DEN05LA137

According to the pilot, he was maneuvering at approximately 25 feet agl. The pilot stated that he heard a grinding noise and the "helicopter yawed to the right a couple of times." He stated that the engine began to overspeed and he initiated an autorotation. The pilot stated that the helicopter impacted the ground hard, resulting in substantial damage. An examination, revealed that all six shearbolts had failed. Examination of the fracture faces of the six shear bolts revealed signatures consistent with a shearing action. The examination of the helicopter airframe, engine, transmission, and remaining systems revealed no anomalies.

On September 13, 2005, approximately 1150 mountain daylight time, a Bell 47G-3B-1, N24SD, operated by Helicopter Roundup Services LLC, and piloted by a commercial pilot, was substantially damaged when it impacted terrain during an autorotation near Rock Springs, Wyoming. Visual meteorological conditions prevailed at the time of the accident. The aerial herding flight was being operated under the provisions of Title 14 CFR Part 91 without a flight plan. The commercial pilot and his passenger were not injured. The local flight departed Rock Springs, approximately 1100.

According to the Pilot/Operator Aircraft Accident Report form submitted by the pilot, "he was maneuvering near the trap with some horses at approximately 25" feet agl. The pilot stated he heard a grinding noise and the "helicopter yawed to the right a couple of times." He stated that the engine began to overspeed and he initiated an autorotation. The pilot stated that the helicopter impacted the ground hard, bending the landing skids and rocking the helicopter forward. The main rotor contacted the tail boom, separating the tail rotor and tail skid from the helicopter.

The helicopter was recovered and relocated to a hangar in Boise, Idaho, for further examination. The helicopter airframe, engine, transmission, and remaining systems were examined under the auspices of an airworthiness inspector from the Federal Aviation Administration, Boise, Idaho, Flight Standards District Office. During the examination, it was noted that all six shearbolts had failed. These bolts were sent to the NTSB laboratory for further examination and testing. The examination of the helicopter airframe, engine, transmission, and remaining systems revealed no anomalies.

Examination of the fracture faces of the six shear bolts revealed a thin crescent shaped portion with surface lines aligned in the direction indicated by the tips of the crescent. The adjacent surface displayed a rough grainy surface also displaying surface lines aligned in the direction indicated by the tips of the crescent. The shank of the bolt adjacent to the fracture face had been deformed in the direction indicated by the tips of the crescent. The features are consistent with a shearing action. Information received from the aircraft manufacturer revealed that the shear bolts should be heat treated to a hardness of 91-98 HRB (Hardness, Rockwell, "B" scale). The hardness of all the shear bolts satisfied the hardness requirement.