On September 4, 1996, about 1400 mountain daylight time, a Bell 206L-3 helicopter, operated by the U.S. Forest Service, collided with trees during an uncontrolled descent near Sheridan, Wyoming, and was destroyed. The commercial pilot, the sole occupant, received minor injuries. Visual meteorological conditions prevailed and a company flight plan had been filed. The public use flight departed from Sheridan in support of forest fire suppression and was conducted under 14 CFR 91. According to an FAA aviation safety inspector from Casper, Wyoming, the helicopter was operating at an altitude of about 8,200 feet above mean sea level (msl) near the vicinity of a forest fire at the time of the accident. The inspector stated that the pilot was in the process of dropping water from a bucket that was sling-loaded from the helicopter when the helicopter began to rotate to the right. The pilot attempted to stop the rotation by reducing power and collective. The helicopter then descended and impacted trees during the recovery attempt. According to the pilot, he had filled the bucket of water about 70 percent full and was returning to an area that he was working in support of fire suppression. He stated that he was in radio contact with ground personnel when he was told that they "needed water badly" in another area because the fire "had slopped over the line at their location." The pilot further stated: Because of smoke in the location I had difficulty seeing the target area. I asked the contact on the ground to guide me over their position. At this time, I received a radio call telling me I was passing over the target.... I was heading parallel to the fire and downwind. In order to line up on the target and approach upwind I made a 20 % turn to my left (up slope) and began a [right] hand race track pattern to line up on the target. As I was completing my right turn and as the nose of the [helicopter] was coming into the wind I experienced a severe yaw to the right. I applied full left pedal to correct the yaw. The pedal application did not slow the turn to the right. I jettisoned the water from the bucket. The turn continued to [accelerate]. When the [helicopter] had entered the third 360 [degree rotation] I entered autorotation to stop the spin. As I was 150 [feet above the ground] when the [helicopter] came out of the spin I had only enough time to [flare] and cushion the [helicopter] into the trees. According to the U.S. Forest Service, "the wind was up to 15 knots with gusts to 25 knots, according to a fire behavior analyst stationed at a nearby ridgetop" and "smoke conditions reduced visibility considerably in some areas." The pilot also reported severe turbulence in the area just prior to the accident. The Safety Board estimated that the density altitude at the accident site about the time of the accident was about 11,600 feet. The U.S. Forest Service stated that the pilot is "highly experienced in helicopters, but at the time of his assignment to the mishap helicopter had no recent mountain flying experience nor any firefighting experience." The pilot reported that he had accumulated 6,473 hours of total flight time in helicopters, including 890 hours in type. According to the FAA aviation safety inspector from Casper, no level clearings were found near the accident site, and the accident site was surrounded by heavily-wooded, mountainous terrain. An examination of the wreckage by the inspector did not reveal any evidence of preimpact mechanical deficiencies.