On October 26, 2003, approximately 1430 eastern standard time, a Schweizer 269C helicopter, N191GC, piloted by a flight instructor and dual student, was substantially damaged during a hard landing following a loss of engine power and subsequent autorotation. The accident occurred approximately two miles southwest of the Ann Arbor Municipal Airport (ARB), Ann Arbor, Michigan. The instructional flight was being operated under the provisions of 14 CFR Part 91 and was not on a flight plan. Visual meteorological conditions prevailed at the time of the accident. The flight instructor and dual student reported no injuries. The local flight departed ARB about 1420. In his written statement, the flight instructor reported that shortly after takeoff, the helicopter suddenly yawed to the left and the engine began to uncontrollably increase and decrease speed. He noted that it sounded as if the throttle was being rolled on and off. He added that no warning lights were illuminated. He stated that he took the control of the helicopter at that time. He stated that he instructed the dual student to apply carburetor heat. However, the engine continued to rev and was "making a popping sound, like it was backfiring." He noted a drop in rotor speed, so he elected to initiate an autorotation. He estimated their altitude at 500 feet above ground level at that point. "The safest spot I could see was a paved road in a subdivision; we struck the ground hard damaging the helicopter," according to the instructor. An individual, who heard the helicopter fly over, reported that it was about 500 feet above ground level, when the engine made three "popping" sounds. He stated the helicopter yawed to the left about 90 degrees and glided down to the pavement. He noted that the engine sounded like it was running fine until the "popping" sounds were heard. A post-accident inspection was conducted. Engine rotation was observed when the starter was engaged at the ignition switch. Fuel was present on-board the aircraft. Its appearance and odor were consistent with 100 low lead aviation fuel. No contamination was observed. Fuel was present at the carburetor inlet and the inlet screen was clean. The magnetos were tested and produced a spark as expected. The engine was removed and torn down. The valve train appeared undamaged and functioned normally. The oil pump was free to rotate and appeared undamaged. The oil filter screen was clean. The bearings and lifter rods appeared to be undamaged and functional. No abnormalities were found. Weather conditions at ARB, recorded at 1453, included a temperature of 9 degrees Celcius and dew point of 1 degree Celcius. Data concerning the probability of carburetor icing was obtained from Transport Canada. The recorded conditions bordered regions of serious icing at any power setting and moderate icing at cruise power.