Clinton

While en route on an IFR flight plan at FL180, the pilot acknowledged instructions to descend to 16,000 feet. Later, the radar controller noticed the aircraft had deviate from course and was not replying with mode 'C' altitude info. The controller requested altitude info and the pilot responded, '. . . I've lost an (awful) lot of altitude, I got mixed up in here and lost altitude, I'm goin' back up . . .' When queried, the pilot reported he was at 9,000 feet and acknowledged instructions to climb to 14,000 feet. Shortly thereafter, the aircraft entered a spiral, then radio and radar contact were lost. An in-flight breakup occurred and the wreckage fell over a widespread area. An exam revealed evidence the wings and right horizontal stabilizer had failed from overload, with the wings separating in an upward direction. A weather study indicated there were cloud layers from about 14,000 feet to 20,000 feet with light to moderate mixed icing in clouds above 14,000 feet. Also, light to moderate turbulences was forecast below 20,000 feet and widely scattered, imbedded thunderstorms were forecast in an area north of the accident area. Witnesses reported clouds and occasional lightning to the east. All four occupants were killed.