Aerospatiale AS.350B2, VH-WFX

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.