Background
The airplane was cruising at flight level 310 (FL310) when the captain observed on radar that the aircraft was approaching a line of thunderstorm cells. These cells had been forecast. In response, the captain advised the first flight attendant to expect turbulence and turned on the fasten seat belt sign. He directed that cabin service be suspended and that service carts be stowed.
Events
The first flight attendant notified the other flight attendants of the captain's instructions and directed them to stop service and take their seats immediately. The flight attendants proceeded to secure the cabin, a process that took a few minutes. However, they had not yet taken their seats when the airplane encountered several jolts of moderate turbulence. According to the flight data recorder, within a half-second period, the vertical acceleration changed from -0.05G to +2.37Gs.
Injuries
During the turbulence, the first flight attendant in the front cabin was knocked off her feet. The three flight attendants in the aft galley were thrown upward but did not strike the ceiling before coming back down and falling to the floor. One of them was seriously injured.
Procedures
The operator's In-Flight Operations Manual required the flight crew to use standard terminology during briefings to the flight attendants, specifically the words "light," "moderate," or "severe" when describing expected turbulence. Neither the captain nor any of the flight attendants reported having used or heard these terms regarding the anticipated turbulence intensity.