On August 5, 2001, at 1655 eastern daylight time, a Robinson R22, N4066G, was substantially damaged when it impacted terrain during a practice autorotation at Long Island Mac Arthur Airport (ISP), Islip, New York. The certificated flight instructor and the private helicopter pilot under instruction were not injured. Visual meteorological conditions prevailed at the time of the accident. No flight plan was on file for the local flight, which originated at Islip. The instructional flight was conducted under 14 CFR Part 91. According to the flight instructor, the private pilot under instruction was practicing autorotations. He first completed 10 straight-in autorotations, then flew 5, 180-degree autorotations. As the helicopter approached the ground during the last practice autorotation, the flight instructor noted that its nose was too low. He took the helicopter's controls and tried to level it, and was able to bleed off some of the vertical and forward airspeed. However, the left skid touched down and dug into the turf, and the helicopter tumbled forward and came to rest on its side. The pilot under instruction had 1,540 helicopter flight hours, with 7 hours in the R22. The flight instructor stated that although he was on or near the controls at all times, he had been confident in the pilot under instruction's ability to "keep it safe." He also noted that prior to touchdown, the helicopter’s nose attitude "changed rapidly for the worse," and that he had little time to take over and complete a safe landing.