Narrative:

All phases of flight prior to this point were normal with one minor exception which will be obvious later. At the top of climb; relief pilot left cockpit and as the PNF; I started to do en route duties which included gross navigation check. This entails checking FMC against a known fix or raw data from a ground based facility to ensure accuracy. I put the sfo VOR in the fix page (FMC) and put sfo on the navigation/rad page (raw data) and proceeded to check bearing and distance. The distances were identical yet the bearings were off by a magnitude of 17-20 degrees. Having done this check multiple times; I was unsatisfied; so I selected several other vors with the same results: distances identical but bearings off by about 15-20 degrees. Rechked my manual to see if I was procedurally correct; tried selecting the heading reference to true and back to normal with no success. First officer and I conferred and he astutely noted that the difference was nearly identical to the magnetic variation for our current position. He also said that as he set up the FMC for a potential engine-out; he noted that the track line was off but was distraction by other duties and subsequently forgot to follow up. At this time we got on the satphone and got dispatch and maintenance on the line to put heads together. Aircraft was navigating ok; position checks and all other relevant checks were ok and I was willing to continue if we were legal. Was concerned with mnps requirements. Finally got one of the fleet gurus who said alternate check (needle through station) was acceptable and we proceeded without further incident. As an aside; we did another check just north of shemya where the variation is almost zero and the anomaly was gone; confirming first officer's theory that data presented on fix page was displayed in 'true' vice the normal magnetic. This could have been a real fiasco were it not for all parties putting their heads together. Had actually considered that this may warrant a return to base for maintenance action. Callback conversation with reporter revealed the following information: the reporter made numerous calls to his fleet manager in an attempt to find out the cause of this anomaly with no explanation.

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Original NASA ASRS Text

Title: B777 CAPT RPTS BEARING ERRORS DURING NAV ACCURACY CHK.

Narrative: ALL PHASES OF FLT PRIOR TO THIS POINT WERE NORMAL WITH ONE MINOR EXCEPTION WHICH WILL BE OBVIOUS LATER. AT THE TOP OF CLB; RELIEF PLT LEFT COCKPIT AND AS THE PNF; I STARTED TO DO ENRTE DUTIES WHICH INCLUDED GROSS NAV CHK. THIS ENTAILS CHKING FMC AGAINST A KNOWN FIX OR RAW DATA FROM A GND BASED FACILITY TO ENSURE ACCURACY. I PUT THE SFO VOR IN THE FIX PAGE (FMC) AND PUT SFO ON THE NAV/RAD PAGE (RAW DATA) AND PROCEEDED TO CHK BEARING AND DISTANCE. THE DISTANCES WERE IDENTICAL YET THE BEARINGS WERE OFF BY A MAGNITUDE OF 17-20 DEGS. HAVING DONE THIS CHK MULTIPLE TIMES; I WAS UNSATISFIED; SO I SELECTED SEVERAL OTHER VORS WITH THE SAME RESULTS: DISTANCES IDENTICAL BUT BEARINGS OFF BY ABOUT 15-20 DEGS. RECHKED MY MANUAL TO SEE IF I WAS PROCEDURALLY CORRECT; TRIED SELECTING THE HDG REF TO TRUE AND BACK TO NORMAL WITH NO SUCCESS. FO AND I CONFERRED AND HE ASTUTELY NOTED THAT THE DIFFERENCE WAS NEARLY IDENTICAL TO THE MAGNETIC VARIATION FOR OUR CURRENT POS. HE ALSO SAID THAT AS HE SET UP THE FMC FOR A POTENTIAL ENG-OUT; HE NOTED THAT THE TRACK LINE WAS OFF BUT WAS DISTR BY OTHER DUTIES AND SUBSEQUENTLY FORGOT TO FOLLOW UP. AT THIS TIME WE GOT ON THE SATPHONE AND GOT DISPATCH AND MAINT ON THE LINE TO PUT HEADS TOGETHER. ACFT WAS NAVING OK; POS CHKS AND ALL OTHER RELEVANT CHKS WERE OK AND I WAS WILLING TO CONTINUE IF WE WERE LEGAL. WAS CONCERNED WITH MNPS REQUIREMENTS. FINALLY GOT ONE OF THE FLEET GURUS WHO SAID ALTERNATE CHK (NEEDLE THROUGH STATION) WAS ACCEPTABLE AND WE PROCEEDED WITHOUT FURTHER INCIDENT. AS AN ASIDE; WE DID ANOTHER CHK JUST N OF SHEMYA WHERE THE VARIATION IS ALMOST ZERO AND THE ANOMALY WAS GONE; CONFIRMING FO'S THEORY THAT DATA PRESENTED ON FIX PAGE WAS DISPLAYED IN 'TRUE' VICE THE NORMAL MAGNETIC. THIS COULD HAVE BEEN A REAL FIASCO WERE IT NOT FOR ALL PARTIES PUTTING THEIR HEADS TOGETHER. HAD ACTUALLY CONSIDERED THAT THIS MAY WARRANT A RETURN TO BASE FOR MAINT ACTION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR MADE NUMEROUS CALLS TO HIS FLEET MGR IN AN ATTEMPT TO FIND OUT THE CAUSE OF THIS ANOMALY WITH NO EXPLANATION.

Data retrieved from NASA's ASRS site as of January 2009 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.