On June 28, 1996, at 1700 mountain daylight time, a Beech B-35, N8860A, sustained substantial damage during descent for landing at Portales, New Mexico. The airline transport certificated pilot was not injured. Visual meteorological conditions prevailed for this personal cross country flight operating under Title 14 CFR Part 91 and no flight plan was filed. The flight departed Albuquerque, New Mexico, at 1530. According to the pilot, he was in a descent passing through 8,000 feet above mean sea level (msl) at 170 miles per hour and approximately 800 to 1,000 feet per minute rate of descent in clear smooth conditions when he felt what appeared to be high speed buffet. He said he slowed about 10 knots and the buffet ceased. After landing, and upon inspecting the aircraft, he found the outer portion of the right ruddervator twisted and buckled. An examination of the aircraft was conducted by two FAA airworthiness inspectors assisted by an investigator from Beech Aircraft. A copy of the examination report, with photographs, conducted by Beech Aircraft and concurred with by the two FAA inspectors, is attached and provides details of the examination. The examination and review of aircraft records revealed that improper maintenance had been performed and that Airworthiness Directive AD-94-20-04 had not been properly complied with. An annual inspection had been performed on September 22, 1995, and approximately 11 flight hours was accumulated from the time the annual was performed until the accident occurred.