Narrative:

Flight left lhr on time at XX00Z. Initially on headings/ vectors by london and scottish control. Occasionally while climbing out we would get message on FMC 'route uplink ready to load.' since we had already done this on the ground and everything looked normal it was just bothersome, but questionable why this FMC (both actually) was doing that. Near glasgow we were at our final, FL310 and the first officer had finished doing all the required checks before entering class ii airspace. IRS checked fine, our VOR check was right on, position check was correct and our magnetic compass, HSI check was correct. I was PF and had aircraft hooked up to autoplt and autothrottle. We were about to receive our oceanic clearance when on the ADI the VNAV annunciation and LNAV annunciation went amber with a yellow line through them. Simultaneously a master caution went off with annunciation of autothrottle disconnect and autoplt disconnect. The autothrottle did disconnect, but the aircraft's autoplt stayed engaged, but without any guidance. Trying to re-engage LNAV and VNAV was unsuccessful. We deselected center autoplt and tried the left and right autoplt with no success. Using heading select I was able to maintain directional guidance and selecting 'flch' was able to maintain airspeed and altitude. Since the autoplt was working, obviously something with LNAV and VNAV was not functioning properly. We had lost our active waypoint, (we were direct 'matik' at time) however all our waypoints were still displayed. The FMC still held our route, but no ETA's were on leg's pages or on progress page. We re-selected 'matik' as our active waypoint, and while very slow the FMC finally reengaged the rtes and we tried 'LNAV' and the aircraft was once again on IRS navigation. The captain then told me 'you fly and talk, we'll work on the problem' meaning the first officer and him. They called maintenance and dispatch and apparently they were of very little help. I did not hear any of their conversation except the first officer and captain were very upset with them. During this time I had scottish confirm our position, and idid some checking on fix page with vors and such. All the time, the FMC display a route, but it was not active, and about this time maintenance had us pull and reset certain circuit breakers. After this the route had remained in the FMC, but it had now become 'mod route' which means it was not active, also the message 'perf/VNAV unavailable' started being displayed. The first officer and I realized almost immediately that that meant we'd lost our performance platform, and that was one reason we did not have ETA's to points. I indexed the FMC and pulled up performance page and sure enough all our performance data had been dumped, probably due to resetting circuit breakers of FMC and other equipment. As soon as I entered all the data the FMC immediately came up with all the proper data, ETA's to fix, time en route, gross weight, etc. We once again did all our checks for entering class ii airspace, and felt comfortable that the problem was fixed. We agreed and we were on our way. During the ocean crossing we were always on course and while over iceland had our position verified with reykevic control. On why the FMC did what it did I have some thoughts. While under radar vectors, the captain kept switching from heading select to LNAV, for as he said 'to keep the clock updated.' often when he did this the message 'not on intercept heading' would come up. Since I was hand flying and keeping a heading it may have confused the FMC on what it was supposed to do. Also I know the captain was going to upload the winds, but I wonder if he accidentally hit 'up load route' instead and that was why we kept getting 'route uplink ready to load' message. We never had FMC failure, nor did we get that message, the route was always in the FMC, and map display was good, it just for some reason, probably human, caused it to overload and go into a degrade mode until it had all the facts. That we were able to restore it points to some glitch, either electronically or human, that caused it to behave the way it did. At no time were we in class ii airspace, and at notime did we lose standard navigation capability (VOR, ADF, ILS). Callback conversation with reporter revealed the following information: the international relief first officer said that he suspected that the captain had initially attempted to enter updated winds by inadvertently pushing the 'upload route' button. The reporter said that later in the flight, the regular first officer and he talked about these events and they agreed that the primary cause of the problem was human error. The reporter also said that one of the other problems was that the fmcs were being overloaded with inputs and were in danger of ceasing to function. This is a common problem and or limitation the reporter said with the B767 and B757 fmcs. He said that you just can't move too fast with your typing or they will quit.

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Original NASA ASRS Text

Title: AN ACR B767 FLC HAS DIFFICULTY WITH THEIR FMCS AND RELATED NAV AND AUTOFLT SYS. THE PROBS WERE FINALLY RESOLVED AND THE SYS ARE RESTORED TO NORMAL OP.

