On February 25, 2001, about 2155 Pacific standard time, Skywest Airlines Flight 7915, an Embraer EMB-120 Brasilia, N288SW, experienced an abrupt pitch change during descent into Monterey, California. The cabin attendant in the aft end of the airplane was thrown to the floor and sustained a broken ankle. Skywest Airlines, Inc., was operating the airplane as a scheduled domestic passenger flight under the provisions of 14 CFR Part 121. The airline transport pilot licensed captain, first officer, and 30 passengers were not injured; however, the one cabin attendant sustained serious injuries. The flight departed Los Angeles, California, about 2100 as a nonstop to Monterey. Visual meteorological conditions prevailed, and an instrument flight rules (IFR) flight plan had been filed. The captain wrote up the sequence of events. He had the autopilot engaged as the airplane descended through 10,000 feet mean sea level (msl). The nose pitched down violently about 10 degrees, and then the nose pitched back up. The captain disengaged the autopilot and hand flew the airplane to touchdown. The flight attendant was standing in the rear of the cabin area. The maneuver threw her to the floor. The captain could not establish contact with the flight attendant. A passenger, who was a nurse, notified the captain of the flight attendant's condition. The captain declared an emergency and requested medical assistance at the gate. Examination revealed that the flight attendant suffered a fractured ankle. Skywest maintenance personnel suspected frozen trim. The elevator trim system of the Embraer 120 has a history of freezing in flight. The Safety Board investigator-in-charge obtained a list of occurrences from the Federal Aviation Administration (FAA) Flight Standards Service, Information Management Section (AFS-624). Service Difficulty Report (SDR) data from August 30, 1990, to February 29, 2000, contained 19 reports of the elevator trim freezing at altitude. Nine of the reports noted that the trim operated normally after descent to lower altitudes and warmer temperatures, which allowed the trim system to thaw. Eleven additional reports referenced binding or stiffness at altitude. Some of the airplanes experienced multiple events. During some of the events, the airplane pitched violently. The manufacturer discovered moisture and corrosion in some of the actuators submitted for repair after these occurrences. They suspected that leaking seals allowed moisture into the actuators, which froze at altitude. They developed a modification that included installation of additional seals inside the actuator. However, Skywest maintenance records indicate that both unmodified (Part Number 5299) and modified (Part Number 5299-1) actuators experienced problems. The Brasilia has two actuators. Neither actuator on the accident airplane was a modified unit. Total time on the airplane was 11,650 hours. DERCO Repair Services, Inc., is a repair station that is certified to overhaul the trim actuators. DERCO disassembled and inspected the Skywest actuators. One unit was dirty, corroded, and had overspray on it. It leaked from the shaft seals and autoloc seal. It failed the non-jamming stop test. It's two shaft housings contained moisture. The chain and pins were corroded, the packing had deteriorated, and the springs were weak. One side of the rollers was not chamfered, and staking warped some screws. The hub was worn and not functioning properly. The second unit was dirty and had overspray on it. The actuator leaked from the shaft seals. The actuator had excessive backlash and failed the tangential free play test. It's two shaft housings contained moisture. The chain, pins, shafts, and housings were corroded. The packing had deteriorated, and the bearings were rough and binding. The rollers only had one chamfered side. The shim was not with the unit.