Event Description
During a log slinging operation, a loud bang was heard, and the aircraft was observed to rapidly descend vertically to the ground. Numerous small fragments fell in the same vicinity.
Investigation Findings
The investigation focused on the synchronization shaft assembly. The #5 synchronization shaft was found to be fractured at its forward end, approximately 9 inches aft of the connection assembly to the #4 shaft. This fracture resulted in an out-of-synchronization condition between the forward and aft rotor blades, as well as a loss of power to the forward rotor.
Metallurgical examination revealed that a fatigue fracture originated in the aft-most ring of rivet holes. The fatigue cracks propagated from multiple origins within these rivet holes, where a "step" was found at the base of the hole, closest to the inside diameter of the shaft.
Metallurgical Examination
Further analysis of the rivet holes in the forward end of the shaft showed that all of them had topographic features similar to those produced in a steel plate by deliberately drilling holes without complete penetrations. This indicates a manufacturing anomaly that contributed to the initiation of fatigue cracks.