On March 25, 1995, at 1400 central standard time, a Hughes 369HS helicopter, N3961H, sustained substantial damage during an autorotation approximately 11 miles north of Burnet, Texas, following a loss of power. The pilot and two passengers received no injuries and no flight plan was filed. Visual meteorological conditions prevailed for this local area sightseeing flight. The flight departed Llano, Texas, 22 minutes prior to the accident. The pilot was providing air tour rides at an air show at Llano and was in need of fuel for the helicopter. He had less than 10 gallons of Jet A on board and Llano did not have jet fuel. According to available information, the pilot added 15 gallons of 100 LL aviation gasoline and took off for Burnet where he could get jet fuel. Approximately 22 minutes into the flight, according to the pilot, a partial power loss occurred and the pilot conducted a full autorotation which terminated in a "hard landing." According to the aircraft operating manual, a mix of jet fuel and aviation gasoline is allowed; however, when this is done the boost pump must be used during flight due to possible vaporization of the aviation gasoline upstream from the fuel control. The pilot did not utilize the boost pump. An examination of the engine was conducted at the facilities of the manufacturer. No evidence was found of preimpact damage which would have prevented the engine from operating.