What happened
The accident occurred when the fork connecting the rotorcraft's blue blade to the main rotor hub separated. This mechanical failure was triggered by a long-term fatigue crack that had propagated through more than 60% of the fork circumference. At the time of the incident, the site experienced strong gusty winds associated with a frontal passage.
The investigation
The investigation revealed that the operator failed to perform the appropriate inspection of the fork unit as delineated in the FAA Airworthiness Directive. This failure was due in part to confusion over the wording of the directive and a misperception that the fork required only a visual inspection rather than a dye penetrant check. The operator believed this lesser standard applied because they thought the original blade fork had a ten thousand series serial number. However, the installed blade fork's serial number was no longer evident on the part, and manufacturer records had been lost, preventing verification of the correct inspection requirements.
Findings
The primary cause was the separation of the fork due to fatigue. Contributing factors included the operator's failure to carry out the appropriate inspection as delineated in the FAA Airworthiness Directive. This failure stemmed from confusion over the wording of the directive and a misperception that the fork required only a visual inspection rather than a dye penetrant check.
Safety message
Operators must ensure they perform the correct type of inspection for all components, even when manufacturer records are lost or serial numbers are not evident. Relying on visual checks alone when a dye penetrant check is required can lead to undetected fatigue cracks and catastrophic failure.