On July 16, 1998, at 2205 hours Pacific daylight time, an Air Tractor AT-400, N2373V, struck a telephone wire and made a forced landing in a tomato field near Tranquillity, California. The aircraft, operated by Western AG Aviation under 14 CFR Part 137, was on an aerial application flight and sustained substantial damage. The commercial pilot was not injured. Visual meteorological conditions existed for the night flight and no flight plan was filed. The pilot stated that this was the first flight at this field, and that he had not visually checked the field before going to spray it. He stated that he had made one pass from the north to the south to check for wires, and did not see any. On his second pass, he saw the steel cable strung between poles, but he was flying right into it and believed it was too late to pull up, so he pushed the nose of the aircraft down to fly underneath the wire. The aircraft was equipped with a wire deflector that hit a telephone wire underlying the steel cable. The pilot stated that the wire deflector broke, as well as the vertical stabilizer. The aircraft started to rotate to the left, as if it were going to roll over, and to stop that from happening he "pushed the nose into the ground," and landed immediately. In an interview with a Federal Aviation Administration inspector from the Fresno, California, Flight Standards District Office, the pilot reported that he flew under a power line, but did not see that there was a telephone line approximately 3-4 feet lower. The rudder of the aircraft struck the telephone line and he made a forced landing. The pilot did not note any mechanical malfunctions with the engine. The night was dark with no moon.