On February 19, 1998, approximately 0900 Pacific standard time, a Grumman-Schweizer G-164B Ag-Cat airplane, N48493, operated by Skagit Ag Service of Mount Vernon, Washington, was substantially damaged when it nosed over in a forced landing in an agricultural field following a loss of engine power while engaged in 14 CFR 137 agricultural aerial application operations south of Stanwood, Washington. The commercial pilot of the single-seat agricultural aircraft was not injured. Visual meteorological conditions existed for the flight, which originated at Skagit Regional/Bay View Airport, Burlington/Mount Vernon, Washington, and no flight plan had been filed. The accident aircraft was equipped with a Pratt & Whitney R-985 radial engine. The operator reported that the accident flight was the first flight of the day, and that the airplane had about 2 hours of fuel on board. He stated the loss of power occurred as the aircraft was maneuvering for a pass over the field it was working. At the time of the power loss, the airplane had been airborne for about 30 minutes, and had been spraying for about 15 minutes. The pilot reported to an FAA inspector who responded to the scene that the throttle was set for his spray passes and was not adjusted from the time he began spraying until the occurrence; he told the FAA inspector that carburetor heat was not being used. The pilot subsequently noted a slow reduction in engine power. The pilot then advanced the throttle, at which point the engine "sputtered" and "popped" a few times, then stopped completely. The aircraft encountered soft terrain in the ensuing forced landing; the aircraft's wheels dug into the ground and the aircraft flipped over. An FAA airworthiness air safety inspector from the Seattle, Washington, Flight Standards District Office (FSDO) responded to the accident scene and conducted an on-scene examination of the aircraft. He reported that "visual inspection of the engine did not reveal any obvious abnormalities", and that "a small amount of cylinder compression resistance was observed when rotating the propeller." The FAA inspector reported that he found the aircraft's fuel tank about one-half full, and that the carburetor heat control in the cockpit was found in the fully in (off) position. The operator reported to the FAA that after the accident aircraft was recovered from the field, he decided to attempt a test run of the engine. The operator reported that during this test run, the engine ran "remarkably well", although engine speeds above 600 RPM caused a heavy vibration due to a slight bend in the propeller. The engine log book indicated that the aircraft's last inspection was an annual inspection in December 1997. At that inspection, the engine had 5,395.5 hours total time and 1,050 hours since major overhaul. The operator reported that the airplane had been flown 8 hours since the last inspection. The automated weather observation station (AWOS) at Arlington, Washington, 10 nautical miles southeast of Stanwood, reported a temperature of 7 degrees C (45 degrees F) and a dew point of 6 degrees C (43 degrees F) at 0857 on the day of the accident. The U.S. Naval Air Station at Whidbey Island, Washington, 13 nautical miles northwest of Stanwood, reported a temperature of 9 degrees C (48 degrees F) and dew point of 6 degrees C (43 degrees F) at 0855. According to FAA carburetor ice hazard prediction data, these combinations of temperature and dew point present a hazard of "serious icing at cruise power." The pilot reported to the FAA inspector on scene that a light drizzle was present in the area during the time frame of the accident flight. Neither the pilot nor the FAA reported that the pilot obtained a weather briefing prior to the flight.