On September 29, 1999, at 1840 mountain daylight time, an Enstrom 280C helicopter, N84DH, sustained substantial damage during a hard landing, after a practice autorotation, at Vance Brand Airport, Longmont, Colorado. The non-helicopter rated commercial pilot and sole occupant was not injured. The flight was a local solo instructional flight operating under Title 14 CFR Part 91 and no flight plan was filed. Visual meteorological conditions prevailed. According to the pilot, he was practicing autorotations and the engine lost power while in an autorotation. During the ensuing hard landing, the main rotor flexed and cut of the tail boom. Density altitude at the time was 9,000 feet. The pilot reported that he had 40 hours flight experience in helicopters and that all his helicopter experience was in the Enstrom 280C. He said he had 11 hours of helicopter experience in the 30 days prior to the accident, and 40 hours in the preceding 90 days. No evidence was developed during the investigation as to the reason for the power loss.