On January 23, 1998, at 1325 Pacific standard time, a Piper PA 24-250, N8422P, operated by the pilot as a 14 CFR Part 91 personal flight, collided with the terrain and subsequently a fence during a forced landing in an open field two miles east of Sutherlin, Oregon. Instrument meteorological conditions prevailed at the time and an instrument flight rules flight plan was filed. The airplane was substantially damaged and the commercial pilot and his passenger were not injured. The flight had departed from Eugene, Oregon, about one hour and ten minutes prior to the accident. During a telephone interview and subsequent written statement, the pilot reported that while cruising at 11,000 feet in light cirrus clouds, the airplane began to pick up light rime icing. The pilot stated that he decided to turn around and return to Eugene. Air traffic control cleared the flight to descend to 8,000 feet. The pilot reported that while at 8,000 feet, the engine was unable to obtain more that 15 inches of manifold pressure and was running rough. The pilot notified air traffic control that he was experiencing engine problems and asked to descend further. Air traffic control cleared the flight to descend to 7,000 feet and when terrain permitted, the flight was cleared to descend to 6,000 feet. At 6,000 feet, the airframe ice began to melt, however, the engine continued to operate at partial power. The aircraft was unable to maintain altitude and after the flight cleared the clouds, the pilot selected an open field for a forced landing. The airplane touched down, gear-up, in a soft field. The airplane collided with a berm and barb wire fence before coming to rest. The aircraft was recovered from the field and transported to a maintenance facility in Roseburg, Oregon. The engine was cleaned to remove mud. It was determined that the engine was in a condition to be test run. The propeller was replaced, a battery was installed, and a fuel supply was rigged. A Federal Aviation Administration Inspector from the Portland, Oregon, Flight Standards District Office was present for the engine run. The inspector reported that the electric fuel boost pump was switched on, with positive results, and indicated a 20 PSI output. The electric boost pump was then switched off and the starter was engaged. The engine started without difficulty and idled normally. After the engine was warmed up, the engine was run-up to 1700 RPM and a magneto check was accomplished. The oil temperature, oil pressure, EGT, and suction pressure were all within normal parameters. The engine was then shut down and secured. The inlet air box was severely damaged during the accident sequence. The alternate air door would operate only part way open, however, the hinge was clear and free. The air filter was a foam Brackett filter that was in good condition, with no sign of deterioration or blockage.