What happened
During the flight, the crew encountered unforecasted clear air turbulence while flight attendants were performing meal service for passengers. The turbulence lasted approximately 3 to 4 seconds. During the encounter, several flight attendants were thrown to the floor. One flight attendant, who was working in the rear of the aircraft, sustained a fractured left shoulder.
The captain described the event as moderate in intensity and noted that its extremely short duration made it feel similar to a wake turbulence encounter. At the time of the event, the autopilot was engaged, and the pilots provided no manual inputs.
The investigation
Statements from the captain and the first officer were reviewed regarding the encounter. The captain reported that there were no other aircraft directly ahead of them and that the nearest thunderstorms were located more than 20 miles to the northwest. The first officer confirmed that the aircraft was clear of all weather and that no turbulence warnings had been received from other aircraft in the area.