WPR14GA281 · American Eurocopter Corp AS350B3 B2

The pilot was positioning the helicopter with an external load into a mountain supply drop zone located along a barren ridge at an elevation of about 8,600 feet mean sea level (msl). The helicopter had completed numerous drops that day; however, the accident drop was the first since the drop zone had been moved about 100 yards downslope, on the leeward side of the ridge. The helicopter was using a 100-foot long line with a 972-pound external load. According to the pilot, as the helicopter approached the drop zone and the load was about 10 ft above the ground, the helicopter started swaying left and right, suddenly dropped in altitude, and began rotating counterclockwise. The pilot attempted to stop the uncommanded left yaw with right pedal input; he also increased collective to stop the sudden loss of altitude. The pilot set the load down and attempted to release it; however, the long line did not completely release from the hook assembly, most likely due to the helicopter's rotation. The helicopter continued to rotate to the left and descend, despite the pilot's continued application of right pedal and collective. It landed hard and rolled over onto its right side, resulting in substantial damage. The engine could be heard winding down, and a small fire erupted out of the engine compartment. Ground personnel assisted the pilot's egress, and the fire extinguished itself. No preaccident malfunctions or anomalies were identified with the helicopter, and the engine had been operating at maximum power during the accident sequence.

Wind observations taken by the ground crew on scene showed surface wind speeds between 5 and 15 knots from the southwest to west at the time of the accident. The wind just above the surface, however, was likely gusting between 5 and 25 knots as a result of an unstable atmospheric layer between the surface and 25,000 feet msl. As the wind flowed down the leeward side of the ridge, the air followed the contour of the terrain and became increasingly turbulent, resulting in downward pressure and reverse wind flow (eddy). The irregular wind conditions, high operating altitude, and the high-power, low airspeed state of the helicopter, were conducive to the development of a loss of tail rotor effectiveness, which subsequently resulted in a loss of directional control.

Verbatim NTSB analysis and probable cause from the NTSB dataset