HISTORY OF FLIGHT On November 23, 1994, about 1234 central standard time, a Fokker F-28-4000 airplane, N479AU, operated by USAIR as Flight 689, experienced a sticking horizontal stabilizer while on approach to the Evansville Airport, Evansville, Indiana. The flight made an uneventful landing at Evansville. None of the 49 passengers or four crew members aboard were injured. The 14 CFR Part 121 flight originated in Pittsburgh, Pennsylvania, and operated in visual meteorological conditions. The captain reported the first officer was flying the airplane. During the descent from cruise into Evansville, with the autopilot engaged, the first officer told the captain that the horizontal stabilizer was not responding to trim inputs. The autopilot was disengaged, and both master caution yellow lights on the hydraulic system flight control panel illuminated. Neither manual or alternate (electric) trim modes had any effect on the horizontal stabilizer. Abnormal checklists were reviewed and followed. Both the captain and first officer found it necessary to exert simultaneous forces on their control yokes in order to accomplish elevator movement and pitch changes for landing. An after landing functional test of the horizontal trim mechanism discovered no obvious anomalies. Both the hydraulic and electric back-up trim systems were able to move the horizontal stabilizer in the absence of flight loads. At the request of the NTSB, an FAA Inspector assisted USAIR maintenance personnel at Evansville during the trouble shooting of the sticking stabilizer. During the inspection of the hydraulic horizontal stabilizer trim actuator, the gear box was drained of fluid. According to the FAA Inspector, the fluid drained was dark brown in color, and had the consistency of "murky sludge." Approximately one ounce of fluid was recovered from the system. The gear box capacity is approximately seven ounces. No hydraulic leaks were observed emanating from the actuator. Several flushes of the gear box was necessary before a clean fluid sample was obtained. The actuator was serviced with the appropriate quantity of fresh hydraulic fluid, and the airplane was ferried to Pittsburgh. The airplane's hydraulic stabilizer trim actuator was removed from the airplane and shipped to the manufacturer, Menasco Aerospace, in Oakville, Canada, for inspection and disassembly. AIRCRAFT INFORMATION N479AU is a Fokker F-28-4000 airplane with approximately 18679 service hours on the airframe. The hydraulic horizontal stabilizer actuator, part number 11100-39AA, serial number MC375, was manufactured (and subsequently overhauled) by Menasco Aerospace. It was a replacement unit installed in the airplane on August 4, 1994. It had accumulated approximately 734 service hours since overhaul at the time of the incident. TESTS AND RESEARCH On December 15, 1994, a functional test and disassembly of the actuator was performed at the Menasco facility at Oakville, Canada, under the direction of the NTSB and the Transportation Safety Board of Canada (TSB). The actuator was placed on a test stand and pressurized to operating pressure. The actuator functioned within allowable parameters in both the primary and alternate modes. Following the functional test, the gear box was drained of hydraulic fluid. The fluid, which was the new hydraulic fluid placed in the actuator at Evansville, was now a deep, reddish-brown in color. Disassembly of the gear box disclosed that the worm screw, bevel gear, clutch plates and bearings were visually corroded. ADDITIONAL INFORMATION The hydraulic horizontal stabilizer actuator is manufactured (or overhauled) and shipped without hydraulic fluid from the manufacturer. Servicing and fill instructions are shipped with the units. The incident operator also has a "job card" which details fill instructions and quantity of the gear box.