On February 21, 2000, at 0958 Pacific standard time, the crew of Skywest Airlines Flight 5353, an Embraer 120, N566SW, reported an elevator trim problem prior to landing at Fresno/Yosemite Airport, Fresno, California. Skywest Airlines, Inc., operated the airplane as a scheduled domestic passenger flight under the provisions of 14 CFR Part 121. The airplane was not damaged. The airline transport pilot licensed captain, first officer, 1 flight attendant, and 17 passengers were not injured. The flight departed Los Angeles, California, about 0850 as a nonstop flight to Fresno. Visual meteorological conditions prevailed, and an instrument flight rules (IFR) flight plan had been filed. The captain reported the circumstances of the flight. During the initial descent from 22,000 feet mean sea level (msl) to 14,000 feet msl, the trim wheel moved to the forward stop. The trim wheel made a "snapping" motion midway through the movement. It then moved aft, making the same snapping motion midway through the movement. The airplane's pitch attitude remained constant throughout the movement of the trim wheel. The trim wheel continued to move back and forth, so the crew disconnected the autopilot and trim. Both crewmembers guarded the flight controls. The captain was flying the airplane and stated that it "wanted to pitch up." He had to apply moderate forward pressure to maintain pitch control. He reported that the trim wheel was still jammed, but one crewmember could control the airplane. At 12,000 feet msl, and approximately 23 degrees Fahrenheit outside air temperature, the trim remained immovable. The crew declared an emergency to the Fresno Approach Control Facility, and advised the flight attendant and passengers of the problem. Between 5,000 feet msl and 4,000 feet msl, the trim wheel became moveable. By the time the airplane reached the traffic pattern altitude, the trim wheel moved freely. The captain noted that the outside air temperature at that time was 46 degrees Fahrenheit. The crew encountered no further difficulties and completed an uneventful landing. Skywest maintenance personnel suspected a frozen elevator trim system. 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. 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 that were 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. Both actuators on the incident airplane were unmodified units. Total time on the airplane was 3,757 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 actuator had leaking shaft seals. Its packing had cracked, and a bearing had seized. The second actuator was dirty, and minor corrosion was on the hardware. The shaft seals leaked, and a rigging device was installed.