Narrative:

Approach to mex runway 5R, smo 2 arrival. First officer flying using RNAV map display outbound 160 degree radial. Captain (PNF) tuned to smo 160 degree radial with expanded VOR display. Passing smo VOR, VOR needle disappeared from view, returned to view approximately 4 1/2 mi south of VOR, but pegged 2 dots right, first officer using map display 20 mi scale, indicated on course, flight director also indicated on course, with smo 160 degree radial superimposed on map display course. Rechked smo VOR setup (frequency, course, identify) and started right turn to recapture VOR radial when approach advised 1 mi east of course. 160 degree radial was recaptured and approach and arrival completed without incident. First officer map display realigned as left turn to final was complete after blanking for a second. Altitude throughout segment greater than 9500 ft as charted. 1) the problem of 'map shift' in areas of unreliable navaids has been previously reported by air carrier crews, hence, the air carrier recommendation that map displays are corroborated by VOR/NDB raw data. 2) it appears that even though the map display can show the computed dotted line of a VOR radial it may not be accurate. Corrective action: recommend in such circumstances, the PF use raw data, and PNF use a map display or raw data for corroboration instead of other way around.

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Original NASA ASRS Text

Title: MLG GETS A MI E OF COURSE BEFORE 'MAP SHIFT' IS RECOGNIZED.

Narrative: APCH TO MEX RWY 5R, SMO 2 ARR. FO FLYING USING RNAV MAP DISPLAY OUTBOUND 160 DEG RADIAL. CAPT (PNF) TUNED TO SMO 160 DEG RADIAL WITH EXPANDED VOR DISPLAY. PASSING SMO VOR, VOR NEEDLE DISAPPEARED FROM VIEW, RETURNED TO VIEW APPROX 4 1/2 MI S OF VOR, BUT PEGGED 2 DOTS R, FO USING MAP DISPLAY 20 MI SCALE, INDICATED ON COURSE, FLT DIRECTOR ALSO INDICATED ON COURSE, WITH SMO 160 DEG RADIAL SUPERIMPOSED ON MAP DISPLAY COURSE. RECHKED SMO VOR SETUP (FREQ, COURSE, IDENT) AND STARTED R TURN TO RECAPTURE VOR RADIAL WHEN APCH ADVISED 1 MI E OF COURSE. 160 DEG RADIAL WAS RECAPTURED AND APCH AND ARR COMPLETED WITHOUT INCIDENT. FO MAP DISPLAY REALIGNED AS L TURN TO FINAL WAS COMPLETE AFTER BLANKING FOR A SECOND. ALT THROUGHOUT SEGMENT GREATER THAN 9500 FT AS CHARTED. 1) THE PROBLEM OF 'MAP SHIFT' IN AREAS OF UNRELIABLE NAVAIDS HAS BEEN PREVIOUSLY RPTED BY ACR CREWS, HENCE, THE ACR RECOMMENDATION THAT MAP DISPLAYS ARE CORROBORATED BY VOR/NDB RAW DATA. 2) IT APPEARS THAT EVEN THOUGH THE MAP DISPLAY CAN SHOW THE COMPUTED DOTTED LINE OF A VOR RADIAL IT MAY NOT BE ACCURATE. CORRECTIVE ACTION: RECOMMEND IN SUCH CIRCUMSTANCES, THE PF USE RAW DATA, AND PNF USE A MAP DISPLAY OR RAW DATA FOR CORROBORATION INSTEAD OF OTHER WAY AROUND.

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.