On July 21, 2003, about 1015 Pacific daylight time, a Rockwell International S-2R, N4250X, registered to and operated by Ag Northwest as a 14 CFR Part 137 aerial application flight, experienced a loss of engine power followed by a forced landing near Hermiston, Oregon. Visual meteorological conditions prevailed at the time and no flight plan was filed for the local flight. The aircraft was substantially damaged and the commercial pilot, the sole occupant, was not injured. During a telephone interview, the pilot reported that he had taken off for a spray application with an estimated one hour to one hour and 20 minutes of fuel remaining in the fuel tanks. The pilot reported that he utilizes a stop watch to keep track of the time to calculate fuel consumption. While returning to the airport for landing, the engine lost power about one mile short of runway 4. Unable to reach the runway, the pilot maneuvered to a field for a forced landing. During the landing roll on rough terrain, the aircraft collided with a rock that damaged the main landing gear and spray boom. The wing sustained substantial damage to ribs and the spar. The pilot reported that after the aircraft came to rest he stopped the stop watch which indicated one hour and three minutes. During aircraft recovery, there was no fuel present in the wing fuel tanks. The header fuel tank contained a small amount of fuel. On July 29, 2003, an Inspector from the Federal Aviation Administration Hillsboro, Oregon, Flight Standards District Office, witnessed an engine run test. The inspector reported that fuel was supplied to the system. The boost pumps were activated and fuel pressure was noted. The engine was then started and run for a few minutes with no anomalies noted before it was shut down.