On February 15, 1999, about 1000 pacific standard time, a Robinson R-22, N8362Q, registered to a private owner and operated by Classic Helicopter Corporation as a 14 CFR 91 local instructional flight, was substantially damaged after it collided with the ground while maneuvering at Boeing Field/King County International Airport, Seattle, Washington. Visual meteorological conditions prevailed and no flight plan was filed. The certified flight instructor received minor injuries and the student pilot was uninjured. There was no fire and no report of an ELT actuating. The flight originated from the Boeing Field airport approximately one hour and thirty minutes prior to the accident. During an interview and subsequent written statement, the flight instructor reported that for the first time in the training program, the student was practicing hovering autorotations. The flight instructor stated that he demonstrated approximately three of the maneuvers. During the fourth maneuver, both he and the student were on the controls. The flight instructor stated that he slowly rolled off the throttle and the helicopter started to drift to the left and descend. The left landing skid caught the grass and the helicopter began to roll to the left. The flight instructor stated that before he could recover, the helicopter rolled over onto its side. No evidence was found to indicate a mechanical failure or malfunction.