Narrative:

On left base visual for runway 28R, ZZZ, speed 200 KTS flaps 2 degrees, 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, requested equipment, proceeded to an area away from airport where we could trouble-shoot and do checklists. Autoplt and autothrottle had already been disconnected long before the visual approach began. Turned yaw damper off, vibration continued. Aircraft control was sluggish, unpredictable. Contacted maintenance thru dispatch. Performed rudder pedal oscillation and airframe vibration checklist. Once B pumps were off, vibration stopped. Vibration began again on final (B pumps on per checklist, speed 180 KTS, flap 15 degrees.) went around again, continued intermittent vibration, consulted maintenance control (no new instructions). Aircraft control seemed more difficult, opted to turn breakers off, leave them off. Cabin had already been prepped via interphone, PA to passenger, configured aircraft for flaps 15 degree landing using alternate flap operation checklist. Vibration did not recur. Brace signal given at 1000 ft. Normal touchdown and reverse/brake operation. Remain seated command given on rollout, turnoff at taxiway F. Emergency equipment crew verified aircraft condition appeared normal. Aircraft taxied to gate where passenger deplaned normally.

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Original NASA ASRS Text

Title: A B737-300 ON APCH AT 7000 FT EXPERIENCED RUDDER KICK AND VIBRATIONS. DECLARED EMER, MADE 2 GARS THEN LANDED. CAUSE UNKNOWN.

Narrative: ON L BASE VISUAL FOR RWY 28R, ZZZ, SPD 200 KTS FLAPS 2 DEGS, FELT A KICK IN R RUDDER THEN A VIBRATION OF LONGER DURATION IN RUDDERS (SIMILAR IN SOUND AND FREQ TO STICK SHAKER EXCEPT RUDDERS WERE VIBRATING). INITIATED GAR, NOTIFIED TWR OF POTENTIAL CTL PROB, DECLARED EMER, REQUESTED EQUIP, PROCEEDED TO AN AREA AWAY FROM ARPT WHERE WE COULD TROUBLE-SHOOT AND DO CHKLISTS. AUTOPLT AND AUTOTHROTTLE HAD ALREADY BEEN DISCONNECTED LONG BEFORE THE VISUAL APCH BEGAN. TURNED YAW DAMPER OFF, VIBRATION CONTINUED. ACFT CTL WAS SLUGGISH, UNPREDICTABLE. CONTACTED MAINT THRU DISPATCH. PERFORMED RUDDER PEDAL OSCILLATION AND AIRFRAME VIBRATION CHKLIST. ONCE B PUMPS WERE OFF, VIBRATION STOPPED. VIBRATION BEGAN AGAIN ON FINAL (B PUMPS ON PER CHKLIST, SPD 180 KTS, FLAP 15 DEGS.) WENT AROUND AGAIN, CONTINUED INTERMITTENT VIBRATION, CONSULTED MAINT CTL (NO NEW INSTRUCTIONS). ACFT CTL SEEMED MORE DIFFICULT, OPTED TO TURN BREAKERS OFF, LEAVE THEM OFF. CABIN HAD ALREADY BEEN PREPPED VIA INTERPHONE, PA TO PAX, CONFIGURED ACFT FOR FLAPS 15 DEG LNDG USING ALTERNATE FLAP OP CHKLIST. VIBRATION DID NOT RECUR. BRACE SIGNAL GIVEN AT 1000 FT. NORMAL TOUCHDOWN AND REVERSE/BRAKE OP. REMAIN SEATED COMMAND GIVEN ON ROLLOUT, TURNOFF AT TXWY F. EMER EQUIP CREW VERIFIED ACFT CONDITION APPEARED NORMAL. ACFT TAXIED TO GATE WHERE PAX DEPLANED NORMALLY.

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.