Narrative:

We took off and climbed to FL360 without event. At cruise altitude FL360 captain turned on flood lights in cockpit. Fukuoka control gave us three consecutive clearances. Cleared direct kre; then a few moments later cleared direct fue; and then a few moments later cleared direct to sadli. Each time proper procedures were used. I selected each waypoint with abeam points; asked for confirmation. Captain responded 'execute'. LNAV and VNAV path was always engaged and verified (in green) on FMA. At that point captain and I began to review the flight planned arrival and we discussed the transitions. Fukuoka called and asked if we were going direct sadli. I looked at the nd and pfd and the aircraft was making an uncommanded shallow bank right turn and now was at least 90 degrees turned off course. LNAV and VNAV path was still green on FMA. The magenta course line with abeam points was behind us. Captain selected heading and turned aircraft back towards sadli. I verified (at captain's request) that the route was accurate and in fact direct sadli had been executed and was indicated; it had. We both remarked that the aircraft was not responding fast enough. Captain disengaged autopilot from left autopilot and began to fly aircraft by hand. I reselected direct sadli again. Confirmed and executed; then once aircraft was heading direct to sadli captain asked me to engage right autopilot command. LNAV was re-engaged. Fukuoka asked us what the problem was. I responded 'we had an autopilot event we did not initially notice.' no further inquiry was made by fukuoka. We never felt the aircraft turning in the shallow bank while we discussed and reviewed the approach; and we had no outside reference due to night and cockpit lights on. There was no perceptible horizon. We were over an undercast. We proceeded to our destination without any further issues. Captain wrote an aircraft maintenance log entry that it had an uncommanded turn while left autopilot was engaged while in LNAV; VNAV path. The mechanic diagnosed the event and found an error logged in aircraft history. Fault history message report. Autopilot flight dir mode operation error in cruise no action required intermittent message >autopilot -caution.

Google
 

Original NASA ASRS Text

Title: B747 flight crew experiences a track deviation while proceeding direct to the next waypoint. The aircraft is turned 90 degrees to the desired track when queried by ATC about their heading. Post flight maintenance reveals an intermittent autopilot fault.

Narrative: We took off and climbed to FL360 without event. At cruise altitude FL360 Captain turned on flood lights in cockpit. Fukuoka Control gave us three consecutive clearances. Cleared direct KRE; then a few moments later cleared direct FUE; and then a few moments later cleared direct to SADLI. Each time proper procedures were used. I selected each waypoint with abeam points; asked for confirmation. Captain responded 'Execute'. LNAV and VNAV PATH was always engaged and verified (in Green) on FMA. At that point Captain and I began to review the Flight Planned arrival and we discussed the transitions. Fukuoka called and asked if we were going direct SADLI. I looked at the ND and PFD and the aircraft was making an uncommanded shallow bank right turn and now was at least 90 degrees turned off course. LNAV and VNAV path was still green on FMA. The magenta course line with abeam points was behind us. Captain selected Heading and turned aircraft back towards SADLI. I verified (at Captain's request) that the route was accurate and in fact direct SADLI had been executed and was indicated; it had. We both remarked that the aircraft was not responding fast enough. Captain disengaged autopilot from left autopilot and began to fly aircraft by hand. I reselected direct SADLI again. Confirmed and executed; then once aircraft was heading direct to SADLI Captain asked me to engage right autopilot command. LNAV was re-engaged. Fukuoka asked us what the problem was. I responded 'We had an autopilot event we did not initially notice.' No further inquiry was made by Fukuoka. We never felt the aircraft turning in the shallow bank while we discussed and reviewed the Approach; and we had no outside reference due to night and cockpit lights on. There was no perceptible horizon. We were over an undercast. We proceeded to our destination without any further issues. Captain wrote an aircraft maintenance log entry that it had an uncommanded turn while left autopilot was engaged while in LNAV; VNAV path. The Mechanic diagnosed the event and found an Error Logged in aircraft history. Fault History Message Report. Autopilot FLT DIR MODE OPERATION ERROR IN CRUISE NO ACTION REQUIRED INTERMITTENT MSG >AUTOPILOT -CAUTION.

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.