What happened
While operating in cruise flight at 38,000 feet, the aircraft encountered a brief area of severe turbulence without warning. Approximately 19 minutes prior to the event, the crew had received clearance to deviate around scattered thunderstorms. The monitoring pilot had contacted the lead flight attendant via interphone to advise that all cabin crew should take their seats and remain seated due to the deviation.
The lead flight attendant reported that while the flight had been bumpy for much of the journey, conditions were smooth at the time of the instruction. She instructed the crew to remain seated for 15 minutes. Approximately 12 to 13 minutes after this instruction, the turbulence occurred. Two flight attendants in the aft galley, who misunderstood the timing of the instruction, were standing while preparing crew meals when the aircraft was struck by turbulence. The impact knocked both crew members to the floor, resulting in one serious injury and one minor injury.
The investigation
Analysis of flight data recorder information revealed that the aircraft experienced a vertical acceleration ranging from -0.4 G to +1.8 G, representing a total change of 2.2 G. This level of acceleration is consistent with severe to extreme turbulence. Both pilots reported that the aircraft was clear of clouds and radar echoes at the time of the encounter.