On December 10, 1998, at 1400 eastern standard time, a Robinson R-22B, helicopter, N2321X, collided with the ground, during a practice autorotation, at the Lantana Airport, in Lantana, Florida. The instructional flight was operated by Aircoastal Helicopter under the provisions of Title 14 CFR Part 91 with no flight plan filed. Visual weather conditions prevailed at the time of the accident. The helicopter sustained substantial damage. The flight instructor and the dual student were not injured. The flight departed Lantana, Florida, at 1350. The flight instructor reported that he and the dual student were conducting autorotation training. The dual student had completed several power recovery autorotations, and the session had progressed to the point where the dual student was ready for a 180-degree autorotation. The flight instructor was demonstrating a 180-degree autorotation, with power recovery, when the accident occurred. The flight instructor stated that "all airplane instruments were in the green, all warning lights were out, and the carburetor was heat on." Also the instructor noted that the carburetor air temperature gauge(CAT) was in the normal range. As the helicopter descended through two-hundred feet, the low rotor RPM horn sounded. The tachometer was at about ninety-two percent. The flight instructor then, "rolled on throttle and went down on the collective simultaneously." The flight instructor attempted a shallow flare, and attempted to cushioned the helicopter with the collective. At this point the flight instructor realized the rate of descent was too great to arrest. The helicopter contacted the ground in a level attitude with the skids aligned with the runway heading. The aircraft slid in the grass, the right skid dug into the ground, and the helicopter rolled over on the left side. The flight instructor turned off the battery as they exited the aircraft. Wreckage examination revealed that the carburetor heat was found in the on position, and the throttle linkage appeared to function normally. Examination of the helicopter and the engine assembly failed to disclose a mechanical malfunction or a component failure. A review of weather data disclosed that conditions were favorable for the formation of carburetor ice.