FLT CREW OF B733 RPTS 2 INCIDENCES OF UNCOMMANDED ROLL WHILE OPERATING ON B AUTOPLT.

Narrative

SMOOTH AIR FL260; AIRSPD 320 KTS; VERT SPD MODE 2000 FPM DOWN; LNAV ENGAGED; AUTOPLT B ENGAGED. ACFT BANKED 30 DEGS L AT MEDIUM TO RAPID RATE; FOLLOWED BY CTL WHEEL STEERING R; SHORTLY AFTER FO DISCONNECTED AUTOPLT AND AUTOTHROTTLES. NO YAW FELT AND RUDDER PEDALS CTRED. ACFT WAS SLOWED TO 280 KTS. HEADING SELECT AND B AUTOPLT WERE REENGAGED AFTER 2 ATTEMPTS. CAPT ASKED ATC FOR RIDE RPTS AND CENTER SAID SMOOTH. HOWEVER; 10 MINS PRIOR PLTS OVERHEARD UNKNOWN ACFT RPT MODERATE TURB; AT FL310 AREA UNKNOWN. THEN AT XA22Z OVER ZZZZZ INTXN; LEVEL FLT 11000 FT; 280 KTS; SMOOTH AIR; ACFT BANKED 15 DEGS L AT A MEDIUM RATE FOR 4 SECONDS; WHILE AT THE SAME TIME ACFT ENCOUNTERED A RHYTHMIC LIGHT CHOP. NO YAW FELT AND RUDDER PEDALS WERE CTRED. AUTOPLT B WAS LEFT ON THROUGHOUT. CAPT ASKED FOR RIDE RPTS; APCH REPLIED SMOOTH. AUTOPLT B REMAINED ON UNTIL 3000 FT. APCH AND LNDG WERE NORMAL. CONCLUSION: DIFFICULT TO TELL IF THE UNCOMMANDED BANK WAS CAUSED BY TURB; WAKE TURB; OR SOME KIND OF AUTOPLT OR CTL PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CAPT ON THE FLT ADVISED THAT; NOT SURPRISINGLY; THE COMPANY MAINT AND FLT SAFETY DEPTS TOOK THIS INCIDENT VERY SERIOUSLY AND; AMONG OTHER CHKS; INVESTIGATED EVIDENCE FROM THE FLT DATA RECORDER. IN CONCERT WITH THE RPTS OF THE PLTS; THE RECORDER INDICATED ONLY ROLL INPUTS AND NOTHING IN THE YAW AXIS. TO THE BEST OF THE CAPT'S CURRENT KNOWLEDGE THE FIX FOR THEIR WRITE-UP WAS TO REPLACE WHAT SHE REMEMBERED AS THE 'AUTOPLT AILERON LIMITER.' SUPPLEMENTAL INFO FROM ACN 585366: ATC ASKED AN RJ ON ARR AHEAD OF US HOW HIS RIDE HAD BEEN IN DSCNT. HE SAID SMOOTH ALL THE WAY. ON BOTH EVENTS; NO ABNORMAL ENG READINGS NOTED. YAW DAMPER INDICATIONS NORMAL. ALL FLT CTL INDICATIONS NORMAL. ALL HYD FLUID LEVELS NORMAL. NO FLAP SETTING CHANGES. NO GEAR CHANGES. NO SPD BRAKE CHANGES. NO THROTTLE ALIGNMENT PROBS OR CHANGES NOTED. NO YAW NOTED. NO RUDDER PEDAL OR DISPLACEMENT NOTED. NO LARGE ACFT IN AREA NOTED (VISIBILITY GOOD ON) OR POINTED OUT BY ATC. ONLY TFC POINTED OUT BY ATC WAS RJ ON ARR AHEAD AND BELOW US. NO ADVERSE TURB RPTS NOTED BY ATC OR ANY OTHER ACFT IN AREA FROM 26000 FT AND BELOW. NO CHANGE IN WIND DIRECTION OR SPD OR SHEARS NOTED ON DSCNT (WIND DIRECTION AND SPD ON EFIS DISPLAY).