LOSS OF ACFT CTL -- ACR ROLLS TO THE L AND THEN TO THE R DURING VECTORS FOR AN APCH. FLC RECOVERS AFTER DISCONNECTING THE AUTOPLT.

Narrative

FO'S TURN TO FLY. AUTOPLT ON IN B CHANNEL; HDG SELECT; 210 KTS; AUTOTHROTTLE ON; FLAPS 1 DEG; NO SIGNS OF ELECTRICAL MALFUNCTIONS; NO SPLIT FLAPS; IN LEVEL FLT WITH NO THRUST OR FLAP INPUTS WITHIN 3 MINS. THE ACFT BEGAN AN UNCOMMANDED ROLL OF 30-45 DEGS TO THE L. BOTH PLTS APPLIED AILERON INPUT TO CORRECT; FO APPLIED R RUDDER AND BALL INDICATOR WENT FULL L THEN THE ACFT ROLLED R 30-45 DEGS R. OUR INPUT WAS UNRESPONSIVE. FO DISCONNECTED THE AUTOPLT AND THE ACFT BECAME RESPONSIVE TO OUR INPUTS. APCH CTL SAID THERE WERE NO OTHER PLANES IN OUR VICINITY. AT THE BEGINNING OF OUR FLT; WE FELT A SLIGHT CONSISTENT VIBRATION AT LEVEL CRUISE AT ALT. WE HAD NO CTL OF THE ACFT DURING THE ROLL. TOTAL TIME OF LOSS OF CTL 4-8 SECONDS. FLT ATTENDANTS IN FORWARD CABIN SAID THEY FELT A DUTCH ROLL FEELING; THE FLT ATTENDANTS IN BACK FELT NOTHING. THE ROLL WAS VERY SMOOTH. WE MAINTAINED ALT AND HDG THROUGHOUT THE ROLL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CAPT WAS FLYING A B737-300 AT THE TIME OF THIS UPSET. THE FO WAS ACTUALLY FLYING THE ACFT; BUT THE CAPT WAS THE FIRST TO ATTEMPT TO COUNTERACT THE ROLL TO THE L. THE ACFT DID NOT RESPOND. IN FACT; NONE OF THE INPUTS WERE SUCCESSFUL UNTIL THE FO DISCONNECTED THE AUTOPLT. AT THAT TIME; FULL CTL WAS REESTABLISHED. LATER THE COMPANY FOUND THAT THE AUTOPLT; THE FMC AND THE YAW DAMPER HAD SOME MAINT PROBS THAT COULD HAVE BEEN A POSSIBLE CAUSE OF PART OF THE PROB; BUT NO DIRECT CAUSE HAS BEEN LOCATED.