Narrative:

We were flying from taipei to seoul, korea, scheduled at XA15 block to block. Periodically, I would notice on the navigation display (nd) that we were not getting VOR/DME updating to the FMC. A check of our raw data confirmed that we were slightly off track. (1/2 to 1 DOT deviation on VOR cde needle.) all proceeded smoothly until we were given a turn to final approach to intercept the localizer and shoot the ILS 14R at sel. I was flying the aircraft. On our map display, it appeared that he (the controller) was taking us through the localizer to rejoin the approach. In actuality, we had suffered a navigation 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 right of the localizer before the captain picked up the runway visually about 4 mi from touchdown. Once I visually acquired the runway, I made a corrective course change to the left and made a smooth, uneventful landing. 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 navigation 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 aircraft 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 navigation display picture can lead to confusion at inopportune times, especially on approach.

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Original NASA ASRS Text

Title: 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.

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.