A pilot witness observed the aircraft in a 20 to 30 degree dive at high speed just prior to the accident. He reported that at an altitude of about 1200 to 1300 feet, the aircraft began a pull-up. As the pull-up was started, the outboard portion of the aircraft's left wing folded upward and the aircraft entered an uncontrolled dive and crashed. The outboard 3 feet of the left wing was found relatively intact. The remainder of the wing was broken into numerous pieces. Also, the left aileron was bent upward about 3 feet from its outboard end. An examination of the wooden structure revealed discoloration in the fracture area. Evidence of wood deterioration and decay was found in localized areas around wing fasteners. A toxicological check of the pilot's blood showed some alcohol (0.04%); however, the pilot's body had not been immediately removed from the wreckage. The amount of alcohol due to putrefaction was not determined. The pilot's last medical certificate was dated 12/18/81.