B747-400
Flight Crew Air Transport Pilot (ATP)
Confusion
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Aircraft Equipment Problem Dissipated; Flight Crew Returned To Clearance; General Maintenance Action
Aircraft; Human Factors
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.
Approximately 1 hour after departure; cruise; FL360. We had just received direct to SADLI. It was the third 'direct clearance' we had received in the past few minutes. I turned up the range because SADLI was approximately 400 miles away; confirmed the waypoint direct clearance; and gave the 'execute' order. I noticed at the time that the First Officer had included abeam points and was thinking I didn't really want that on the screen; but we were not using CPDLC; so I decided to leave it. The clearance was basically straight ahead; so there wasn't much; if any turn involved. We had previously been cleared direct to KRE; then FUE and a few minutes later to SADLI. The First Officer verbalized 'LNAV AVAILABLE' and I confirmed 'LNAV ENGAGED'. A few minutes later; we were discussing the arrival and approach into our destination. We both had our charts out; and had turned the lights up to discuss the procedure. There was no horizon; there was an undercast and it was very dark outside; so there was no visual cue that we apparently had started a turn to the right at some point and we did not notice it. We both noticed at about the time Fukuoka Center called asking us to confirm we were going direct to SADLI. I can't be sure; but it could be that we noticed it because center was calling. The first response to Center was 'correcting to course'. I did notice that LNAV was still displayed on the FMA; but we selected heading in an attempt to get the aircraft back on course. When we first noticed the problem; we were approximately 90 degrees off course; in a shallow bank right turn; and still diverging to the right. The aircraft did not respond to the heading select; or possibly just didn't respond fast enough for me; so I disengaged the autopilot and manually turned back toward the course. I directed the First Officer to check the FMC to see that our course was still there; and then directed him to re-enter direct to SADLI. As I turned back to course; we could see the magenta line was still displayed. After I returned the aircraft to the proper course; I directed the First Officer to engage the right autopilot. We had previously been using the left. We then re-verified all the subsequent waypoints; and had no further incidents. The Center called back and asked the nature of our problem; and the First Officer responded that we had an autopilot malfunction and that we were back on course at this time. There was no further communication with ATC; other than normal clearance to the next Center. There was no EICAS indication of an autopilot problem. I wrote up the 'uncommanded turn' in the log book; and Maintenance discovered a 'mode operation error' on the autopilot. Not being familiar with the maintenance computer; I'm not exactly sure what that means; but apparently whatever he did was enough to clear and sign off the logbook entry. We continued the next leg using the right autopilot with no further incident. [We needed] increased vigilance on the flight deck to more quickly detect an insidious deviation from course. Replace or repair malfunctioning auto-pilot; if that was in fact the cause.
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.