Narrative:

Aircraft maintenance history revealed an FMS map shift 3 days prior to our flight. The aircraft had flown 5 legs this day prior to our flight with no problem nor any noted after the write-up. Nevertheless, in addition to normal SOP map confirmation prior to using FMS for navigation I monitored our course intercept, on the buckeye two departure phx, with raw data displayed using LNAV engaged. Everything indicated the FMS was operating correctly. No problems were noted en route. On the left turn to intercept runway 25L at lax from pdz 277 degree radial, however, socal told us to continue our turn to 220 degrees (249 degrees is the course) to intercept the localizer. I switched to raw data to find the localizer off scale. Raw data was not used to this point because we had not been cleared for the approach and we were more than 25 mi from the field which requires both radios be in automatic to insure FMC update. The system (FMS) was checked by maintenance at lax with no problem found. On the next leg en route navigation LNAV worked fine, yet we did note a map shift shortly after we went from automatic to manual for the final approach into oak (approximately 1 mi). Why this happened at lax with both VOR switches in automatic can only be postulated that both radios lost VOR lock and the IRS drifted. Supplemental information from acn 451004: on paradise 3 arrival into lax, FMC map display drifted approximately 5 NM. Not recognized until approach control vectored us back to final. FMC displayed on course which was incorrect. We flew remainder in raw data. May have been caused by a software bug in the recently updated database. FMC map errors are somewhat insidious.

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Original NASA ASRS Text

Title: B737 WITH A HISTORY OF MAP SHIFTS REQUIRES VECTOR FROM ATC WHEN FMS DOES NOT PROVIDE ACCURATE COURSE INTERCEPT.

Narrative: ACFT MAINT HISTORY REVEALED AN FMS MAP SHIFT 3 DAYS PRIOR TO OUR FLT. THE ACFT HAD FLOWN 5 LEGS THIS DAY PRIOR TO OUR FLT WITH NO PROB NOR ANY NOTED AFTER THE WRITE-UP. NEVERTHELESS, IN ADDITION TO NORMAL SOP MAP CONFIRMATION PRIOR TO USING FMS FOR NAV I MONITORED OUR COURSE INTERCEPT, ON THE BUCKEYE TWO DEP PHX, WITH RAW DATA DISPLAYED USING LNAV ENGAGED. EVERYTHING INDICATED THE FMS WAS OPERATING CORRECTLY. NO PROBS WERE NOTED ENRTE. ON THE L TURN TO INTERCEPT RWY 25L AT LAX FROM PDZ 277 DEG RADIAL, HOWEVER, SOCAL TOLD US TO CONTINUE OUR TURN TO 220 DEGS (249 DEGS IS THE COURSE) TO INTERCEPT THE LOC. I SWITCHED TO RAW DATA TO FIND THE LOC OFF SCALE. RAW DATA WAS NOT USED TO THIS POINT BECAUSE WE HAD NOT BEEN CLRED FOR THE APCH AND WE WERE MORE THAN 25 MI FROM THE FIELD WHICH REQUIRES BOTH RADIOS BE IN AUTO TO INSURE FMC UPDATE. THE SYS (FMS) WAS CHKED BY MAINT AT LAX WITH NO PROB FOUND. ON THE NEXT LEG ENRTE NAV LNAV WORKED FINE, YET WE DID NOTE A MAP SHIFT SHORTLY AFTER WE WENT FROM AUTO TO MANUAL FOR THE FINAL APCH INTO OAK (APPROX 1 MI). WHY THIS HAPPENED AT LAX WITH BOTH VOR SWITCHES IN AUTO CAN ONLY BE POSTULATED THAT BOTH RADIOS LOST VOR LOCK AND THE IRS DRIFTED. SUPPLEMENTAL INFO FROM ACN 451004: ON PARADISE 3 ARR INTO LAX, FMC MAP DISPLAY DRIFTED APPROX 5 NM. NOT RECOGNIZED UNTIL APCH CTL VECTORED US BACK TO FINAL. FMC DISPLAYED ON COURSE WHICH WAS INCORRECT. WE FLEW REMAINDER IN RAW DATA. MAY HAVE BEEN CAUSED BY A SOFTWARE BUG IN THE RECENTLY UPDATED DATABASE. FMC MAP ERRORS ARE SOMEWHAT INSIDIOUS.

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.