On August 18, 2006, about 1645 central daylight time, a Cessna A188B, N9224R, piloted by a commercial pilot, was substantially damaged during a forced landing in a wheat field following a loss of engine power while performing an aerial-application flight near Rolla, North Dakota. Visual meteorological conditions prevailed at the time of the accident. The flight was operating under the provisions of 14 Code of Federal Regulations (CFR) Part 137 without a flight plan. The pilot reported no injuries. The local area flight departed the Rolla Municipal Airport, Rolla, North Dakota, about 1600. The pilot reported that during an aerial-application turn, the engine lost power and the windshield was sprayed with oil. The pilot performed an immediate forced landing into a nearby wheat field. The airplane was damaged when it encountered the wheat crop, spun around, and impacted terrain. The on-site investigation revealed that the engine crankshaft had fractured and the propeller had separated from the engine. The propeller was never located and is believed to be in a nearby slough or a densely wooded area. The engine was disassembled and the fractured crankshaft was examined by the National Transportation Safety Board Materials Laboratory in Washington, D.C. The crankshaft fracture was just forward of the oil slinger flange. Fracture features and deformation associated with the fracture were typical of an overstress bending failure. The final area of fracture was at the region with the greatest bending deformation. The location of the fracture initiation was not diametrically opposite from the final fracture region, consistent with some amount of clockwise (as viewed from the aft) rotation during the fracture. The oil slinger flange had a circumferential crack at the base of the flange, around approximately 1/3 of the circumference of the flange. The cracked portion was deformed forward relative to the remainder of the piece. The crack in the oil slinger flange was approximately opposite the fracture origin area. One of the thrust washer halves contained impact and smearing damage on one face, consistent with hard contact from the oil slinger flange. The overstress bending fracture of the crankshaft and the crack and the deformation in the oil slinger were consistent with ground impact. During impact, the large aft load on the propeller generated bending stresses in the crankshaft and forced the crankshaft aft, cracking the oil slinger flange.