On June 24, 2008, about 1345 mountain daylight time, a Schweizer 269C, N432MS, collided with terrain during takeoff near Buffalo, Wyoming. Northern Skies Aviation, Inc., was operating the helicopter under the provisions of 14 Code of Federal Regulations Part 135. The certificated airline transport pilot and passenger were not injured; the helicopter sustained substantial damage. The local passenger-repositioning flight departed from a landing zone 22 miles east of Buffalo, just prior to the accident. Visual meteorological conditions prevailed, and no flight plan had been filed. In a written statement, the pilot reported that the purpose of the flight was to pickup a biologist who was conducting land surveys. He reported arriving at the landing zone and circling the area to assess the wind conditions. He noted the landing zone was within a 3/4-mile diameter bowl like depression. He landed to pickup the passenger and noted that he transitioned out of effective translational lift (ETL) earlier than expected. He attributed this phenomena to a reduced headwind caused by the enclosed terrain. He loaded the passenger and they departed; however, during acceleration to ETL speed the pilot reported that the rotor speed began to decay. He observed that his ground speed was faster than he anticipated. He lowered the collective, and added forward cyclic, but could not maintain enough rotor speed for sustained flight. He flew the helicopter 1/4 mile further trying to locate a safe landing area. With airspeed and rotor speed decaying, the pilot elected to land the helicopter ahead on a 30-degree uphill slope. After landing, the helicopter rolled back and the tail rotor made contact with the ground causing substantial damage to the tail rotor drive system. After the accident the pilot walked up an embankment above the accident site to observe the wind conditions. He noted that the winds had shifted 180 degrees from his initial pre-accident observation. The accident occurred at an altitude of approximately 4,300 feet mean sea level (msl). The closest official weather observation station was Johnson County Airport, Buffalo, located about 24 miles west of the accident site. The elevation of the weather reporting station was 4,968 feet msl. An aviation routine weather report was issued at 1353. It stated: winds from 010 degrees at 13 knots, gusting to 19 knots; visibility 10 statute miles; clear skies; temperature 28 degrees Celsius; dew point 12 degrees Celsius; altimeter 30.01 inches of mercury. The pilot stated that the helicopter and engine had no mechanical failures or malfunctions during the flight.