Accident Overview
During an aerial application flight, the pilot reported initiating a turn over power lines at about 60 feet above ground level. As the helicopter crossed the lines, a loud pop was heard accompanied by extreme vibration. The pilot maneuvered away and initiated a forced landing to an open field. Control became marginal during descent, and shortly before touchdown the helicopter drifted left uncommanded. The pilot lowered the collective, but the helicopter touched down hard, pivoted left, and rolled onto its right side.
Post-Accident Examination
Examination revealed that a control rotor had separated from the rotor system. The separated rotor was found in the area where the vibration began. The tubular spar of the blue control rotor was circumferentially fractured at the outboard cuff bolt location. Magnified examination showed surface features consistent with fatigue progression in two fronts around the spar. Fatigue initiation locations were identified on opposite sides of one outboard bolt hole. One origin was on the bore surface of the bolt hole; the other was mechanically damaged, preventing exact determination. The bore surface was rough with no obvious corrosion. The fatigue propagated almost entirely around the spar, with small overstress regions connecting to the opposite bolt hole. No corrosion was found in the originating bolt hole, but the exterior spar surface exhibited widespread pitting corrosion and fretting on faying surfaces with the cuff, and localized corrosion was noted on the interior faying surface of the cuff.
Maintenance History
Maintenance records indicated the most recent 100-hour inspection was conducted about 77 hours before the accident. The logbook entry for that inspection stated compliance with Airworthiness Directive AD 97-10-16, which requires repetitive inspections for corrosion or cracks in the blade spar tube and cuff, and in bolt holes for elongation, corrosion, burrs, pitting, or fretting, with repair as necessary.