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

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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.