On May 31, 2004, at approximately 1215 mountain daylight time, an Ayres Corporation S-2R, N5686X, was substantially damaged when it impacted terrain during a forced landing to a field approximately 1 nautical mile north of Beaver, Utah. The commercial certificated pilot, the sole occupant on board, was not injured. Visual meteorological conditions prevailed at the time of the accident. The local aerial application flight was being conducted under the provisions of Title 14 CFR Part 137 without a flight plan. The flight originated at approximately 1120. According to the pilot, he had finished spraying a field and was returning to Beaver Municipal Airport (U52), for another load. He said that he had half a tank of fuel remaining and the engine was producing "excellent power." A few moments later, the engine ran rough for about a second and then stopped. He did not notice anything coming out of the exhaust. The pilot tried twice to restart the engine but was unsuccessful and he attempted a forced landing to an open field. During the landing, the airplane impacted terrain and struck a fence. Both wings were displaced aft and the right main landing gear was collapsed. A visual examination of the engine revealed a 360 degree crack around the P3 air pressure tube, causing a separation of the tube between the engine plenum and fuel control unit. The crack was approximately one-quarter inch aft of the "B" nut, which connects the P3 air pressure tube to the fuel control unit. There was a "notable" amount of carbon build-up in the area around the crack. According to a maintenance representative, failure of the tube in flight would have resulted in a "significant reduction in fuel scheduling which could result in an engine flameout." An attempt to restart the engine in flight would "likely have failed," however; an attempt would have accounted for the "notable" amount of carbon build-up around the break in the tube. No other aircraft anomalies were noted.