Accident Sequence
A witness reported observing a helicopter maneuvering for an aerial application flight. During the maneuver, one of the main rotor blades separated from the helicopter. Subsequently, the helicopter nosed down and collided with terrain.
Post-Accident Examination
Examination of the separated main rotor blade sections revealed that the origin area contained several crack arrest positions and small ratchet marks, indicating fatigue cracking initiated from multiple locations. The thickness of the larger spar was measured and found to be slightly below the manufacturer's specified measurements. Scratches or scoring marks, described as having a sanded appearance, were noted on the lower surface where the thickness was reduced. The spar wall thickness continued to decrease further outboard of the fracture. Corrosion was also visible on the blade.
Regulatory Requirements
The applicable Airworthiness Directive (AD) requires performance of inspections according to a Service Bulletin (SB) to prevent main rotor blade failure due to cracking of the spar or delamination of the trailing edge skin from the spar. The SB specifies the types of inspections, intervals for accomplishment, and the methods for performing inspections to detect cracks or delamination. It indicates that a chemical compound should be used to strip paint from the surface to facilitate inspection.
Maintenance Records
Review of maintenance records indicated that inspections were performed as required; however, no cracks were detected prior to the accident.