Hawker Beechcraft King Air B300, Tuscaloosa, Ala., April 4, 2008–The King Air encountered severe turbulence that apparently caused substantial damage to the wing skins and spar. Nearing the destination airport, the pilot-in-command saw a 200- to 300-foot-thick cloud with what seemed to be virga. The base was about 500 feet above the airplane’s altitude and appeared dark. Under the cloud, the airplane encountered violent and rapid turbulence. The pilot started a 180-degree turn while reducing power. The airplane descended several hundred feet and did not appear to have experienced any type of wing overloading. After an uneventful landing, no damage was noted during the preflight inspection for the next flight. When the damage was later discovered, the flight to Tuscaloosa appeared to be the most likely one on which the airplane sustained the damage.