Narrative: FLT LEFT LHR ON TIME AT XX00Z. INITIALLY ON HDGS/ VECTORS BY LONDON AND SCOTTISH CTL. OCCASIONALLY WHILE CLBING OUT WE WOULD GET MESSAGE ON FMC 'RTE UPLINK READY TO LOAD.' SINCE WE HAD ALREADY DONE THIS ON THE GND AND EVERYTHING LOOKED NORMAL IT WAS JUST BOTHERSOME, BUT QUESTIONABLE WHY THIS FMC (BOTH ACTUALLY) WAS DOING THAT. NEAR GLASGOW WE WERE AT OUR FINAL, FL310 AND THE FO HAD FINISHED DOING ALL THE REQUIRED CHKS BEFORE ENTERING CLASS II AIRSPACE. IRS CHKED FINE, OUR VOR CHK WAS RIGHT ON, POS CHK WAS CORRECT AND OUR MAGNETIC COMPASS, HSI CHK WAS CORRECT. I WAS PF AND HAD ACFT HOOKED UP TO AUTOPLT AND AUTOTHROTTLE. WE WERE ABOUT TO RECEIVE OUR OCEANIC CLRNC WHEN ON THE ADI THE VNAV ANNUNCIATION AND LNAV ANNUNCIATION WENT AMBER WITH A YELLOW LINE THROUGH THEM. SIMULTANEOUSLY A MASTER CAUTION WENT OFF WITH ANNUNCIATION OF AUTOTHROTTLE DISCONNECT AND AUTOPLT DISCONNECT. THE AUTOTHROTTLE DID DISCONNECT, BUT THE ACFT'S AUTOPLT STAYED ENGAGED, BUT WITHOUT ANY GUIDANCE. TRYING TO RE-ENGAGE LNAV AND VNAV WAS UNSUCCESSFUL. WE DESELECTED CTR AUTOPLT AND TRIED THE L AND R AUTOPLT WITH NO SUCCESS. USING HEADING SELECT I WAS ABLE TO MAINTAIN DIRECTIONAL GUIDANCE AND SELECTING 'FLCH' WAS ABLE TO MAINTAIN AIRSPD AND ALT. SINCE THE AUTOPLT WAS WORKING, OBVIOUSLY SOMETHING WITH LNAV AND VNAV WAS NOT FUNCTIONING PROPERLY. WE HAD LOST OUR ACTIVE WAYPOINT, (WE WERE DIRECT 'MATIK' AT TIME) HOWEVER ALL OUR WAYPOINTS WERE STILL DISPLAYED. THE FMC STILL HELD OUR RTE, BUT NO ETA'S WERE ON LEG'S PAGES OR ON PROGRESS PAGE. WE RE-SELECTED 'MATIK' AS OUR ACTIVE WAYPOINT, AND WHILE VERY SLOW THE FMC FINALLY REENGAGED THE RTES AND WE TRIED 'LNAV' AND THE ACFT WAS ONCE AGAIN ON IRS NAV. THE CAPT THEN TOLD ME 'YOU FLY AND TALK, WE'LL WORK ON THE PROB' MEANING THE FO AND HIM. THEY CALLED MAINT AND DISPATCH AND APPARENTLY THEY WERE OF VERY LITTLE HELP. I DID NOT HEAR ANY OF THEIR CONVERSATION EXCEPT THE FO AND CAPT WERE VERY UPSET WITH THEM. DURING THIS TIME I HAD SCOTTISH CONFIRM OUR POS, AND IDID SOME CHKING ON FIX PAGE WITH VORS AND SUCH. ALL THE TIME, THE FMC DISPLAY A RTE, BUT IT WAS NOT ACTIVE, AND ABOUT THIS TIME MAINT HAD US PULL AND RESET CERTAIN CIRCUIT BREAKERS. AFTER THIS THE RTE HAD REMAINED IN THE FMC, BUT IT HAD NOW BECOME 'MOD RTE' WHICH MEANS IT WAS NOT ACTIVE, ALSO THE MESSAGE 'PERF/VNAV UNAVAILABLE' STARTED BEING DISPLAYED. THE FO AND I REALIZED ALMOST