Overview
A helicopter under contract to the U.S. Forest Service collided with a tree while flying over a mountain ridgeline in Idaho. The aircraft was assigned to Krassel Helibase (24K). The pilot was transporting a relief Fire Lookout to a lookout tower on Williams Peak, approximately 4 nautical miles away with an elevation gain of 2,844 feet. Several forest fires were active in the area.
Flight Details
The relief Fire Lookout reported that visibility in the lower valley was about 1 nautical mile but improved as the helicopter ascended the ridge. Two Forest Service personnel were on board to assist with loading and unloading equipment, supplies, and refuse from two open-top external cargo baskets secured to the helicopter's skid gear. Cargo was held in place by multiple bungee cords. The return flight would have been flown directly into the late afternoon sun.
Accident Sequence
No witnesses observed the accident. Evidence on the ridgetop included 13 cubitainers (5-gallon plastic water containers), two large plastic refuse bags, yellow crepe paper, and several chips of green helicopter paint near a freshly downed, uprooted snag (a dead, defoliated conifer) estimated to be 90 feet tall. A tail rotor paddle that had separated at its cuff was found further down the mountainside. The burned wreckage of the helicopter was located on a Forest Service road approximately 2,000 feet down the mountain, with a 1,230-foot elevation loss from the initial impact point.
Postaccident Findings
Examination of the engine revealed that the power turbine blades were sheared from their disc, indicating an overspeed event. This would occur with a main rotor blade strike or sudden stoppage, compromising the engine-to-main rotor power train. Additionally, 11 of the containers that had been in one cargo basket exhibited 3- to 9-inch slashes consistent with tail rotor contact.
Interviews
Interviews with four Helitack crew members assigned to the helicopter indicated that the pilot occasionally engaged in "showy" flying but would always ask passengers if they were comfortable. He would sometimes "buzz" a ridge and perform a maneuver he called the "sleigh ride," described as topping a ridge, then dropping the collective and lowering the nose. No preimpact engine or airframe anomalies that might have affected performance were identified.