On February 26, 2000, approximately 1045 mountain standard time, a Mooney M-20C, N90MR, was substantially damaged during a forced landing near Boulder, Colorado. The instrument rated commercial pilot and his passenger were not injured. The airplane was being operated by Artell, Inc., of Broomfield, Colorado, under Title 14 CFR Part 91. Visual meteorological conditions prevailed for the cross-country personal flight which originated from Broomfield, Colorado, approximately 45 minutes before the accident. No flight plan had been filed. According to the pilot, he departed Jeffco Airport and flew to Longmont's airport for a touch and go landing. He then flew to Boulder for more practice landings. The pilot said that he entered Boulder's traffic pattern with a 45 degree track to mid-field downwind. When he got abeam the runway numbers, he pulled on the carburetor heat, reduced the throttle to 13 inches, put some flaps down, and lowered the landing gear. The pilot said he heard another pilot calling 2 mile final, so he continued flying an extended downwind to runway 26 at approximately 85 miles per hour (mph) while looking for the other airplane. After extending his downwind for approximately 2 miles, the other airplane flew by him on final. The pilot said that as he turned base to final, his engine began to steadily lose power. He checked and verified that his carburetor heat was pulled on, and then decided to go missed approach. He advanced the throttle to full, pushed the propeller full forward, and put the mixture full rich, however, the power continued to drop. The pilot performed a forced landing to a field, and the airplane impacted two fences and came to rest in a large irrigation ditch. The pilot reported that the temperature at the accident site was 42 degrees Fahrenheit, and the National Weather Service at Jeffco Airport was reporting a temperature of 43 degrees Fahrenheit and a dew point of 3 degrees Fahrenheit during the same time period. Although these weather conditions fall outside the carburetor icing conditions indicated on the carburetor icing chart (see attached chart), a review of the literature on carburetor icing indicates the unpredictable nature of this phenomenon. One source stated that carburetor ice could manifest from 20 degrees to 90 degrees with "high" humidity. Another source stated that carburetor icing could occur from 10 degrees to 100 degrees with relative humidity greater than 20 percent. The U.S. Department of Transportation FAA Flight Training Handbook states in the section under "Descents (Maximum Distance Glides)" that during "power-off descents, the engine should be cleared periodically to prevent excessive cooling and fouling." The engine manufacturer, in one of their Operator's Manuals, states that "carburetor heat is available only at engine outputs well above idle." The Federal Regulations Part 23.1093, which dictates requirements for airworthiness certification, states that an induction de-icing and anti-icing system must "provide a preheater which is capable of providing a heat rise of 90 degrees Fahrenheit when the engine is operating at 75 percent of its maximum continuous power." The engine was test run on its airframe. According to the manufacturer representative, who observed the test run, the engine "start up was immediate, and the engine ran smoothly."