On August 2, 1996, at 1630 central daylight time, a Beech 77, N18259, sustained substantial damage when it impacted a ditch and nosed over during a forced landing in South Elgin, Illinois. The solo student pilot reported a partial loss of engine power. He reported minor injuries. The local, personal flight originated about 1600 and was conducted under 14 CFR Part 91. Visual meteorological conditions prevailed and no flight plan was filed. The pilot reported that he performed a "normal" preflight and engine run up. He was at 1,700 feet mean sea level and initiated a climb. He advanced the throttle to "75 to 80 percent" and the "rpm dropped fast." He pulled the carburetor heat on and left it on for 30 to 45 seconds. He pushed the mixture to rich, tried each magneto separately, and turned the fuel pump on but the engine did not regain power above about 1,500 rpm. He reported that he flew a "regular pattern," and made a "beautiful soft field landing" in a field. During the landing roll, at an indicated airspeed of about 40 to 50 knots, the airplane impacted a ditch. The nose dropped, the airplane traveled over a road, slid about 25 feet and nosed over. Examination of the airplane revealed substantial damage to the right wing, fuselage, and vertical stabilizer. Approximately 5 gallons of clean blue fuel was drained from the right fuel tank and approximately 3 gallons from the left tank. The flaps were in the up position. The Federal Aviation Administration (FAA) Inspector who examined the airplane reported that the engine operated normally during a test run at low power settings. All engine controls functioned except the carburetor heat control was stuck in the off position. The air box carburetor heat valve exhibited evidence of impact damage and no evidence of preimpact malfunction. Approximately one to two tablespoons of water was drained from the carburetor bowl. Examination of the carburetor, exhaust system, magnetos, and fuel system revealed no evidence of preimpact malfunction.