On February 14, 1994, at 1520 eastern standard time, a Robinson R22 Beta, N591PD, registered to the City of Fort Lauderdale, landed hard at Pompano Beach Airpark, Pompano Beach, Florida, following loss of engine power during recovery from a autorotation landing. The flight was a 14 CFR Part 91, public-use flight. Visual meteorological conditions prevailed at the time and no flight plan was filed. The aircraft received substantial damage and the airline transport-rated flight instructor was not injured. The commercial-rated dual student received minor injuries. The flight originated at Fort Lauderdale, Florida, on February 14, 1994, about 1445. Both pilots stated the instructor pilot was demonstrating a 180-degree autorotation with power recovery. The dual student was following on the flight controls. They had completed two previous autorotations. As they flared at the completion of the autorotation they heard the engine running at idle; however, when they applied power to recover, the engine did not respond. The helicopter touched down hard and the main rotor blades severed part of the tailboom and the tail rotor. The helicopter spun to the left several rotations and the right skid collapsed. The helicopter came to rest upright, leaning to the right. They stated the engine was at idle power for about 30 to 40 seconds during each autorotation and that they were not applying carburetor heat. After the accident the main rotor blades were removed and the tail rotor drive was disconnected from the transmission. The engine was started and operated to 3,000 rpm where it ran rough and popped. A magneto check was performed and the right magneto had over a 200 rpm drop. The left mag had a normal drop. The no. 2 cylinder bottom plug, which operates off the right magneto, was found to be fouled by oil and the ignition lead was not firing. All cylinders developed normal compression. The engine was removed from the helicopter and the exhaust valve guide clearance was measured. The nos. 1, 2, and 4 exhaust valve guides were found worn beyond maximum limits. No 3 cylinder exhaust valve guide was at maximum limit. The engine was placed in a test cell and operated to maximum power with no evidence of failure or malfunction. During initial running the carburetor fuel shutoff needle valve stuck partially open. This corrected itself shortly after start of the engine run. The no. two cylinder bottom spark lead from the right magneto operated normally during this test run. A review of carburetor icing probability curves indicate that for the conditions present at the time of the accident, temperature 72 degrees F and dewpoint temperature 55 degrees F, the helicopter was operating in conditions conducive to serious icing at glide power. The FAA approved flight manual for the Robinson R22 states " when conditions conducive to carburetor ice are known or suspected, such as fog, rain, high humidity, or when operating near water, use carb heat as follows: during autorotation or reduced power below 18 inches MP apply full Carb Heat regardless of CAT gage temperature." See attached icing probability curves and pages from the Robinson Flight Manual.