AN ACR WDB HAD AN FMC MAP SHIFT OF ABOUT 4 MI. THEY NOTICED THIS AFTER INTERCEPTING THE ILS.

Narrative

WE WERE FLYING FROM TAIPEI TO SEOUL; KOREA; SCHEDULED AT XA15 BLOCK TO BLOCK. PERIODICALLY; I WOULD NOTICE ON THE NAV DISPLAY (ND) THAT WE WERE NOT GETTING VOR/DME UPDATING TO THE FMC. A CHK OF OUR RAW DATA CONFIRMED THAT WE WERE SLIGHTLY OFF TRACK. (1/2 TO 1 DOT DEV ON VOR CDE NEEDLE.) ALL PROCEEDED SMOOTHLY UNTIL WE WERE GIVEN A TURN TO FINAL APCH TO INTERCEPT THE LOC AND SHOOT THE ILS 14R AT SEL. I WAS FLYING THE ACFT. ON OUR MAP DISPLAY; IT APPEARED THAT HE (THE CTLR) WAS TAKING US THROUGH THE LOC TO REJOIN THE APCH. IN ACTUALITY; WE HAD SUFFERED A NAV DISPLAY MAP SHIFT; PROBABLY DUE TO THE FACT THAT WE WERE NOT GETTING CONSISTENT UPDATES FROM VORS TO THE FMC. WE ACTUALLY TURNED A PARALLEL COURSE TO THE R OF THE LOC BEFORE THE CAPT PICKED UP THE RWY VISUALLY ABOUT 4 MI FROM TOUCHDOWN. ONCE I VISUALLY ACQUIRED THE RWY; I MADE A CORRECTIVE COURSE CHANGE TO THE L AND MADE A SMOOTH; UNEVENTFUL LNDG. TO PRECLUDE THIS PROBLEM IN THE FUTURE; WE DECIDED THAT IT IS EXTREMELY IMPORTANT TO HAVE THE PNF BACK UP THE PF WITH RAW DATA; ESPECIALLY IF A POTENTIAL MAP SHIFT OF THE NAV DISPLAY IS POSSIBLE. AS ROUTINE AS THIS FLYING BECOMES; IT IS EASY TO GET IN A TRAP OF TRUSTING THE 'MAGIC' OF THE GLASS COCKPITS INSTEAD OF THE OLD RELIABLE RAW DATA. BUT I ALSO FEEL CREWS OF THESE ACFT MUST BE REPEATEDLY MADE AWARE TO MAP SHIFTS THAT CAN BE CAUSED BY LACK OF DME OR VOR UPDATING; DUE TO SOME FAULT IN THE SOFTWARE OF THE FMC. IT IS EASY TO MISS AND A FALSE RELIANCE ON THE NAV DISPLAY PICTURE CAN LEAD TO CONFUSION AT INOPPORTUNE TIMES; ESPECIALLY ON APCH.