On September 2, 1998, at 1050 central daylight time (cdt), a Cessna 172N, N6703E, piloted by a commercial pilot, was substantially damaged during a forced landing following a total loss of power during cruise flight. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 91 Civil Air Patrol aerial observation flight was not operating on a flight plan. The pilot and passenger reported no injuries. The flight departed Hutchinson, Minnesota, at 0740 cdt. According to the pilot's written statement, the pilot said he was preparing to land at an airport and was doing the landing checklist. He said the engine began running "...very rough..." when he pushed the mixture control in. The pilot said he immediately applied the carburetor heat "...but the engine continued to run rough." He said he switched the fuel tank selector to the wing's "RIGHT" fuel tank. "[The] engine smoothed out momentarily and quit." The pilot said he made a forced landing in an alfalfa field that had about a 15-degree downslope incline. The pilot said the right wing had about "...1-quart of fuel in the right tank." He said he didn't look in the left tank but could hear "...fuel sloshing around..." in the tank when he rocked the wings. Before departing on the accident flight, the pilot said another pilot had filled the fuel tanks. The accident pilot said he told the other pilot to fuel the accident airplanes fuel tanks to "...the filler neck and watched as he did." The airplane did not fly after it had been refueled until the accident flight. During the accident flight's preflight inspection the pilot said he did not visually check the fuel tanks. He made sure the fuel caps were on tight and noted that both fuel tank gauges showed "FULL." The Federal Aviation Administration Principal Maintenance Inspector (PMI) said he did not observe fuel in N6703E's fuel tanks.