What happened
Following an annual inspection, a helicopter was prepared for flight with the installation of three overhauled main rotor blades. During the initial takeoff attempt, which was intended to track the main rotor blades in a hover, the aircraft encountered ground resonance. The resulting forces caused the destruction of the helicopter.
The investigation
The investigation focused on the maintenance performed during the recent annual inspection. It was determined that while the main rotor blade dampers had been disassembled, inspected, and reassembled, the internal sequence of the friction discs was incorrect during the reassembly process. This error led to improper and non-uniform damping action between the blades.
Additionally, the investigation found that the shock strut oleos had not been serviced. A post-accident inspection of the aircraft revealed low pressure in the three remaining intact struts.