B767-300 EXPERIENCES MAP SHIFT ON ARRIVAL.

Narrative

CONDITIONS: SCATTERED CLOUDS; VISIBILITY 7 MI (BY ATIS). PHASE OF FLT: VISUAL APCH TO RWY 1R IAD. (ILS OFF AIR.) PROB: FMC MAP SHIFT. WERE SE OF ARPT ON BARIN 1 STAR; WHEN APCH CLRED US DIRECT TO TILLE (OM RWY 1R). SHORTLY AFTER THAT; WAS CLRED TO VISUAL FOR RWY 1R. (ILS WAS OFF AND RWY IN USE WAS RWY 19L AND RWY 19R; S OP.) FMC WAS; AT THIS POINT IN FLT; SHOWING RWY 1R AND TILLE IN THEIR CORRECT POS. AS WE DSNDED AND GOT IN CLOSER TO TILLE (ABOUT 20 MI FROM TILLE) WE RECEIVED A SIGNIFICANT MAP SHIFT OF WHAT I ESTIMATE WAS 5-7 MI TO E OF ACTUAL TILLE POS; AND AUTOFLT; FLT DIRECTOR COMMANDED A R TURN TO ABOUT A 030 DEG HDG TO MAINTAIN DIRECT TRACK TO TILLE AND COMPLY WITH OUR ATC CLRNC OF 'DIRECT TILLE.' I MAINTAINED OLD COURSE AS I WAS LOOKING AT RWY 1R DIRECTLY AHEAD. BUT WAS AWARE THAT THERE WAS A POSSIBILITY THAT WE COULD BE TRACKING TO RWY 1L INSTEAD OF RWY 1R. THERE WAS THIS DOUBT ONLY BECAUSE FMC WAS SHOWING TILLE AND RWY 1R WELL TO R OF ACFT COURSE. AT THIS POINT WE WERE LOW ENOUGH (OM ALT OF ABOUT 1700 FT) WHERE WE WERE UNABLE TO SEE OTHER RWYS. ALSO; RWY LIGHTS AND APCH LIGHTS; VASI WERE ALL OFF; OR WERE SET FOR RWY 19L; NOT RWY 1R. AFTER A BRIEF DISCUSSION WITH MY CREW AND TWR; ELECTED TO ABORT APCH AND REQUEST AN ILS. TWR RECLRED US TO L DOWNWIND FOR RWY 19L (WHICH HAD THE ILS AND LIGHTS ON; AT THAT TIME OF MORNING). WE PROCEEDED NE HDG ON A L DOWNWIND LEG TO RWY 19L. THE WHOLE TIME THE FMC WAS SHOWING ARPT WELL TO OUR E WHEN IN FACT IT WAS WELL TO OUR W. AFTER POSITIVELY IDENTING RWY 19L; WE WERE CLRED TO A VISUAL RWY 19L AND LANDED. SHORTLY BEFORE LNDG; BELOW 1000 FT; WE RECEIVED INTERMITTENT GPWS WARNINGS. AND CONTINUED RECEIVING THESE WARNINGS AFTER EXITING RWY UNTIL TERRAIN SWITCH WAS DISARMED. AFTER STOPPING ON TXWY; WE DETERMINED THAT FMC WAS STILL SHOWING A 6 MI ERROR/DISAGREEMENT IN THE POS PAGE. RWY 1R WAS STILL SHOWING 6 MI TO E OF ACFT POS. EVEN THOUGH WE WERE CORRECTLY PROCEEDING TO RWY 1R IN VISUAL CONDITIONS; THE FMC CREATED A SIGNIFICANT ELEMENT OF DOUBT AS TO WHICH RWY WE WERE REALLY LOOKING AT; RWY 1L OR RWY 1R. SINCE I DID NOT WANT TO DSND BELOW 1700 FT WITHOUT KNOWING FOR SURE THAT WE WERE LOOKING AT CORRECT RWY; I ELECTED TO ABORT APCH AND REQUEST ANOTHER APCH. I FURTHER BELIEVE WE GOT AN ERRONEOUS GPWS WARNING BECAUSE THE AIRPLANE THOUGHT WE WERE LNDG OFF ARPT; AND DID NOT KNOW WHERE WE WERE. AS IT TURNS OUT; FMC WAS IN ERROR. THE AIRPLANE HAS A HISTORY OF FMC PROBS. ALSO RECEIVED SEVERAL 'ERROR' CODES: 'PURGE UPDATES ON FMC' FROM L FMC; AFTER ON GND IN IAD. FMC AND MAP SHIFT WAS WRITTEN UP. HAD WE BEEN IN ANYTHING LESS THAN VISUAL CONDITIONS; THIS COULD HAVE BEEN A SERIOUS PROB. EVEN WHILE RECEIVING ILS TO RWY 19L; FMC CONTINUED TO SHOW RWY COURSE 6 MI E OF ACTUAL LOC. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER HAS RECEIVED NO FEEDBACK FROM MAINT REGARDING THE ANOMALY. HE STATED THERE WAS SOME NOTAM REGARDING ECG VOR THAT HAD ESCAPED THEIR NOTICE WHICH MAY HAVE AFFECTED THE IRU UPDATING UPON ARRIVING OVERLAND. THE PROBLEM; HOWEVER; WAS THAT THE POSITION APPEARED ACCURATE INITIALLY AND THE MAP SHIFT RESULTED IN A SIGNIFICANT ERROR; RATHER THAN A MORE PRECISE POSITION AS WOULD BE EXPECTED FROM AN IRU UPDATE. THE EGPWS WARNINGS WOULD HAVE BEEN A SIGNIFICANT ISSUE HAD THEY NOT HAD VISUAL CONTACT WITH THE GROUND. EVEN MORE POTENTIALLY DISTRESSING WAS THAT THEY HAD LITTLE RESERVE FUEL TO ALLOW FOR TROUBLESHOOTING HAD THEY BEEN FORCED TO PERFORM A GAR DUE TO THE TERRAIN WARNINGS AS IS REQUIRED WHEN IMC.