The AS350B2 Squirrel helicopter was being operated on a private flight with the pilot and two passengers on board. The pilot reported that shortly after lifting the helicopter into an approximately 1.2 metre hover, he noted that the main rotor system had a pronounced vertical, once per revolution, vibration. The pilot then elected to terminate the hover and land the helicopter. He further reported that when the skid landing gear touched the ground, the helicopter began to oscillate violently. The pilot then activated the emergency fuel cut-off. Subsequently, the engine and main rotor revolutions per minute (RPM) began decreasing. The pilot and passengers reported that the oscillations of the helicopter became more violent and pronounced as the main rotor RPM decreased. Once the main rotor ceased rotation, the occupants exited the helicopter. One passenger received minor injuries.
The helicopter sustained substantial damage to the main rotor assembly, the right landing gear skid, the forward cargo mirror mount bracket, the left and right structure keel beams, and the right rear passenger seat support. The principle damage to the main rotor assembly consisted of the fracture and separation of the yellow and blue starflex rotor arm outboard segments.
An examination of the helicopter main rotor head and blades did not reveal any anomalies, other than the separated starflex rotor arm outboard segments, that could have resulted in the vertical vibration reported by the pilot. A witness near the helicopter during its hover flight did not report any foreign objects or birds in the area of the main rotor disc during the flight.
The damage to the main rotor starflex rotor assembly was consistent with the damage documented in a technical report compiled by the Australian Defence Science and Technology Organisation relating to a previous military AS350 helicopter occurrence. That report indicated that the starflex rotor arms failed due to severe upward bending due to excessive loading. That investigation determined that the circumstances of the accident were consistent with a ground resonance event.
No evidence was found to suggest that the helicopter was not serviceable prior to the occurrence. No mechanical issues could be established as contributing to the occurrence. The reason for the anomaly that led to the vertical vibration reported by the pilot could not be established. The excessive loading indicated by the starflex rotor arm fractures would not be expected to be encountered under normal service conditions, but could occur during a ground resonance event. The damage to the helicopter components was consistent with damage previously documented as a result of a ground resonance event.