Narrative:

Turning on left base visual runway xx right speed 200K flaps 2 captain felt a kick in right rudder then a vibration of longer duration in rudders (similar in sound and frequency to stick shaker except rudders were vibrating). Initiated go around, notified tower of potential control problem. Declared emergency, req equipment, proceeded to an area northwest of airport where we could troubleshoot and do checklists. Autoplt and automatic throttle had already been disconnected before the visual approach began. Turned yaw damper off--vibration recurred. Turned hydraulic B switch to stby rudder position, vibration continued. Aircraft control seemed sluggish, unpredictable. Contacted mx thru dispatch and preformed rudder pedal oscillation and airframe vibration checklist. With B pumps off--vibration stopped. Vibration began again on final (B pumps on per checklist, speed 180, flaps 15). Went around again, cont intermittent vibration, consulted mx (no new instructions). Aircraft control seemed more difficult, opted to turn off B's and leave off. Cabin attendant had already been prepped via interphone, PA made to passenger, configured aircraft for flaps 15 landing using alternate flap operation checklist. Vibration did not reoccur. Brace signal given at 1000 ft. Normal touchdown and reverse/brake operation. Remain seated command given on rollout, turnoff at taxiway. Emergency equipment crew verified aircraft condition appeared normal. Aircraft taxied to gate where passenger deplaned normally. Callback conversation with reporter revealed the following information: the reporter stated the air carrier removed the aircraft from service. Mechanics and air carrier engineers were sent to the non maintenance station and conducted a complete inspection of the rudder system and aircraft hydraulic system. The rudder power control unit was replaced. The next morning an air carrier flight test crew arrived to give the aircraft a complete flight test. The aircraft was ferried to a maintenance base of the air carrier. The aircraft was then given to the manufacturer for testing and analysis. After extensive work by the manufacturer the aircraft was returned to service. The manufacturer returned the rudder PCU to the factory for testing and disassembly. The reporter heard some air carrier engineers talking about a possible feedback loop from the aircraft brake system. Apparently this can cause this type of problem. The reporter stated the aircraft had all the latest logbook releases and ad's complied with, however the aircraft did not have the extensive rudder system redesign accomplished.

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Original NASA ASRS Text

Title: B737-300 CREW HAD UNCOMMANDED R RUDDER KICK AND RAPID RUDDER VIBRATION AND OSCILLATION. ONLY TURNING OFF THE B HYD PUMPS WOULD STOP THE PROB.

Narrative: TURNING ON L BASE VISUAL RWY XX RIGHT SPEED 200K FLAPS 2 CAPT FELT A KICK IN R RUDDER THEN A VIBRATION OF LONGER DURATION IN RUDDERS (SIMILAR IN SOUND AND FREQUENCY TO STICK SHAKER EXCEPT RUDDERS WERE VIBRATING). INITIATED GAR, NOTIFIED TWR OF POTENTIAL CTL PROB. DECLARED EMER, REQ EQUIP, PROCEEDED TO AN AREA NW OF ARPT WHERE WE COULD TROUBLESHOOT AND DO CHKLISTS. AUTOPLT AND AUTO THROTTLE HAD ALREADY BEEN DISCONNECTED BEFORE THE VISUAL APCH BEGAN. TURNED YAW DAMPER OFF--VIBRATION RECURRED. TURNED HYD B SWITCH TO STBY RUDDER POSITION, VIBRATION CONTINUED. ACFT CTL SEEMED SLUGGISH, UNPREDICTABLE. CONTACTED MX THRU DISPATCH AND PREFORMED RUDDER PEDAL OSCILLATION AND AIRFRAME VIBRATION CHKLIST. WITH B PUMPS OFF--VIBRATION STOPPED. VIBRATION BEGAN AGAIN ON FINAL (B PUMPS ON PER CHKLIST, SPD 180, FLAPS 15). WENT AROUND AGAIN, CONT INTERMITTENT VIBRATION, CONSULTED MX (NO NEW INSTRUCTIONS). ACFT CTL SEEMED MORE DIFFICULT, OPTED TO TURN OFF B'S AND LEAVE OFF. CAB HAD ALREADY BEEN PREPPED VIA INTERPHONE, PA MADE TO PAX, CONFIGURED ACFT FOR FLAPS 15 LNDG USING ALTERNATE FLAP OP CHKLIST. VIBRATION DID NOT REOCCUR. BRACE SIGNAL GIVEN AT 1000 FT. NORMAL TOUCHDOWN AND REVERSE/BRAKE OP. REMAIN SEATED COMMAND GIVEN ON ROLLOUT, TURNOFF AT TXWY. EMER EQUIP CREW VERIFIED ACFT CONDITION APPEARED NORMAL. ACFT TAXIED TO GATE WHERE PAX DEPLANED NORMALLY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACR REMOVED THE ACFT FROM SVC. MECHANICS AND ACR ENGINEERS WERE SENT TO THE NON MAINT STATION AND CONDUCTED A COMPLETE INSPECTION OF THE RUDDER SYSTEM AND ACFT HYDRAULIC SYS. THE RUDDER POWER CTL UNIT WAS REPLACED. THE NEXT MORNING AN ACR FLIGHT TEST CREW ARRIVED TO GIVE THE ACFT A COMPLETE FLT TEST. THE ACFT WAS FERRIED TO A MAINT BASE OF THE ACR. THE ACFT WAS THEN GIVEN TO THE MANUFACTURER FOR TESTING AND ANALYSIS. AFTER EXTENSIVE WORK BY THE MANUFACTURER THE ACFT WAS RETURNED TO SVC. THE MANUFACTURER RETURNED THE RUDDER PCU TO THE FACTORY FOR TESTING AND DISASSEMBLY. THE RPTR HEARD SOME ACR ENGINEERS TALKING ABOUT A POSSIBLE FEEDBACK LOOP FROM THE ACFT BRAKE SYSTEM. APPARENTLY THIS CAN CAUSE THIS TYPE OF PROB. THE RPTR STATED THE ACFT HAD ALL THE LATEST LOGBOOK RELEASES AND AD'S COMPLIED WITH, HOWEVER THE ACFT DID NOT HAVE THE EXTENSIVE RUDDER SYSTEM REDESIGN ACCOMPLISHED.

Data retrieved from NASA's ASRS site as of July 2007 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.