On April 12, 2003, about 2030 central daylight time, an Aero Commander S2R, N5519X, piloted by a commercial pilot, sustained substantial damage when it nosed over during a forced landing near Benson, Minnesota. The airplane was spraying a plowed field when the engine lost power at the start of a spray run. The pilot landed in the field and the airplane subsequently nosed over. The 14 CFR Part 137 aerial application flight was operating in visual meteorological conditions without a flight plan. The pilot was not injured. The flight originated from the Benson Municipal Airport, Benson, Minnesota, about 1900. An examination of the airplane's engine was conducted by a Federal Aviation Administration inspector. The engine was able to be rotated. Valve function and "thumb" compression were verified. The magneto timing was checked and the fuel screens were examined with no anomalies noted. No deficiencies were found that would have precluded normal engine operation.