What happened
During a flight through an area containing thunderstorms with cloud tops reaching 45,000 feet, the pilot was notified of significant heavy precipitation along the route. While descending to 11,000 feet, the pilot requested a deviation to the right to avoid the weather. During this descent, which averaged a rate of approximately 2,571 fpm, the aircraft's calibrated airspeed increased from 190 knots to roughly 265 knots, reaching the aircraft's design dive speed (Vd).
Upon encountering a level 5 weather echo, the aircraft entered a high rate of descent. This was followed by a structural failure in which both horizontal stabilizers, including the elevators, and the outer portions of both wings failed in a downward direction. No pilot reports were submitted prior to the event.
Findings
Post-accident inspections of the engines, propellers, flight control cables, and the fracture surfaces of the wings and horizontal stabilizers showed no signs of malfunction or failure occurring prior to the impact. The investigation noted that the aircraft's design maneuvering speed is 169 knots indicated, and the Pilot's Operating Handbook (POH) recommends reducing speed to this level when encountering turbulence. The increase in airspeed to 265 knots was a critical factor during the encounter with severe weather.