During departure, the pilot climbed in IFR conditions to 6000 ft. After leveling, a frequency change was made and he was cleared to 8000 ft, which he acknowledged. About 2 min later, the controller radioed to the pilot that he was about 4 mi north of the airway, but there was no reply from the pilot. Shortly after that, radar contact was lost and the plane crashed in mountainous terrain. Several residents in the area reported hearing extremely loud engine noises followed by complete silence. An investigation revealed that both wings had separated outboard of their respective engine nacelle assemblies. An examination of the fractures revealed the wings had separated from positive overload. The leading edges of both wings had ballooned upward, indicative of extreme airspeed. The empennage had separated in a yaw and from right torsional overload. Parts of the aircraft were found within a 1 mi arc, north of the main impact area. The pilot's recent instrument experience could not be verified. Turbulence was forecast below 8000 ft.