The air carrier passenger flight encountered severe turbulence while in cruise flight. Three passengers were seriously injured and nine passengers and three flight attendants received minor injuries during the turbulence encounter. The remaining passengers and crew were not injured. The airplane was traveling in a predominately northern direction at 37,000 feet altitude during the encounter. Weather products obtained showed that extreme intensity thunderstorms existed in the area where the turbulence was encountered. Weather data and aircraft position radar data show that the airplane was 9 miles south-southwest of a cloud buildup that extended to 39,000 feet. Additionally, the airplane was 5 miles west of an extreme intensity radar echo associated with the thunderstorms in the area. The airline's General Operations Manual states that thunderstorms that are identified as severe, or giving a intense radar echo should be avoided by at least 20 miles. No National Weather Service aviation weather advisories were in effect for the location and time of the turbulence encounter. Communications transcripts show that the flight crew requested and were granted a course deviation for weather about 10 minutes prior to the upset. However, the Digital Flight Data Recorder shows that the seat belt sign was illuminated only 10 seconds prior to the encounter.
The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
The flight crew's failure to follow weather avoidance procedures and their delay in activating the seat belt sign. Factors were the turbulent thunderstorm weather conditions, and the failure of the National Weather Service to issue an applicable in-flight weather advisory.