On July 10, 1993, about 1900 eastern daylight time, a McDonnell Douglas DC-10-40, N158US, registered to and operated by Northwest Airlines, Inc., as Flight 48, experienced an uncommanded rudder oscillation while climbing through 20,000 feet (mean sea level). The anomaly was corrected when the crew disconnected the number one inertial navigation system (INS). The airplane returned to Detroit, Michigan, where it landed without incident. The flight crew of three, cabin crew of eight, and 92 passengers reported no injuries. The airplane sustained no damage. The 14 CFR Part 121 flight was a domestic flight operating between Detroit, Michigan, and Boston, Massachusetts. The flight was on an IFR flight plan; however, VFR conditions existed at Detroit, Michigan. The flight departed at 1842 and returned at 1930. The airplane was examined by the NTSB, FAA, and Northwest Airlines personnel in Detroit on July 11, 1993. The #1 INS display unit and the Captain's attitude indicator were replaced. The cockpit voice recorder (CVR) and flight data recorder (FDR) were removed and taken to the NTSB laboratories in Washington, DC for readout. A review of the airplane's design indicated that the #1 INS provides bank angle information to the yaw flight guidance computer through the attitude monitoring and switching unit. The bank angle information is then transmitted from the yaw flight guidance computer to the lower rudder hydraulic actuator to move the rudder. On July 11, 1993, while the airplane was at Northwest maintenance facilities in Detroit, MI, the examination of the #1 INS began with a cockpit test to identify fault codes recorded in the INS. Fault codes were recorded consistent with a #1 INS failure. The #2 INS and standby INS were examined and both appeared to be operational. The airplane's hydraulic systems were powered via the APU. All flight controls were operated with no anomalies. The yaw damper system self-test was completed with no anomalies. Following the tests and examinations, the #1 INS and controller were removed along with the Captain's ADI. These components were sent to Northwest Airlines facilities for further evaluation. Following a preflight inspection, the airplane was flight tested by Northwest Airlines flight crew. The flight test involved extensive maneuvering of the airplane and a thorough operation of all flight control systems and INS operations. The flight was accomplished successfully. On or about July 16, 1993, bench testing of the #1 INS, by Northwest personnel, indicated that the internal gyro generating bank-axis (lateral) reference information had failed. The gyro output varied as the gyro tumbled, which produced a bank angle signal. The INS warning light that illuminated on the unit during the incident event correctly identified a failure of the unit. Northwest Airlines personnel reported that testing verified there were no anomalies with the Captain's ADI. The Digital Flight Data Recorder Factual Report is attached as an addendum to this report.