Incident Overview
A flight attendant suffered a broken ankle and tibia during a turbulence encounter while the airplane was descending. The captain reported that the seat belt sign had been turned on as the aircraft descended through 18,000 feet. Approximately 15 seconds of moderate turbulence occurred as the airplane passed through 16,000 feet. The captain noted no weather returns in the area, and the ride returned to smooth air after the encounter.
Crew Actions
The first officer stated that he instructed the flight attendants to take their seats as soon as the turbulence began. He was preparing to call the flight attendants afterward to ensure their well-being when the cockpit received a call from the cabin reporting that a flight attendant had injured her ankle. The first officer stated that paramedics were requested to meet the flight upon landing.
Flight Attendant Account
The injured flight attendant reported being in the aft galley when the turbulence commenced. She attempted to reach the jumpseat but could not due to the motion. She heard another flight attendant tell a passenger in the aft galley to sit on the floor and decided to do the same. As she was sitting down, she stated that the "plane dropped," causing her to fall. Once the turbulence subsided, the other flight attendant and a passenger helped her off the floor and into a seat.
Warning and Weather Conditions
None of the flight attendants recalled receiving any advance warning of the turbulence. At the time, rapidly building convective activity was evident along the upper Mississippi Valley. Convective tops averaging FL250 were moving north-northeast at 60 knots. A Severe Weather Advisory (15S) valid from 1731 UTC to 2301 UTC covered the route from the Mississippi River to ORD. The severe thunderstorm watch included forecasts for hail, surface wind gusts to 60 knots, maximum cloud tops to FL450, and storm motion vector of 240 degrees at 35 knots. Convective Sigmet 25C, valid until 2255 UTC, called for an area of embedded thunderstorms moving from 190 degrees at 20 knots, with tops to FL300.
Flight Data
Data from the digital flight data recorder showed that the airplane experienced vertical acceleration fluctuations from a minimum of 0.437 g to a maximum of 1.994 g and back to 0.576 g in less than 3 seconds.