On December 13, 1998, at 1840 hours Pacific standard time, a Boeing 737-300, N328AW, encountered turbulence during descent for landing at Sacramento, California. One flight attendant was seriously injured. The other 4 crewmembers and 70 passengers were not injured, and the aircraft was not damaged. Visual meteorological conditions prevailed for the regularly scheduled domestic passenger flight. The flight was operated on an instrument flight rules flight plan under 14 CFR Part 121 by America West Airlines as flight 378. The flight departed Santa Ana, California at 1750. According to statements provided by America West Airlines, prior to descent from cruise, the cockpit crew advised the second flight attendant, who was later injured, and the third flight attendant by intercom that turbulence was anticipated during descent for landing and for them to secure the cabin and be seated. The second attendant notified the first attendant who directed her attention toward securing first class. The second flight attendant proceeded through the cabin from front to rear, with a trash bag, picking up service items and ensuring that passenger's seat belts were fastened. As she reached the rear of the cabin, turbulence was encountered and both she and the third flight attendant, who was working in the rear galley, took seats and fastened their seatbelts. After 1 or 2 minutes of moderate turbulence, the turbulence ceased and the second and third flight attendants got up to finish their work in the aft galley. Additional turbulence was then encountered which caused the second flight attendant to fall to the floor of the galley. The third flight attendant also fell but landed on the foot of the second attendant, which broke the second attendant's ankle. The Safety Board investigator reviewed the aircraft's flight data recorder output. As the aircraft descended from flight level 310 (approximately 31,000 feet) the vertical acceleration values recorded remained between 0.85 and 1.06 g's until descending through flight level 200 (approximately 20,000 feet). Between flight level 200 and 7,500 feet mean sea level (msl) in the descent, vertical acceleration values ranged from a low of 0.53 to a high of 2.02 g's. The peak values occurred 5 seconds from one another while the aircraft was descending through 19,832 feet at time 33:41. The vertical acceleration dropped to 0.65 g's and 1 second later increased to 2.02 g's. In the next 4 seconds, values of 1.23, 1.16, 1.45 and 0.53 g's were recorded. Below 7,500 feet the vertical acceleration values returned to the 0.85 to 1.19 range.