On October 22, 1993, about 1530 central daylight time (CDT), a Hiller UH12E helicopter; N4026S, was destroyed when the left skid hit the ground, and the aircraft caught fire in Sioux Rapids, Iowa. The Aerial application flight, originating from Webb, Iowa, was operating under 14 CFR Part 137 in visual meteorological conditions with no flight plan on file. The commercial certificated helicopter pilot reported he was in the process of spraying a pasture and was traveling north for a pass up a sloping valley when the engine began to run rough and started losing power. Because of the terrain, the pilot decided he could not set the helicopter down at that point and was trying to continue up the hill to flat ground. The aircraft continued to lose power and the left skid made contact with the hillside, yawed to the right, rolled on it's right side and was destroyed by a post-impact, on-ground fire. The pilot escaped without any injury. The pilot stated he had sprayed several loads and had fueled the helicopter several times during the morning of the 22nd. He had not experienced any problems during the morning spraying. After lunch, he fueled the helicopter with about 20 gallons of fuel, conducted a walk around, and drained the carburetor bowl in accordance with the pilot's operating handbook. About 10 minutes into the accident flight, he experienced the power loss. On scene investigation found the platform section 15 feet north and about 3 feet to the right of the initial ground scar. The platform section was laying on its right side with the nose pointed to the west and the main rotor shaft pointing to the north. The tail boom was intact but was twisted 90 degrees clockwise around the longitudinal axis at the seam just forward of the first universal joint of the tail rotor shaft. The spray tank which was mounted on the belly of the platform section had all the attach points broken. The chemical hoses were destroyed by fire and the tank separated from the platform and toppled down the hill approximately 25 feet and burned itself out. The fire that destroyed the helicopter burned an area of the pasture in an arch from about 15 feet west of the platform to 30 feet north of the platform to about 15 feet east of the platform. There is a diagram attached. The Pilot's Aircraft Accident Report was received July 15, 1994, was unsigned and the section "Narrative History of Flight" was not filled in. The lower end of the engine and engine mount was dislodged from its mount. All magnesium parts and a large portion of aluminum parts were melted down. The engine and transmission oil sumps, accessory case, magnetos, carburetor, fuel pumps, cooling fan, portions of engine case, cylinder heads, and portion of the transmission were totally destroyed by the fire. Because of the destruction caused by the fire there was very little documentation of various engine components and engine accessories that could be obtained. Of those controls that were not destroyed by the fire it appeared that they were all attached and safetied properly. The cockpit area and all instrumentation was destroyed. A review of the maintenance records indicate that the aircraft received an annual inspection 09/20/93. In addition to the inspection, various components were replaced on that date with freshly overhauled units including the engine. There were approximately 49 hours accumulated flight time since the overhaul of the engine, major components, and the annual inspection.