What happened
A few seconds after takeoff, approximately 20 feet above the ground, the helicopter encountered a total loss of drive to the rotor system. This resulted in a hard touchdown, causing substantial damage to the aircraft.
The investigation
The investigation focused on recent maintenance performed by company mechanics, which included work on the rotor drive shaft and a flex coupling. Under standard procedures, this flex coupling is secured using three bolts and self-locking nuts. The manufacturer's maintenance instructions require that after the nuts are torqued during installation, cotter pins must be inserted.
Upon examination of the wreckage, investigators found that the bolts had sheared. Additionally, there was no evidence that cotter pins had been installed on the assembly. While the manufacturer stated that bolts should not become loose if properly torqued even without cotter pins, the failure occurred during this flight phase.