On April 14, 2008, about 1038 mountain daylight time, an Artic Aircraft Corporation S-1B2 single engine airplane, N73WY, collided with terrain while maneuvering at low altitude near Rock River, Wyoming. The aircraft was owned and operated by TS Aviation, Worland, Wyoming, and was being used for survey of geese for the Wyoming Department of Game and Fish at the time of the accident. According to the Director of Operations for the company, TS Aviation does not have a 90-day exclusive use contract with the State of Wyoming, and was operating the aircraft under the provisions of 14 CFR Part 91 of the Federal Aviation Regulations and not as a public-use operation. The airplane sustained substantial damage. The certificated commercial pilot incurred serious injuries while the passenger, a biologist employed by the Wyoming Department of Game and Fish, sustained minor injuries. Visual meteorological conditions prevailed and a company flight plan was filed. The flight departed Worland at an undetermined time for the survey mission. The pilot made consistent statements to various law enforcement, medical response personnel, and the company that he has no memory of the accident sequence. The biologist was interviewed by a Federal Aviation Administration inspector, and he submitted a written statement. The biologist said they had been flying about 100 feet agl following a creek over low rolling terrain while he observed and took a census of the geese population. On previous flights, the pilot had flown too fast for the biologist to accurately count the birds of interest and he asked the pilot to fly slower. On this flight, the pilot extended the flaps and was flying slower than on previous flights at a speed he estimated at 50 to 60 miles per hour. They were following the creek and the pilot was in a turn to the right and then he turned back to the left. The biologist was looking away when he then heard the pilot call out an expletive. When he looked forward, the airplane was in a right wing and nose down attitude descending toward the ground. The pilot managed to level the airplane out just before ground impact. The biologist said he did not hear a change in the engine sounds during the sequence and had not heard the pilot voice any concerns about the way the airplane was handling.