Narrative:

First officer's turn to fly. Autoplt on in B channel, heading select, 210 KTS, autothrottle on, flaps 1 degree, no signs of electrical malfunctions, no split flaps, in level flight with no thrust or flap inputs within 3 mins. The aircraft began an uncommanded roll of 30-45 degrees to the left. Both pilots applied aileron input to correct, first officer applied right rudder and ball indicator when full left then the aircraft rolled right 30-45 degrees right. Our input was unresponsive. First officer disconnected the autoplt and the aircraft became responsive to our inputs. Approach control said there were no other planes in our vicinity. At the beginning of our flight, we felt a slight consistent vibration at level cruise at altitude. We had no control of the aircraft during the roll. Total time of loss of control 4-8 seconds. Flight attendants in forward cabin said they felt a dutch roll feeling, the flight attendants in back felt nothing. The roll was very smooth. We maintained altitude and heading throughout the roll. Callback conversation with reporter revealed the following information: the captain was flying a B737-300 at the time of this upset. The first officer was actually flying the aircraft, but the captain was the first to attempt to counteract the roll to the left. The aircraft did not respond. In fact, none of the inputs were successful until the first officer disconnected the autoplt. At that time, full control was reestablished. Later the company found that the autoplt, the FMC and the yaw damper had some maintenance problems that could have been a possible cause of part of the problem, but no direct cause has been located.

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Original NASA ASRS Text

Title: 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 WHEN 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.

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.