What happened
The captain reported that the seat belt sign remained illuminated for the entire duration of the flight from Mexico City to Miami. Approximately 50 miles from Key West, the aircraft initiated a descent from flight level 370. While passing through approximately 11,500 feet, the aircraft encountered severe turbulence while traversing a cumulus cloud. At that moment, the airspeed was recorded at 270 knots.
One passenger sustained serious injuries, while four passengers and four flight attendants received minor injuries. The injuries occurred when occupants were thrown about the cabin or against loose seat belts. Passengers stated that no previous announcements were made to warn them of possible turbulence, except for the activation of the seat belt sign. The flight crew continued to Miami International Airport and landed without further incident.
The investigation
A weather study revealed a large area of level 2 to level 5 thunderstorms along the route. There was a convective SIGMET for an area of thunderstorms with tops to 45,000 feet.