On August 29, 1996, at 1620 central daylight time (cdt), a Piper PA-28-180, N8456W, operated by a commercial pilot sustained substantial damage when during cruise flight the airplane's engine lost partial power. The pilot subsequently attempted a precautionary landing in a hay field near Grant City, Missouri. During the landing the airplane struck a hay bale and nosed over. Visual meteorological conditions prevailed at the time of the accident. The personal flight was being conducted under 14 CFR Part 91. A VFR flight plan was on file. The pilot sustained serious injuries. The passenger reported minor injuries. The flight originated at Hays, Kansas, at 1320 cdt, and was en route to Ottumwa, Iowa. In his written statement, the pilot said that the airplane was in cruise flight at 5,500 feet mean sea level (MSL), and three hours into the flight, when he noticed that the oil pressure indicator read zero. "I scanned the area, looking for a satisfactory place for a forced landing. There was a circular hay field directly below of approximately 1000 feet in diameter, which contained numerous rolled bales of hay." The pilot closed the throttle, applied two notches of flaps and did a 720-degree turn to the left which eventually lined the airplane up on his intended touchdown point in the field. "I planned to land on an upgrade, which I did, halfway up. As I topped the grade, two hay bales appeared. I opened the throttle, with no response, hoping to clear the bales of hay." The airplane's right wing struck one of the hay bales and nosed over. In his written statement, the passenger on board the airplane said that he "noted that there was no indication of oil pressure and that the engine RPM had dropped a couple of hundred RPM, from approximately 2500 RPM to 2300 RPM." The passenger said that the engine was still operating, but from the sound and the zero oil pressure indication, his perception was that it the engine would not continue operating for much longer and that landing the airplane was imminent. The Federal Aviation Administration (FAA) inspector who examined the wreckage at the accident site found the airplane main wreckage resting inverted in a field. It was approximately 102 feet from a large round hay bale and oriented on a southwesterly heading. The airplane's right wing was broken off and resting 30 feet south of the main wreckage. The empennage was bent and twisted to the left. The horizontal stabilator was bent upward and aft. Fuel was observed dripping from the left wing tank. The propeller exhibited torsional bending. Flight control continuity was confirmed. Examination of the engine, engine controls and other airplane systems revealed no anomalies. Oil was observed on the ground beneath the airplane's inverted cowling. No signs of in-flight oil loss was observed. In a telephone conversation on September 11, 1996 at 1000 cdt, the pilot told the FAA inspector that the airplane's engine was consuming 1.5 to 2 quarts of oil an hour. The Lycoming O-360-A3A operator's manual states that the maximum oil consumption for this engine in a one hour time period at normal power (2,700 RPM) is .80 quarts.