What happened
The pilot was en route under an instrument flight rules (IFR) flight plan at flight level 180 when he acknowledged air traffic control instructions to descend to 16,000 feet. Subsequently, the radar controller observed that the aircraft had deviated from its assigned course and ceased transmitting mode C altitude information. Upon requesting altitude data, the pilot responded that he had lost a significant amount of altitude, stating he was confused by the situation and intended to climb back up. When further queried, the pilot reported his position at 9,000 feet and acknowledged instructions to climb to 14,000 feet.
Shortly thereafter, the aircraft entered a spiral, and both radio and radar contact were lost. An in-flight breakup occurred, with wreckage scattered over a widespread area. Examination of the debris revealed that the wings and right horizontal stabilizer had failed due to overload, with the wings separating in an upward direction.
The investigation
A weather study indicated cloud layers extending from approximately 14,000 feet to 20,000 feet, containing light to moderate mixed icing conditions above 14,000 feet. Light to moderate turbulence was forecast below 20,000 feet, and widely scattered embedded thunderstorms were predicted in an area north of the accident site. Witnesses reported clouds and occasional lightning to the east.
Findings
The structural failure was consistent with overload conditions. The presence of icing and turbulence likely contributed to the pilot's loss of situational awareness and subsequent control difficulties.