On September 5, 1998, about 1117 eastern daylight time, a Grumman AA-5B Tiger, N28395, registered to a private individual, operating as a 14 CFR Part 91 instructional flight, made a forced landing to a grass field following a loss of engine power near Ridgeway, South Carolina. Visual meteorological conditions prevailed and no flight plan was filed. The airplane received substantial damage and the private-rated pilot and a noncurrent instructor/safety pilot were not injured. The flight left Columbia, South Carolina, about 1036. The pilots stated, having finished performing practice instrument approaches at Columbia Metropolitan Airport, and en route to their home base in Lancaster County, South Carolina, in cruise flight at an altitude of 2,500 feet msl, they experienced a gradually worsening rough running engine. They performed emergency procedures for a rough running engine, including use of carburetor heat, turned toward the Winnsboro NDB, colocated at Fairfield County Airport, Winnsboro, South Carolina, the closest airfield, and declared an emergency with Columbia ATC. They were given an immediate vector for Fairfield County Airport, reported to be 10 miles from their location, but the roughness and loss of power continued until they felt their only option was an emergency landing to a grass pasture short of the runway. They further stated that as they neared their touchdown point, they realized the grass was much higher that anticipated, and hid some rough spots in the terrain. Touchdown resulted in bouncing that did most of the damage to the airframe. Subsequent examination of the engine and airframe by FAA personnel revealed no obvious cause for the loss of engine power. All engine controls were checked for continuity and functioned normally. Uncontaminated fuel was confirmed contained in both fuel tanks, as well as the electric fuel boost pump and the carburetor. All components of the ignition system functioned normally. The engine was started and run to near takeoff power, on the airplane, with the fuel supply that was aboard, precrash. A carburetor heat and magneto functional check proved satisfactory. The engine was shut down and re-examined with no defects noted. The report of the FAA examination is included in this report under, "Reports from Other Federal Agencies". Reference to carburetor icing probability charts reveals that an ambient temperature of 86 degrees F and a dew point temperature of 68 degrees F, as reported by Columbia AFSS for the time period of the accident, would put the flight's probability well within the region labeled, "visible icing @glide and cruise power". A copy of the chart is included in this report under, "Other Pertinent Forms and Reports".