The pilot reported that he descended to approximately 150 ft AGL as he was approaching Bellburn airstrip, and decided to overfly the windsock. He noted that the wind was an easterly breeze of about 7 knots. The pilot then intended to carry out a left turn, fly a downwind leg, and terminate into wind. He then rolled on about 50 degrees of bank, and shortly afterwards the helicopter began shaking violently. The helicopter then rotated through 180 degrees and adopted a nose down attitude. The pilot said he applied full power and full collective control, but was unable to prevent the helicopter striking the ground.
A witness located near the parking area saw the helicopter bank steeply and then begin to descend. The tail rotor then hit a patio attached to an office building and the main rotor struck a tree just before the helicopter impacted the ground.