Narrative:

Smooth air FL260, airspeed 320 KTS, vertical speed mode 2000 FPM down, LNAV engaged, autoplt B engaged. Aircraft banked 30 degrees left at medium to rapid rate, followed by control wheel steering right, shortly after first officer disconnected autoplt and autothrottles. No yaw felt and rudder pedals centered. Aircraft was slowed to 280 KTS. Heading select and B autoplt were reengaged after 2 attempts. Captain asked ATC for ride reports and ZBW said smooth. However, 10 mins prior pilots overheard unknown aircraft report moderate turbulence, at FL310 area unknown. Then at XA22Z over bronc intersection, level flight 11000 ft, 280 KTS, smooth air aircraft banked 15 degrees left at a medium rate for 4 seconds, while at the same time aircraft encountered a rhythmic light chop. No yaw felt and rudder pedals were centered. Autoplt B was left on throughout. Captain asked for ride reports, bos approach replied smooth. Autoplt B remained on until 3000 ft. Approach and landing were normal. Conclusion: difficult to tell if the uncommanded bank was caused by turbulence, wake turbulence, or some kind of autoplt or control problem. Callback conversation with reporter revealed the following information: the captain on the flight advised that, not surprisingly, the company maintenance and flight safety departments took this incident very seriously and, among other checks, investigated evidence from the flight data recorder. In concert with the reports of the pilots, the recorder indicated only roll inputs and nothing in the yaw axis. To the best of the captain's current knowledge the fix for their write-up was to replace what she remembered as the 'autoplt aileron limiter.' supplemental information from acn 585366: ATC asked an rj on arrival ahead of us how his ride had been in descent. He said smooth all the way. On both events, no abnormal engine readings noted. Yaw damper indications normal. All flight control indications normal. All hydraulic fluid levels normal. No flap setting changes. No gear changes. No speed brake changes. No throttle alignment problems or changes noted. No yaw noted. No rudder peddle or displacement noted. No large aircraft in area noted (visibility good on arrival and into bos) or pointed out by ATC. Only traffic pointed out by ATC was rj on arrival ahead and below us. No adverse turbulence reports noted by ATC or any other aircraft in area from 26000 ft and below. No change in wind direction or speed or sheers noted on descent (wind direction and speed on EFIS display).

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Original NASA ASRS Text

Title: 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 ZBW SAID SMOOTH. HOWEVER, 10 MINS PRIOR PLTS OVERHEARD UNKNOWN ACFT RPT MODERATE TURB, AT FL310 AREA UNKNOWN. THEN AT XA22Z OVER BRONC 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, BOS 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 PEDDLE OR DISPLACEMENT NOTED. NO LARGE ACFT IN AREA NOTED (VISIBILITY GOOD ON ARR AND INTO BOS) 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 SHEERS NOTED ON DSCNT (WIND DIRECTION AND SPD ON EFIS DISPLAY).

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.