On March 10, 1998, at 1425 mountain standard time (mst), a Mooney M20C, N6235U, was substantially damaged during a forced landing, approximately 4 miles northwest of White River, South Dakota, following a total loss of engine power. The private pilot and one passenger reported no injuries. The personal 14 CFR Part 91 flight was operating on a special VFR flight clearance in instrument meteorological conditions. A VFR flight plan was on file. The flight departed Rapid City, South Dakota, at 1312 mst, with an intend destination of Sioux City, Iowa. According to the pilot's written statement, after receiving a special VFR clearance he departed Rapid City and maintained an altitude of 500 feet to 1,200 feet above the ground as he followed Interstate 90. The pilot reported that at 1415 mst, he adjusted the fuel mixture to correct a slightly rough running engine. He continued to reported that at 1425 mst, the "...engine ran rough for approximately 15 seconds; I pulled carburetor heat and left it on; the engine quit with the propeller windmilling; I switched fuel tanks back to the fullest tank(right)- no change with airspeed and altitude dropping, I searched for a place to land." The pilot also stated in the safety recommendation section that "...better scan of engine of engine instruments, especially manifold pressure regular application of carburetor heat." The pilot said he suspects it was carburetor ice that caused the engine to lose power. A Federal Aviation Administration Principal Maintenance Inspector (PMI), represented the NTSB on-scene. The PMI said the engine examination revealed after the air element was removed, the element did not have snow or ice on it. He then used a light and an inspection mirror to look inside the airbox where he found four pieces of ice lying inside of the airbox. Two pieces were the size of a quarter and two pieces the size of a dime.