While undertaking a demonstration autorotational descent during an instructional flight test in a Robinson R22 Beta II helicopter, the student pilot and flight instructor noted an unusual mechanical noise, followed by the onset of severe vibrations from the main rotor system. After immediately landing the helicopter, it was found that the skin from the underside of one main rotor blade had disbonded from the leading edge spar over a length of approximately 450 mm from the blade tip.
The skin separation was found to be associated with abrasion and loss of the rotor blade leading edge paint across the bond line between the skin and leading edge spar. Erosion along the bond line had produced an undercutting effect and a feathering of the skin edge. Associated with random voids and pores in the adhesive that filled the gap between skin and spar recess edges, it was probable that the erosion had produced localised stresses within the adhesive joint, promoting the lifting of the feathered edges and the subsequent peeling separation of the skin.
As a result of a number of similar failures in both R22 and R44 main rotor blades, the helicopter manufacturer published a series of safety alerts, service letters and service bulletins, recommending the regular inspection of the blades for evidence of skin disbonding and the refinishing of blades showing abrasion of the leading edge paint to, or beyond, the skin bond line. Airworthiness directives from the US Federal Aviation Administration and the Civil Aviation Safety Authority subsequently mandated the initial and repeat inspection of R22 and R44 main rotor blades for this issue. Those airworthiness directives became effective in January 2008.
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