On April 9, 2004, about 1319 Pacific daylight time, a Piper PA-38-112, N25645, collided with the airport perimeter fence after a loss of engine power during departure from South County Airport of Santa Clara County, San Martin, California. The owner operated the airplane under the provisions of 14 CFR Part 91. The commercial pilot, the sole occupant, was not injured; the airplane sustained substantial damage. Visual meteorological conditions prevailed, and a flight plan had not been filed. The personal local flight was originating at the time of the accident. In a written statement, the pilot reported that the airplane had not been flown for about 6 weeks. The owner and pilot completed a thorough preflight inspection of the airplane, with special attention to the fuel samples. The pilot was unable to remember when the airplane's fuel tanks were last filled, but opined that it was also nearly 6 weeks previous. The pilot reported the quantity of the fuel tanks as more than 1/2 full, and the fuel samples were clear of water and debris. The pilot operated the engine for about 20 minutes, and then performed a before takeoff check. The pilot stated that all engine and fuel system indications were normal. During initial climb after takeoff, the airplane's engine experienced a loss of power, resulting in a forced landing. The pilot attempted to land the airplane in an open field and collided with the airport perimeter fence. In a telephone interview with a National Transportation Safety Board investigator, an airport operations employee, and certified pilot, recalled witnessing the accident. The witness stated that he was performing maintenance on the airport perimeter when he heard sputtering from the airplane's engine. He watched the pilot perform a right turn and make a forced landing in a field. The witness added that during recovery while moving the airplane, he noted that one of the fuel tanks contained fuel. The other fuel tank was ruptured in the accident and he noted evidence of fuel leakage nearby. The owner of the airplane stated in a telephone conversation that before he purchased the airplane, an annual inspection was performed on May 30, 2003. During the positioning flight from the purchase location in Indiana, to the owner's home base in California, in September 2003, the owner requested additional maintenance after experiencing less than full power during flight. An engine inspection found deterioration in the muffler and a damaged magneto; both were replaced. The pilot operated the airplane an additional 25 hours between September 2003, and the day of the accident. No further maintenance was performed.