Loss of control involving a Hughes Helicopters 269C, VH-DBL, Bankstown Aerodrome, New South Wales, on 4 June 1995

The helicopter was being landed at the completion of a training exercise. The weather was reported as being fine and clear, with little or no wind. During the landing, shortly after making a normal touchdown, the helicopter commenced to shake violently. The transition from normal operation was very rapid and did not allow the instructor time to take any remedial action before control of the helicopter was lost.

The most probable reason for the loss of the aircraft resulted from the rapid onset of ground resonance. This problem is associated with fully articulated rotor systems and is the result of geometric imbalance of the main rotor system. This imbalance of the rotor causes an oscillation which is transmitted throughout the entire helicopter, giving movement from side to side, as well as fore and aft. This action can become violent enough to cause the helicopter to roll over or incur major structural damage. Although not determined in this investigation, the onset of ground resonance can be aggravated by a number of factors, including incorrect landing gear strut inflation pressures, and incorrectly adjusted friction type blade dampers.