Raytheon Aircraft Company B300 N900WP accident near Tuscaloosa, Alabama

Probable cause

The pilot's encounter with severe turbulence during descent into the airport environment, which resulted in an exceedence of the aircraft's design stress limit and structural damage.

NTSB analysis

As the airplane entered the vicinity of the destination airport, the pilot observed a 200- to 300-foot-thick cloud ahead that looked as though it was precipitating virga. He noted that the base was about 500 feet above the airplane's altitude and appeared very dark. As the airplane passed under the dark cloud, at about 3,000 feet mean sea level, the flight immediately encountered a violent and rapid turbulence event. During the turbulence episode the airplane descended several hundred feet, but the pilot was able to maintain control. The pilot performed an uneventful landing and no damage was noted during the preflight inspection for the subsequent flight. Damage was noted during a later preflight inspection 4 days after the upset. An engineer from the airplane's manufacturer completed an evaluation of the damage incurred during the upset. He stated that the damage sustained by the left wing structure is indicative that the airplane experienced loads in excess of its design limits. A review of weather radar data indicated that a squall line was close to the airplane's location at the time of the upset. According to an analysis done by the Safety Board's meteorology specialist, the airplane most likely flew under either a "roll cloud" or a "shelf cloud." Although no exact magnitudes of shear velocities associated with roll or shelf clouds encountered could be determined, severe or extreme turbulence should always be expected in vicinity of these cloud types. In addition, the radial velocity and spectrum width data showed signatures of potential severe or extreme turbulence in the area where the airplane likely encountered the phenomena.

Text from the NTSB. Open the NTSB docket for LAX08LA105 for the full record.