Narrative:

On our leg first officer was pilot flying. Climbing thru FL240 I noticed about a 20 degree left aileron displacement and asked the first officer to trim with rudder to get the aileron out. Also noted that the ball was about 1/2 DOT out to the left. After trimming; the aileron was level and the ball was centered. Climbing thru FL270 the aircraft began an uncommanded roll to the right. The first officer and I both noticed that the rudder was moving to the right on its own. We stopped our climb and disengaged the autopilot and performed the uncommanded yaw/roll memory items. We were IMC at the time of the incident and I think that we only rolled approximately 10 degrees to the right before the first officer took control. The rudder continued to push to the right for a while but we had control of the aircraft. We had a company pilot onboard the jumpseat so I handed him the QRH and he ran thru the procedures in the checklist while the first officer and I directed our attention to aircraft control. This was a big help! I also asked him if there was anything he thought we missed. After we were sure that we had control of the aircraft; I contacted dispatch via ACARS and then thru company radio and explained our problem our need to return to ZZZ. He agreed and we were given clearance back to ZZZ. One thing I missed in our haste was copying an amendment to our dispatch release for the return. I delayed for a short time declaring an emergency because I wanted to make sure we could get turn back to ZZZ without complications and not have the distractions of the controllers asking all the emergency question. Once we got turned around and began a descent; I declared an emergency and we continued to ZZZ. During the descent we had one more slight incident of rudder displacement; but it was just a momentary event and the aircraft was controllable. The rest of the flight and landing were normal. I wrote up everything I knew at the time in the logbook and talked with maintenance and dispatch and chief pilot about the incident then we continued our day. The first officer and I talked about what caused this and we are not sure. The aircraft had a slightly different feel to it. The ailerons were a little heavy; and the rudder pedals were a little light. Both these conditions were only very slight and not really a concern to me. I and the first officer felt that we have felt this many times before and would attribute this to aircraft rigging; older aircraft etc. In no way did the aircraft feel sloppy.callback conversation with reporter acn 764836 revealed the following information: the reporter stated that the rudder input was gradual; and the rudder trim was not moving. The flight crew was able to overcome the input; but substantial resistance was encountered on the rudder pedals. This dissipated; and the aircraft was 'hand flown' for the remainder of the flight. An additional rudder displacement occurred on arrival while hand flying; but this encounter was not as pronounced. The PCU was replaced after the event; although no defects were discovered with the unit that was replaced.

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Original NASA ASRS Text

Title: B737-300 UNCOMMANDED RUDDER INPUT ROLLED ACFT WHILE ON AUTOPILOT. FLT CREW DISENGAGED AUTOPILOT AND WAS ABLE TO OVERCOME RUDDER ANOMALY; DECLARED EMER; AND RETURNED FOR NORMAL LNDG.

