Narrative:

Aircraft ord-oak on descent to oak on the madwin 3 arrival between locke intersection and sunol intersection, descending with the 'B' autoplt on, LNAV engaged, 250 KTS selected in level change, flight attendants and passenger seated. Aircraft gross weight 102000 pounds. At 8000 ft MSL descending to 6000 ft MSL, the aircraft experienced an uncommanded yaw event lasting approximately 1.5 seconds. The aircraft yawed to the right, the yaw was accompanied by a loud rumble and bang. The first officer thought we hit something. I thought we had flown through the wake of another aircraft. I checked the yaw damper switch, it was on, and asked oak approach if we were following another aircraft -- we were not. The 'B' autoplt and autothrottles remained on throughout the event. The autoplt and autothrottles were disconnected around 6000 ft MSL, all engine indications were normal. A visual approach was flown to runway 29 at oakland. On approximately 1 mi final the first officer (PF) thought it felt like the yaw damper was not working. I bugged 5 KTS extra to the target airspeed, we landed at oakland uneventfully. Callback conversation with reporter revealed the following information: reporter said that the initial roll of the aircraft was like a wake turbulence encounter. There was no reported traffic. There is research being done to see if there was departure traffic that had passed above but not under control of TRACON in the very recent past. Windshear was reported in that geographic area and altitude during that day. Yaw damper was checked on final and indications were that it was normal. Reporter has been debriefed by company safety, flight standards, union safety representatives and maintenance. No maintenance problems discovered, however, several components were changed. All involved are treating it as a serious incident.

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Original NASA ASRS Text

Title: B737-300 CREW HAS UNEXPLAINED YAW EVENT DURING ARR TO OAK, CA.

Narrative: ACFT ORD-OAK ON DSCNT TO OAK ON THE MADWIN 3 ARR BTWN LOCKE INTXN AND SUNOL INTXN, DSNDING WITH THE 'B' AUTOPLT ON, LNAV ENGAGED, 250 KTS SELECTED IN LEVEL CHANGE, FLT ATTENDANTS AND PAX SEATED. ACFT GROSS WT 102000 LBS. AT 8000 FT MSL DSNDING TO 6000 FT MSL, THE ACFT EXPERIENCED AN UNCOMMANDED YAW EVENT LASTING APPROX 1.5 SECONDS. THE ACFT YAWED TO THE R, THE YAW WAS ACCOMPANIED BY A LOUD RUMBLE AND BANG. THE FO THOUGHT WE HIT SOMETHING. I THOUGHT WE HAD FLOWN THROUGH THE WAKE OF ANOTHER ACFT. I CHKED THE YAW DAMPER SWITCH, IT WAS ON, AND ASKED OAK APCH IF WE WERE FOLLOWING ANOTHER ACFT -- WE WERE NOT. THE 'B' AUTOPLT AND AUTOTHROTTLES REMAINED ON THROUGHOUT THE EVENT. THE AUTOPLT AND AUTOTHROTTLES WERE DISCONNECTED AROUND 6000 FT MSL, ALL ENG INDICATIONS WERE NORMAL. A VISUAL APCH WAS FLOWN TO RWY 29 AT OAKLAND. ON APPROX 1 MI FINAL THE FO (PF) THOUGHT IT FELT LIKE THE YAW DAMPER WAS NOT WORKING. I BUGGED 5 KTS EXTRA TO THE TARGET AIRSPD, WE LANDED AT OAKLAND UNEVENTFULLY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR SAID THAT THE INITIAL ROLL OF THE ACFT WAS LIKE A WAKE TURB ENCOUNTER. THERE WAS NO RPTED TFC. THERE IS RESEARCH BEING DONE TO SEE IF THERE WAS DEP TFC THAT HAD PASSED ABOVE BUT NOT UNDER CTL OF TRACON IN THE VERY RECENT PAST. WINDSHEAR WAS RPTED IN THAT GEOGRAPHIC AREA AND ALT DURING THAT DAY. YAW DAMPER WAS CHKED ON FINAL AND INDICATIONS WERE THAT IT WAS NORMAL. RPTR HAS BEEN DEBRIEFED BY COMPANY SAFETY, FLT STANDARDS, UNION SAFETY REPRESENTATIVES AND MAINT. NO MAINT PROBS DISCOVERED, HOWEVER, SEVERAL COMPONENTS WERE CHANGED. ALL INVOLVED ARE TREATING IT AS A SERIOUS INCIDENT.

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.