What happened
While cruising at FL310, the captain identified a line of forecasted thunderstorm cells on the radar. In response, the captain notified the first flight attendant to expect turbulence, activated the fasten seat belt sign, and ordered that cabin service be suspended and service carts be stowed.
The first flight attendant relayed these instructions to the rest of the crew, directing them to stop service and take their seats immediately. While the flight attendants were in the process of securing the cabin, the aircraft encountered several jolts of moderate turbulence.
Data from the flight data recorder showed that within a 0.5-second interval, vertical acceleration shifted from -0.05G to +2.37Gs. This sudden movement caused the first flight attendant in the front cabin to be knocked off her feet. In the aft galley, three flight attendants were thrown upward; while they did not strike the ceiling, they fell back to the floor upon impact. One flight attendant sustained serious injuries.
Findings
The investigation noted that the operator's InFlight Operations Manual required flight crews to use specific standard terminology—specifically the words "light," "moderate," or "severe"—when briefing flight attendants on expected turbulence intensity. Neither the captain nor the flight attendants reported using or hearing these specific descriptive terms during the briefing for the anticipated weather.