Incident Overview
A helicopter was destroyed when it encountered ground resonance during its first takeoff following an annual inspection. The aircraft had been reassembled with three overhauled main rotor blades installed. During the maintenance, the main rotor blade dampers were disassembled, inspected, and reassembled.
Findings
Post-accident examination revealed that during reassembly of the dampers, the internal sequence of friction discs was incorrect. This error resulted in improper and non-uniform damping action between the main rotor blades. Additionally, the shock strut oleos were not serviced during the inspection; after the accident, low pressure was found in all three remaining intact struts.
Sequence of Events
The helicopter was intended to perform a hover to track the main rotor blades, as is standard after such maintenance. During the first takeoff, the helicopter experienced ground resonance, a condition where the rotor system vibrates excessively in phase with the airframe. The resulting oscillations led to the destruction of the aircraft.
Probable Cause
The internal sequence of friction discs in the main rotor blade dampers was incorrect, causing improper and non-uniform damping action between the blades.