On December 16, 2000, about 1450 eastern standard time, an Aero Vodochody L39C, N139CG, registered to Glastonbury Aviation LLC, and operated as a Title 14 CFR Part 91 personal flight, had its landing gear collapse during landing at Hilton Head Airport, Hilton Head, South Carolina. Visual meteorological conditions prevailed, and no flight plan was filed. The aircraft incurred substantial damage, and the commercial-rated pilot, the sole occupant of the aircraft was not injured. The flight originated the same day, about 1420. The pilot stated that he had been flying for about 20 minutes east of the Hilton Head Airport, over the ocean, when he became concerned that fog may be moving into the airport area, so he returned for a landing. The pilot further stated that during the landing preparations he did not extend the landing gear on the downwind leg as he normally did, and he realized he had not done so late, just prior to landing. He said the landing gear touched the runway before it was fully extended and locked. According to the pilot, the landing gear collapsed, and the aircraft slid to a stop on the runway, incurring substantial damage. The pilot said that no mechanical malfunctions existed with either the aircraft, or its systems prior to the accident.