Rotorway A600 N602RW accident near Chandler, Arizona

Probable cause

A design modification that changed the fasteners and components used to attach the main rotor drive pulley to the engine, which resulted in fatigue failure of those fasteners and a complete loss of power to the main rotor.

NTSB analysis

The flight instructor and a helicopter-rated pilot were conducting the flight for the purpose of transitioning the pilot to the piston-engine experimental helicopter. About 30 minutes into the flight, while flying straight and level, the pilots felt a jolt followed by loud noise and vibration. The flight instructor reduced power and began looking for a place to land. Erratic engine tachometer indications and additional power reduction then prompted the flight instructor to enter an autorotation for a forced landing. The helicopter had some forward velocity at touchdown on the firm, smooth ground. The skids dug in and the helicopter rolled over. Both pilots were uninjured.

The helicopter used a system of pulleys and sprockets to enable the engine to drive the main rotor at the desired rotational speed. The accident helicopter incorporated a design change for the main drive pulley attachment to the engine flywheel. The original design used three 3/8-inch diameter socket-head bolts to attach the pulley to the flywheel and one other drive component; the revised design used four 1/4-inch diameter cross-slot (Phillips) screws to attach the pulley only to the flywheel. Postaccident examination of the components revealed that the four attach screws had failed due to fatigue. Laboratory examination of other screws from the same manufacturing lot indicated that the screws were in compliance with their design specifications. The failed screws had about 20 hours in service. The torque that could be applied to the cross-slot screws was limited by the slippage of the screwdriver in the screw head, which in turn limited the preload on the screws and the induced friction in the pulley-to-flywheel joint. Those conditions contributed to the cyclic loading of the fasteners, which then resulted in their fatigue failure. Subsequent to the accident, the manufacturer reverted to its original flywheel-pulley attach method and replaced the newer design pulley assemblies with the original configuration.

Text from the NTSB. Open the NTSB docket for WPR11LA167 for the full record.