A B747-200 autopilot 'B' entered an uncommanded left turn and did not respond to heading mode inputs...

Narrative:

During cruise at FL340; first officer was in first officer's seat. Autopilot 'B' engaged. I went to the bathroom. Aircraft entered moderate turbulence. I notice as I returned to my seat that aircraft was in a 30 degree bank turn to right. First officer was hand-flying airplane while autopilot 'B' still engaged in command. First officer told me the aircraft rolled by itself left. At first; first officer selected heading mode to correct deviation. There was no response; aircraft continued to roll left. He recovered from the impending upset and returned the aircraft back to course hand flying it. Altitude remained at FL340 +/- 50 ft. There was no warning: (wailer or light). All GPS parameters were normal; hydraulic system #2 (power to autopilot B) normal. ATC asked us if all was ok; we told them yes. First officer engaged autopilot 'a' and we proceeded with out incidence. There was no warning: (wailer or light). All GPS parameters were normal; hydraulic system #2 (power to autopilot B) normal.

Original NASA ASRS Text

Title: A B747-200 autopilot 'B' entered an uncommanded left turn and did not respond to Heading Mode inputs so autopilot 'A' was selected which corrected the deviation.

Narrative: During cruise at FL340; First Officer was in First Officer's seat. Autopilot 'B' engaged. I went to the bathroom. Aircraft entered moderate turbulence. I notice as I returned to my seat that aircraft was in a 30 degree bank turn to right. First Officer was hand-flying airplane while autopilot 'B' still engaged in command. First Officer told me the aircraft rolled by itself left. At first; First Officer selected Heading Mode to correct deviation. There was no response; aircraft continued to roll left. He recovered from the impending upset and returned the aircraft back to course hand flying it. Altitude remained at FL340 +/- 50 FT. There was no warning: (WAILER or LIGHT). All GPS parameters were normal; Hydraulic System #2 (power to autopilot B) normal. ATC asked us if all was OK; we told them yes. First Officer engaged autopilot 'A' and we proceeded with out incidence. There was no warning: (WAILER or LIGHT). All GPS parameters were normal; Hydraulic System #2 (power to Autopilot B) normal.

Data retrieved from NASA's ASRS site as of July 2013 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.

Attributes
ACN	996257
Time
Date	201202
Local Time Of Day	0601-1200
Environment
Light	Daylight
Aircraft 1
Make Model Name	B747-200
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Autopilot
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Track / Heading All Types Inflight Event / Encounter Loss Of Aircraft Control Inflight Event / Encounter Weather / Turbulence

A B747-200 autopilot 'B' entered an uncommanded left turn and did not respond to heading mode inputs so autopilot 'a' was selected which corrected the deviation.