On April 28, 1996, at 1500 central daylight time, a Boeing 727- 222A, N7443U, operated by United Airlines, Incorporated, of Chicago, Illinois, as Flight Number 392, and piloted by an airline transport and commercially certificated crew, encountered moderate turbulence during a descent into Chicago, Illinois. The 14 CFR Part 121 flight was operating on a IFR flight plan. Instrument meteorological conditions prevailed at the time of the accident. The 3 person flight crew, 3 flight attendants, and 129 passengers were not injured. A fourth flight attendant received minor injuries. The flight departed San Jose, California, at 1004 pacific daylight time. The flight was at Flight Level 240 when the Federal Aviation Administration's Chicago Air Route Traffic Control Center controller (controller) advised the flight's captain of possible moderate chop starting at Flight Level 220. The captain said he advised the flight attendants that they had a couple of minutes to finish closing the galleys and take a seat. About 2 to 3 minutes later, the controller instructed Flight 392 to descend to Flight Level 190. The captain turned the seatblt sign on and requested the pasengers take their seats. During the descent the flight encountered turbulence as it entered a cloud layer at Flight Level 220. A flight attendant was dealing with a passenger who had gotten out of their seat. The flight attendant fell against the passenger's seat arm and then onto the floor during the turbulence encounter. The airline reported that the flight attendant suffered a severly sprained back. The injured flight attendant was assigned seat number 1, left, outboard.