On June 2, 2001, at 1400 eastern daylight time, a Cessna R172-K, N1278V, registered to and operated by the private pilot, experienced a hard landing at Benjamin Rivera Noriega Airport, Culebras Island, Puerto Rico. The personal flight was operated under the provisions of Title 14 CFR Part 91 and visual flight rules. Visual meteorological conditions prevailed and no flight plan was filed. The airplane sustained substantial damage, and the pilot and passenger reported no injuries. The flight departed Culebras Island, Puerto Rico, at 1340. According to the pilot, he was on approach to Flamenco Beach when he encountered a downdraft approximately 15 feet over the runway. He stated," that the downdraft pushed the airplane down hard onto the runway." After the hard landing he inspected the airplane and found no visible damage to the landing gear, wheel pants or fairings. Weather data did not reveal any gust or downdrafts in the area at the time of the accident. Examination of the airplane revealed that the lower part of the engine firewall was buckled, with buckling of the skin under the fuselage behind the firewall. A review of the FAA form 337 repair, showed that the lower firewall assembly, stringer assembly and the lower engine mount on the left hand side had been replaced. There were no mechanical problems reported by the pilot prior to the accident.