On October 26, 2001, at approximately 1225 mountain daylight time, an Irwin Thorp T-18, N2819L, was substantially damaged when it impacted terrain during a forced landing near Longmont, Colorado. The airline transport pilot, the sole occupant on board, was not injured. Visual meteorological conditions prevailed and no flight plan was filed for this local test flight being conducted under Title 14 CFR Part 91. The flight originated at Vance Brand Airport, Longmont, Colorado, at approximately 1100. According to the pilot, he was conducting a test flight to do a further "run-in" on the engine, which had 3.0 hours since major overhaul. He stated that the engine ran smoothly and that all pressures and temperatures were normal. When the airplane was approximately 5 miles northeast of the airport, the engine "temporarily faltered." At that point, the pilot was able to reestablish power by manipulating the mixture control lever. He was concerned about his position over a densely populated area and modified his approach by circling south of the airport. At approximately 4 miles south of the airport, the engine began to run rough and quit, "similar to a fuel starvation failure." The pilot was unable to regain engine power and he landed the airplane in a field. During the landing roll, he applied the brakes, and the airplane tipped nose down. The airplane sustained damage to the propeller, left flap assembly, left main landing gear and substantial damage to the rear spar of the left wing. The pilot stated, "the problem was definitely fuel related," but the fuel samples taken prior to the flight and after the recovery of the airplane revealed no evidence of fuel contamination. Weather conditions at the time of the accident, obtained from Jeffco Airport, Broomfield, Colorado, located approximately 14 statute miles to the southwest, were: airfield elevation, 5,670 feet mean sea level; temperature, 12 degrees Celsius; dew point, minus 8 degrees Celsius; altimeter setting, 30.51. The calculated density altitude was 6,102 feet above mean sea level.