On January 8, 1998, at 1145 central standard time, a Cessna 340A, twin engine airplane, N26956, was substantially damaged following a loss of control during landing on runway 17L at the Wiley Post Airport, Bethany, Oklahoma. The aircraft was operated by Sierra Aviation, Inc., of Bethany, under Title 14 CFR Part 91. The commercial pilot, sole occupant, was not injured. Instrument meteorological conditions prevailed for the repositioning flight and an IFR flight plan was filed. The flight departed Tradewind Airport, Amarillo, Texas, at 0915. During a telephone interview, conducted by the investigator-in-charge, the pilot reported that he transported a passenger to Amarillo during a morning flight under Title 14 CFR Part 135. Before the departure at Amarillo, the pilot obtained a weather briefing which included a "chance of icing in clouds and precipitation" with freezing temperatures existing from the cloud tops (4,000 to 5,000 feet msl) to the ground. During the en route phase of the return flight, the skies were clear above 5,000 feet msl. When the flight arrived in the Oklahoma Approach Control Airspace, the flight received radar vectors for the ILS approach. During the approach descent, the aircraft encountered moderate rime and mixed icing in the clouds. At about the outer marker, the pilot cycled the deicing boots and then extended the gear and flaps. The pilot recalled that the tip tanks and the unheated portion of the windshield had ice accumulations. The approach airspeed was blue line plus 10 knots with a power setting at 23 inches of manifold pressure. The aircraft was over the runway at 25 feet agl for the landing flare/touchdown when the pilot reduced the manifold pressure approximately 2 inches. The airplane "dropped" to the runway. The pilot stated that he had encounter icing conditions several months back on a flight into Denver, Colorado. The FAA inspector and the director of maintenance for the operator examined the airplane and found structural damage at the right gear attachment bracket and the right wing. The wing and fuselage were twisted and buckled. The right and left engines were canted downward 7 1/2 degrees and 5 degrees, respectively. The pilot obtained a standard weather briefing for the round trip flight from the Wiley Post Airport to the Tradewind Airport, Amarillo, Texas. A review of the weather briefing tapes by the IIC revealed that the pilot received the airmet for moderate rime or mixed icing across Oklahoma from 6,000 feet to FL 180. A pilot report from the crew of a Boeing 737 departing the Oklahoma City area reported moderate mixed icing below 3,500 feet msl during the initial departure climb. At the time that N26956 departed the Wiley Post Airport, 1 mile visibility with freezing rain and mist was being reported at the airport. During the return flight, the pilot filed an IFR flight plan for the descent into the Oklahoma City area. The weather briefer verified that the pilot was aware of the airmet for moderate icing from the freezing level to FL 180. The briefer informed the pilot that the freezing level was at the surface.