manufacturer, even if the cotter pins are not installed, if the bolts are adequately torqued, they should not become loose.