On April 30, 1994, at 1229 central daylight time (cdt), a Piper PA-24-250, N7605P, registered to James Webb Sales of Dayton, Ohio, and piloted by an instrument rated private pilot, was substantially damaged during an off airport forced landing. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 91 flight had been operating on an IFR flight plan. The pilot reported no injuries. The flight departed Dayton, Ohio, at 1147 eastern daylight time. Before departure, the pilot obtained a full weather briefing from the Federal Aviation Administration (FAA) Flight Service Station (FSS). During the briefing the pilot was advised that an advisory for moderate rime or mixed icing had been issued for his route of flight. The advisory said that icing was possible between the freezing level and 20,000 feet mean sea level (msl). The pilot asked what the freezing level was for his route of flight and was told that it was "...probably about nine." The FSS weather briefer advised the pilot that the air temperature near Green Bay, Wisconsin, was minus three degrees centigrade at 6,000 feet msl. The Area Forecast for Chicago called for a mixture of rain and snow after 1800 cdt. While en route the pilot called the Chicago Flight Watch at 1130 cdt. He was advised that there was level one rain showers from his present position to his destination. The briefer told the pilot that there were strong winds from the east and northeast. The briefer asked the pilot if he were "...familiar with the advisory for icing at four thousand and above." The pilot replied that he was, and asked for weather information related to Madison, Wisconsin, and Rockford, Illinois. The pilot was advised that the surface temperature at the Greater Rockford Airport was 39 degrees Fahrenheit with light rain. He told the pilot that a turboprop airplane reported "...light mixed icing during (its) climb off Rockford above 2,200 feet... ." The briefer told the pilot that Madison's airport was reporting light snow and fog. As the pilot concluded his conversation with Flight Watch at 1132 cdt, he reported an "...outside temperature [of] plus eight degrees centigrade... ." At 1133 cdt the pilot reported back to the FAA Air Route Traffic Control Center (ARTCC). The pilot requested an amended clearance to Rockford, Illinois. The ARTCC controller issued a clearance to Madison, Wisconsin. According to air traffic control transcripts, at 1212 cdt, the pilot "...attempts contact with the ARTCC sector and is told to standby... ." The controller handling N7605P stated, "I was working the PLANO sector during a busy ORD (Chicago O'Hare International Airport, Chicago, Illinois) arrival rush. During a break in the transmissions N7605P attempted to talk to me, and [I] requested N7605P to standby." At 1213 cdt the pilot called ARTCC and told the controller that he was descending to 3,000 feet due to icing. The controller contacted Rockford Airport's control tower cab radar controller at 1214 cdt. The radar controller was told that N7605P's pilot reported he had encountered icing conditions. N7605P's pilot contacted the radar controller at 1217 cdt. The controller responded by giving the pilot an altimeter setting. The pilot asked approach for clearance to land at Rockford and was given instructions for the ILS runway 01 at the airport. During the resulting conversation, the pilot requested a lower altitude and was told that the lowest he could be cleared to was 2,700 feet msl. The pilot responded by saying, "Okay, down to two point seven, hope it helps." The pilot stated that after contacting Rockford approach control, "...my engine began to run rough. I pulled the carb heat which helped some... . Finally the engine seemed to smooth out, and I put the carb heat back in. A minute or so later, [the] engine [ran] rough again. [I] applied carb heat for several minutes. Then engine quit." Approximately nine minutes after contacting approach control, the pilot reported that the airplane's engine was losing power and becoming intermittent. He advised the approach controller that he was descending without power. The pilot stated he "...broke out of the clouds at about 800 feet and landed in soft muddy corn field." A written statement from the pilot revealed the airplane began to pick up ice on the wings "...about 20 miles from Rockford... ." The ice accretion stopped after the pilot descended to 3,000 feet msl. He stated he heard three other airplanes being handled by Rockford approach control, "...none complained about icing." According to an icing probability curve serious carburetor ice is probable at cruise power with a given temperature/dewpoint of 35 degrees and 34 degrees F. respectively. Post accident inspection of the airplane and engine failed to disclose any mechanical failure or malfunction which would have resulted in the loss of engine power.