IMMEDIATELY THAT THAT MEANT WE'D LOST OUR PERFORMANCE PLATFORM, AND THAT WAS ONE REASON WE DID NOT HAVE ETA'S TO POINTS. I INDEXED THE FMC AND PULLED UP PERFORMANCE PAGE AND SURE ENOUGH ALL OUR PERFORMANCE DATA HAD BEEN DUMPED, PROBABLY DUE TO RESETTING CIRCUIT BREAKERS OF FMC AND OTHER EQUIP. AS SOON AS I ENTERED ALL THE DATA THE FMC IMMEDIATELY CAME UP WITH ALL THE PROPER DATA, ETA'S TO FIX, TIME ENRTE, GROSS WT, ETC. WE ONCE AGAIN DID ALL OUR CHKS FOR ENTERING CLASS II AIRSPACE, AND FELT COMFORTABLE THAT THE PROB WAS FIXED. WE AGREED AND WE WERE ON OUR WAY. DURING THE OCEAN XING WE WERE ALWAYS ON COURSE AND WHILE OVER ICELAND HAD OUR POS VERIFIED WITH REYKEVIC CTL. ON WHY THE FMC DID WHAT IT DID I HAVE SOME THOUGHTS. WHILE UNDER RADAR VECTORS, THE CAPT KEPT SWITCHING FROM HDG SELECT TO LNAV, FOR AS HE SAID 'TO KEEP THE CLOCK UPDATED.' OFTEN WHEN HE DID THIS THE MESSAGE 'NOT ON INTERCEPT HDG' WOULD COME UP. SINCE I WAS HAND FLYING AND KEEPING A HEADING IT MAY HAVE CONFUSED THE FMC ON WHAT IT WAS SUPPOSED TO DO. ALSO I KNOW THE CAPT WAS GOING TO UPLOAD THE WINDS, BUT I WONDER IF HE ACCIDENTALLY HIT 'UP LOAD RTE' INSTEAD AND THAT WAS WHY WE KEPT GETTING 'RTE UPLINK READY TO LOAD' MESSAGE. WE NEVER HAD FMC FAILURE, NOR DID WE GET THAT MESSAGE, THE RTE WAS ALWAYS IN THE FMC, AND MAP DISPLAY WAS GOOD, IT JUST FOR SOME REASON, PROBABLY HUMAN, CAUSED IT TO OVERLOAD AND GO INTO A DEGRADE MODE UNTIL IT HAD ALL THE FACTS. THAT WE WERE ABLE TO RESTORE IT POINTS TO SOME GLITCH, EITHER ELECTRONICALLY OR HUMAN, THAT CAUSED IT TO BEHAVE THE WAY IT DID. AT NO TIME WERE WE IN CLASS II AIRSPACE, AND AT NOTIME DID WE LOSE STANDARD NAV CAPABILITY (VOR, ADF, ILS). CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE INTL RELIEF FO SAID THAT HE SUSPECTED THAT THE CAPT HAD INITIALLY ATTEMPTED TO ENTER UPDATED WINDS BY INADVERTENTLY PUSHING THE 'UPLOAD RTE' BUTTON. THE RPTR SAID THAT LATER IN THE FLT, THE REGULAR FO AND HE TALKED ABOUT THESE EVENTS AND THEY AGREED THAT THE PRIMARY CAUSE OF THE PROB WAS HUMAN ERROR. THE RPTR ALSO SAID THAT ONE OF THE OTHER PROBS WAS THAT THE FMCS WERE BEING OVERLOADED WITH INPUTS AND WERE IN DANGER OF CEASING TO FUNCTION. THIS IS A COMMON PROB AND OR LIMITATION THE RPTR SAID WITH THE B767 AND B757 FMCS. HE SAID THAT YOU JUST CAN'T MOVE TOO FAST WITH YOUR TYPING OR THEY WILL QUIT.

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.