Accident Overview
During a low-altitude fish survey, a helicopter experienced a separation of a control rotor while leveling off from a climb to approximately 400 feet above ground level (AGL). The pilot initiated an emergency descent; however, due to severely reduced cyclic controllability resulting from the separation, he was unable to avoid power lines in the flight path. The helicopter struck the power lines at about 50 feet AGL and fell to the ground, landing hard but upright.
Examination Findings
Post-accident examination disclosed a fatigue failure in the cuff that retains the control rotor blade. The fatigue originated on two opposite sides of a retaining bolt hole that appeared to have been unevenly hand chamfered or deburred during original manufacture of the cuff. Gouging and sharp-bottomed dents were visible at the origins of the fatigue.
Maintenance Issues
A company mechanic had signed off an Airworthiness Directive (AD) requiring recurring inspection of the cuff approximately two months and 56 flight hours before the accident. However, post-accident examination disclosed evidence indicative of inadequate inspection and maintenance of the assembly. This included old dried grease between the cuff and control rotor blade spar, large areas of the spar missing required paint with associated corrosion areas, and zinc chromate primer in corrosion pits. The FAA-approved Hiller service bulletin referenced by the AD contains procedures for inspection of spar tube retention bolt holes for elongation, corrosion, burrs, pitting, or fretting and associated repair procedures. However, it does not contain any instructions to inspect or repair bolt holes in the control rotor cuff for those same conditions.