Narrative: ON OUR LEG FO WAS PILOT FLYING. CLIMBING THRU FL240 I NOTICED ABOUT A 20 DEGREE LEFT AILERON DISPLACEMENT AND ASKED THE FO TO TRIM WITH RUDDER TO GET THE AILERON OUT. ALSO NOTED THAT THE BALL WAS ABOUT 1/2 DOT OUT TO THE LEFT. AFTER TRIMMING; THE AILERON WAS LEVEL AND THE BALL WAS CENTERED. CLIMBING THRU FL270 THE AIRCRAFT BEGAN AN UNCOMMANDED ROLL TO THE RIGHT. THE FO AND I BOTH NOTICED THAT THE RUDDER WAS MOVING TO THE RIGHT ON ITS OWN. WE STOPPED OUR CLIMB AND DISENGAGED THE AUTOPILOT AND PERFORMED THE UNCOMMANDED YAW/ROLL MEMORY ITEMS. WE WERE IMC AT THE TIME OF THE INCIDENT AND I THINK THAT WE ONLY ROLLED APPROXIMATELY 10 DEGREES TO THE RIGHT BEFORE THE FO TOOK CONTROL. THE RUDDER CONTINUED TO PUSH TO THE RIGHT FOR A WHILE BUT WE HAD CONTROL OF THE AIRCRAFT. WE HAD A COMPANY PILOT ONBOARD THE JUMPSEAT SO I HANDED HIM THE QRH AND HE RAN THRU THE PROCEDURES IN THE CHECKLIST WHILE THE FO AND I DIRECTED OUR ATTENTION TO AIRCRAFT CONTROL. THIS WAS A BIG HELP! I ALSO ASKED HIM IF THERE WAS ANYTHING HE THOUGHT WE MISSED. AFTER WE WERE SURE THAT WE HAD CONTROL OF THE AIRCRAFT; I CONTACTED DISPATCH VIA ACARS AND THEN THRU COMPANY RADIO AND EXPLAINED OUR PROBLEM OUR NEED TO RETURN TO ZZZ. HE AGREED AND WE WERE GIVEN CLEARANCE BACK TO ZZZ. ONE THING I MISSED IN OUR HASTE WAS COPYING AN AMENDMENT TO OUR DISPATCH RELEASE FOR THE RETURN. I DELAYED FOR A SHORT TIME DECLARING AN EMERGENCY BECAUSE I WANTED TO MAKE SURE WE COULD GET TURN BACK TO ZZZ WITHOUT COMPLICATIONS AND NOT HAVE THE DISTRACTIONS OF THE CONTROLLERS ASKING ALL THE EMERGENCY QUESTION. ONCE WE GOT TURNED AROUND AND BEGAN A DESCENT; I DECLARED AN EMERGENCY AND WE CONTINUED TO ZZZ. DURING THE DESCENT WE HAD ONE MORE SLIGHT INCIDENT OF RUDDER DISPLACEMENT; BUT IT WAS JUST A MOMENTARY EVENT AND THE AIRCRAFT WAS CONTROLLABLE. THE REST OF THE FLIGHT AND LANDING WERE NORMAL. I WROTE UP EVERYTHING I KNEW AT THE TIME IN THE LOGBOOK AND TALKED WITH MAINT AND DISPATCH AND CHIEF PILOT ABOUT THE INCIDENT THEN WE CONTINUED OUR DAY. THE FO AND I TALKED ABOUT WHAT CAUSED THIS AND WE ARE NOT SURE. THE AIRCRAFT HAD A SLIGHTLY DIFFERENT FEEL TO IT. THE AILERONS WERE A LITTLE HEAVY; AND THE RUDDER PEDALS WERE A LITTLE LIGHT. BOTH THESE CONDITIONS WERE ONLY VERY SLIGHT AND NOT REALLY A CONCERN TO ME. I AND THE FO FELT THAT WE HAVE FELT THIS MANY TIMES BEFORE AND WOULD ATTRIBUTE THIS TO AIRCRAFT RIGGING; OLDER AIRCRAFT ETC. IN NO WAY DID THE AIRCRAFT FEEL SLOPPY.CALLBACK CONVERSATION WITH RPTR ACN 764836 REVEALED THE FOLLOWING INFO: THE REPORTER STATED THAT THE RUDDER INPUT WAS GRADUAL; AND THE RUDDER TRIM WAS NOT MOVING. THE FLT CREW WAS ABLE TO OVERCOME THE INPUT; BUT SUBSTANTIAL RESISTANCE WAS ENCOUNTERED ON THE RUDDER PEDALS. THIS DISSIPATED; AND THE ACFT WAS 'HAND FLOWN' FOR THE REMAINDER OF THE FLT. AN ADDITIONAL RUDDER DISPLACEMENT OCCURRED ON ARRIVAL WHILE HAND FLYING; BUT THIS ENCOUNTER WAS NOT AS PRONOUNCED. THE PCU WAS REPLACED AFTER THE EVENT; ALTHOUGH NO DEFECTS WERE DISCOVERED WITH THE UNIT THAT WAS REPLACED.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.