On October 21, 2000, at 0905 central daylight time, a Zenair CH2000, N420ZA, owned and piloted by an airline transport pilot, sustained substantial damage when it collided with a fence during a forced landing following a loss of engine power after takeoff from runway 31 (7,000 feet by 150 feet dry asphalt) at the Capital Airport, Springfield, Illinois. The 14 CFR Part 91 personal flight was operating in visual meteorological conditions. No flight plan was on file. The pilot, who was the sole occupant, reported no injuries. The local flight was originating at the time of the accident. In a written statement, the pilot said that about 450 feet agl, "...the engine started to run very erratic with an appreciable loss of thrust...." The pilot said that the engine was not producing enough thrust to maintain level flight. The pilot attempted a forced landing on an airport road that leads from the airport boundary to the control tower. The aircraft touched down short of the road in a plowed field and then traveled across a perimeter road and onto the road leading to the tower. The aircraft stuck a security gate and then a fence before coming to rest. The pilot listed no mechanical malfunction in his written report. A Federal Aviation Administration Inspector conducted a postaccident examination of the aircraft. The aircraft engine was examined and the nut that secures the carburetor heat control cable housing was found loose. According to the inspectors report, "... full travel of the carburetor heat valve could not be achieved." A postaccident engine run was conducted and the engine ran at idle power with "slight roughness". Engine speed was increased to 1700 rpm and a magneto check performed with the right magneto showing about a 150-rpm drop, and the left showing about a 125-rpm drop. The engine produced 2400 static rpm at full throttle. The weather report for the Capital Airport at 0854 reported the temperature and dew point as 13 degrees Celsius and 11 degrees Celsius respectively. According to the Carburetor Icing chart from Transport Canada, this temperature and dew point are in the range for serious icing potential at all power settings.