AN A330-300 WITH A NAV HDG DISCREPANCY WAS NOT PERMITTED ON AN OCEANIC TRACK AND RETURNED TO ITS DEP POINT.

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WHILE ON CLRNC DIRECT TO 46N 50W ISSUED BY GANDER CTR; CTR ASKED IF WE WERE FOLLOWING THE DIRECT TO 46N 50W. I LOOKED AT MY NAV DISPLAY AND OBSERVED THE ACFT ON DIRECT COURSE TO 46N 50W; AND ANSWERED YES. GANDER CTR REPLIED HE SHOWED OUR COURSE MISSING 46N 50W. I WENT TO BASIC HDG MODE; CHKED MY NAV DISPLAY WITH COMPASS HDGS THEN RE-ENTERED DIRECT 46N 50W. CTR INFORMED US WE WERE STILL OFF COURSE; AND OUR ACFT WOULD NOT BE ALLOWED TO ENTER THE NAT SYS. AFTER COM WITH COMPANY DISPATCH; WE RETURNED TO ZZZ1. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THE FMGC DISPLAYED THE TRACK FROM THE ACFT'S PRESENT POS TO THE SELECTED FIX (46N 50W) BUT IN REALITY IT WAS TRACKING TO NEXT FIX (46N 40W) AND SO WAS GOING TO MISS THE FIX TO WHICH THEY WERE CLRED. GANDER OCEANIC GAVE THE CREW THE OPPORTUNITY TO MAKE A 360 DEG TURN BACK ON TRACK TO FIGURE OUT WHAT WAS WRONG; BUT IF THEY DID THE DELAY TURN; THEY WOULDN'T HAVE LEGAL FUEL TO CONTINUE THE TRIP. IN THE END; 2 WKS AFTER THIS INCIDENT; THE CONTENTS OF THE FMGC WERE LOADED INTO A TRAINING SIMULATOR; IT WAS DISCOVERED THAT THE RTE WAS CORRECT; BUT AN ANOMALY EXISTED THAT CAUSED THE ACFT TO BYPASS THE FIRST FIX AND DISPLAY THE FIRST FIX AS THE DESIRED TRACK. AIRBUS INDUSTRIES KNOWS ABOUT THE ANOMALY.