On February 20, 1996, about 1505 eastern standard time (EST), an Embraer EMB-120, N129AM, experienced an uncommanded movement of the rudder to the fully deflected, trailing edge right, position, during takeoff at the Raleigh-Durham Airport in Raleigh, North Carolina. There were no injuries to the three crewmembers or five passengers on board the airplane. The airplane, Flight 1555, was operated under the provisions of Title 14 CFR Part 135, as a scheduled, domestic, passenger flight, by Great Lakes Aviation, Ltd., DBA Midway Connection. Visual meteorological conditions existed at the time, and an instrument flight rules flight plan was in effect for the flight. The flight departed Raleigh, North Carolina (RDU), with a destination of Wilmington, North Carolina (ILM), minutes before the occurrence. According to the pilots, shortly after takeoff, at an altitude of about 200 feet above ground level, the airplane's rudder pedals deflected fully right without pilot input. The pilots stated that the event lasted about two seconds, and that during the event, they could not force the rudder pedals back to the neutral position. The aircraft nose deviated to the right about 20 degrees prior to the event ending, and control recovery by the flight crew. The flight crew then turned the rudder hydraulic power switches to the "OFF" position, and returned and landed safely at the Raleigh-Durham Airport. The cockpit voice recorder (CVR) was forwarded to the NTSB CVR laboratory in Washington, DC for read-out where it was discovered that the only audio on the tape referred to de-fueling of the airplane. The digital flight data recorder (DFDR) was also examined. The DFDR data indicated that the recorder experienced a track switching problem while recording. Field Service Bulletin 027, issued by Loral Data Systems, which identified a track switching problem in the applicable type of recorder and recommended a modification, had not been not accomplished on the recorder. Additionally, the recorder read and write heads were worn and dirty. The incident flight could not be located on the transcription of the recorder. Subsequent testing and examination of the airplane's mechanical rudder control system determined that the system was intact and could be operated as designed. The rudder power control unit, and both rudder actuators were shipped to the Southwest Regional Office for their assistance in the examination of the units at the manufacturer's facilities in Irvine, California. A report of that examination is attached to this report. Additionally, hydraulic fluid samples from various locations including the airplane's hydraulic system reservoirs, the power control unit, and the actuators were analyzed for particulate contamination level. The test results showed particulate contamination in the reservoirs and the power control unit that exceeded the limits recommended in the applicable Embraer maintenance manual (excerpt attached). The maintenance manual states that the highest acceptable limit for hydraulic fluid contamination is Class 9, per NAS 1638. Particulate contamination was found at level 10, 11, and 14. The Operations Specifications issued by the FAA to the operator state that aircraft contained in the operator's listing may be operated if certain conditions are met. Those conditions include: the items identified as "on condition" shall be maintained in a continuous airworthy condition by periodic inspections, checks, service, repair, and/or preventative maintenance, and that the procedures for those various checks shall be described in the certificate holder's manual. The operator's General Operating Manual (GOM)states that any service or manufacturers maintenance manual referenced in the GOM are considered a part of the GOM. The Embraer 120 Brasilia Maintenance manual is listed in the GOM. Correspondence from the operator's FAA principal maintenance inspector indicated that the operator's maintenance program did not contain provisions or schedules for analyzing hydraulic fluid, periodically.