On July 25, 1998, about 1400 Alaska daylight time, a skid equipped Enstrom F-28F helicopter, N8622U, sustained substantial damage during a forced landing about 30 miles south of Skwentna, Alaska. The helicopter is owned by Mark E. Miller, dba Talaheim Lodge and Air Service, of Skwentna. The helicopter was being operated as a visual flight rules (VFR) on-demand air taxi flight under Title 14, CFR Part 135, when the accident occurred. The certificated commercial helicopter pilot, and the one passenger aboard were not injured. Visual meteorological conditions prevailed, and company flight following procedures were in effect. The flight originated about 1330, at the operator's remote fishing lodge located about 5 miles south of the accident site. During a telephone conversation with the National Transportation Safety Board investigator-in-charge, on July 25, the pilot reported that he was conducting a salmon survey for the State of Alaska, Department of Fish & Game. This operation required him to hover the helicopter over the stream while the passenger, a wildlife biologist employed by the State of Alaska, noted the number of fish. The pilot stated that while in a hover above the stream, he felt a slight vibration followed by a partial loss of engine power. He said that while hover taxiing the helicopter to a suitable landing area, the engine continued to lose power, and he was forced to make a downwind autorotation. He said that as the helicopter touched down, the main rotor blades flexed downward, and struck the tail boom. The helicopter's tail boom, and tail rotor gearbox sustained substantial damage. The operator reported that during a postaccident inspection of the engine, a fractured intake manifold flange was discovered on the number three cylinder assembly.