Incident Overview
An air carrier passenger flight experienced severe turbulence while in cruise flight. The aircraft was traveling in a predominantly northern direction at an altitude of 37,000 feet. During the encounter, three passengers sustained serious injuries, while nine passengers and three flight attendants received minor injuries. The remaining passengers and crew were not injured.
Weather Conditions
Weather products obtained indicated that extreme intensity thunderstorms were present in the area where the turbulence occurred. Weather data and aircraft position radar data showed that the airplane was located 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. No National Weather Service aviation weather advisories were in effect for the location and time of the turbulence encounter.
Flight Crew Actions
Communications transcripts revealed that the flight crew requested and were granted a course deviation for weather approximately 10 minutes prior to the upset. However, the Digital Flight Data Recorder indicated that the seat belt sign was illuminated only 10 seconds before the turbulence encounter.
Operator Procedures
The airline's General Operations Manual states that thunderstorms identified as severe or those producing an intense radar echo should be avoided by at least 20 miles.