20020816020729I

(-23) On August 16, 2002 the o'Hare flight standards district office received a notice from the great lakes regional control center that united airlines flight 1133, a B-737, enroute and descending into ord, reported an uncommanded roll event. An investigation into this incident was conducted and the following narrative contains the facts of this event: while the aircraft was descending, downwind of ord runway 4 right, and not following any heavy aircraft, with no turbulence through 7000 feet at 250KTS, a slight airframe rumble was heard followed by the right rudder pedal moving slightly forward. The "a" auto pilot was engaged at that time. The aircraft then began to gradually roll to the left, at which time the captain disconnected the auto pilot at 15 degrees left roll. The aircraft continued to roll left 20 to 25 degrees before it was manually corrected using the aileron control wheel. The aircraft recovered and no other control problems were noted. The speed brake handle was down, there were no trim inputs prior to the event and no unusual wheel or rudder forces were noted. The crew went through the abnormal procedure, disconnected the yaw damper, but did not shut off hydraulics due to this incident not being a "rudder hardover." United airlines ord maintenance, in coordination with line engineering and flight safety troubleshot and conducted a thorough and exhaustive check and inspection of all related flight control systems and components. The primary guidance for this troubleshooting was boeings service letter number 737-SL-27-110-A, dated 14 September 2001. Subject: "unexpected roll and yaw event troubleshooting." As part of the troubleshooting procedures and resulting inspection and checks of the systems and components, the following components were removed and replaced per the maintenance manual: flight data recorder, main rudder power control unit. Standby rudder actuator and the yaw damp coupler. In addition, during the rudder actuation check, a binding pulley was noted in the forward cargo pit ceiling and it was replaced. All related hydraulic filters were replaced and fluid samples from the a and b hydraulic systems were taken for analysis. During the system check outs and troubleshooting of the rudders (pcu) main power control unit, it was noted that it did not have a consistent neutral. After "shake-out" the rudder would return to a differentneutral each time with a tendency toward the right 3/4 inch. In addition during this check out the stand by rudder actuator was found to have an inconsistent free-play and a neutral 3/4 inch to the left. Both of these units were subsequently replaced. In conclusion, the rudder power control unit, yaw damp coupler, and the standby rudder actuator were replaced due to discrepancies and as a precaution as recommended by united airlines flight safety and engineering. No definite conclusions have yet been reached as to the specific cause for this uncommanded roll. All of the data has not been evaluated yet, such as the radar data, voice recorder, or qar (quick access recorder). The removed components have been sent to united airlines shops for tear down and evaluation, a final report with a conclusion will be provided by united airlines at a later date. The subject aircraft, N307UA, B737-322 was released for service on August 19, 2002 and at the time of this report had experienced no difficulties.