On September 16, 1998, at 0833 central daylight time, an Enstrom 480, N142CC, operated by a private pilot collided with the terrain while practicing autorotations at the Straubel International Airport, Green Bay, Wisconsin. The pilot and a pilot rated passenger were not injured. The helicopter was substantially damaged. Visual meteorological conditions prevailed and no flight plan was filed for the 14 CFR Part 91 flight. The flight originated from Green Bay at 0800 cdt. The pilot reported that while practicing autorotations the helicopter set down hard. The pilot rated passenger reported the helicopter "...ran out of rotor rpm before it could be cushioned to the surface." An Inspector from the Federal Aviation Administration Milwaukee, Wisconsin Flight Standards District Office reported the pilot was practicing autorotations terminating with power. One the accident autorotation the pilot initiated the flare at an altitude of 30 to 40 feet above the ground. Power was applied at an altitude of approximately 8 feet above the ground and the engine responded but not in enough time to arrest the descend.