On September 1, 2001, at 1825 central daylight time, a Cessna 172N, N5593E, operated by Midwest Flying Service Incorporated, was substantially damaged during a forced landing on a cornfield one mile northwest of the Sheldon Municipal Airport (SHL), Sheldon, Iowa. The pilot reported a rough running engine while on approach to runway 33 (4,199 feet by 75 feet, concrete). Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 91 personal flight was operating on a visual flight rules flight plan. The private pilot received no injuries and passenger received minor injuries. The flight departed from the Des Moines International Airport, Des Moines, Iowa, at 1705, en route to SHL. The pilot reported the following in a written statement, "...I was coming back from a cross-country trip originated from Des Moines, IA. Where we refueled full tanks. I believe I opened my VFR flight plan about 5:05 CST. ... I tried to get AWOS for altimeter setting, wind direction and velocity information when about 10 miles of the airport but I didn't work for some reason. So I decided to cross the field at higher altitude than pattern altitude as recommended to find out the wind direction from the windsock. (Refer to figure attached, #1). The wind was from about 180 degrees. I needed then to land on runway 15. So I went on heading 150 degrees close to the runway (#4), at point #5 the engine started to shake real bad, I decided to proceed with an emergency landing downwind. But I was too close to the runway, high and too fast to make it on runway 33. But still I tried (#6). At point #7, I decided to go-around realizing for sure that I wouldn't have enough runway to land safely. At point #8, at less than 400' AGL, the engine quit while I started a climb for a go-around. At that point all I could do was to put the plane in gliding attitude with appropriate angle of attack, correct airspeed and find a suitable field to land. The cornfield was my best choice (#9). We landed safely on the main landing gear, the plane rolled for a while; then the cornstalk resistance made the plane flips upside down..." During a postaccident telephone interview, the pilot stated that approximately 25 nm from SHL the engine began to run rough but thinking that it was carburetor ice he selected carburetor heat which stopped the engine roughness. He added that the engine began to run rough approximately 1/4 mile from the approach end of runway 33 and at an altitude of approximately 3,000 feet msl. He selected a flap setting of 20 degrees for a landing flap setting on runway 33. During the go around, the engine quit and the pilot landed on a reportedly dry corn field with approximately 6 foot tall corn stalks. The airplane nosed over and came to rest. SHL elevation is 1,419 feet msl. Advisory Circular 61-21A, Flight Training Handbook, "Emergency Approaches (Simulated)", states, "...Slipping the airplane, using flaps, varying the position of the base leg, and varying the turn onto final approach should be stressed as ways of correcting for misjudgment of altitude and glide angle. Eagerness to get down is one of the most common faults of inexperienced pilots during simulated forced landings. In giving way to this, they forget about speed and arrive at the edge of the field with too much speed to permit a safe landing. Too much speed may be just as dangerous as too little; it results in excessive floating and overshooting the desired landing spot..." The pilot reported a total flight time of 119 hours all of which were in the make and model of the accident airplane. Examination of the engine by the Federal Aviation Administration (FAA) revealed a missing exhaust valve from the number two cylinder. The valve was not found in the exhaust system or elsewhere in the engine. The piston dome was damaged but not punctured. The Lycoming O-320-H2AD, serial number RL-1767-76T, engine accumulated a total time of 1,595.6 hours since overhaul.