LATE TURN STARTED DUE TO NAVAID PROB. NIGHT OP.

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PLTS ARRIVING MEX WERE BOTH AWARE OF RPTED MAP SHIFTS ON THE FMC AT MEX. COMPANY NOTAM WARNED OF 'FALSE LOCK-ONS' 'NO LOCK-ONS' OF ILS. ALL THIS WAS BRIEFED PRIOR TO ARR; AS WELL AS HIGH TERRAIN E AND SE OF THE ARR OUTBOUND LEG. ACCORDINGLY; THE FOLLOWING ACTIONS WERE TAKEN PRIOR TO XING MEX VOR. FMC AND INS LATITUDES/LONGITUDES XCHKED TWICE. ADF TUNED AND IDENTED. CAPT'S RADAR TUNED TO DISPLAY MOUNTAIN. VORS TUNED AND IDENTED. ILS TUNED AND IDENTED. AGREED THAT VOR/ILS RAW DATA WOULD BE USED TO XCHK POS AT ALL TIMES. XING MEX VOR IN LNAV THE ACFT TRACKED OUT THE 072 DEG RADIAL IN MAP MODE. I SHIFTED TO VOR MODE AND VERIFIED ON CTRLINE 072 DEG RADIAL. I SHIFTED BACK TO MAP WHILE CAPT STAYED IN VOR MODE. ARR/APCH CHART REQUIRES US TO TRACK OUTBOUND TO 7 DME MEX AND MAKE A L TURN COURSE REVERSAL TO INTERCEPT ILS COURSE INBOUND. WHILE OUTBOUND WAITING FOR 7 DME MEX AND MAKE A L TURN COURSE REVERSAL TO INTERCEPT ILS COURSE INBOUND. WHILE OUTBOUND WAITING FOR 7 DME; OTHER INDICATIONS FROM HSI DISPLAY; ADF; RADAR AND A '6TH SENSE' OF TIMING TOLD US IT WAS TIME TO TURN IN; EVEN THOUGH DME SHOWED 5-6 MI. WE TURNED AT 6.2 MI. HALFWAY THROUGH THE TURN MEX APCH CTL ASKED US TO START THE TURN. ESTABLISHED INBOUND THE CTLR VERY POLITELY REMINDED US THAT WE SHOULD HAVE STARTED THE TURN BY 7 MI. I BELIEVE THAT APCH CTL WAS RIGHT AND THAT WE WERE OUTSIDE 7 MI BECAUSE OF THE TIME IT TOOK US TO TRACK BACK TO VASOS (THE FAF) WHEN WE HAD COMPLETED OUR TURN. EXCEPTIONAL CARE WAS TAKEN IN BRIEFING; PLANNING AND EXECUTING THIS APCH. THE APCH CTLRS WERE ON TOP OF THE SIT; SPOKE EXCELLENT ENGLISH; AND WERE VERY HELPFUL. NEVERTHELESS; EVERY ARR AT MEX SEEMS TO BECOME AN 'EVENT' FOR NAV REASONS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE ACR HAS A COMPANY BULLETIN OUT REGARDING NAV PROBS WITHIN THE MEX AIRSPACE BUT DEALING MOSTLY WITH MAP SHIFTS OR LOC PROB. THERE WAS NO MENTION OF VOR/DME PROBS IN THIS BULLETIN. NEITHER RPTR KNEW OF ANY FURTHER PROBS WITH THE DME; WITH EACH HAVING MADE A SECOND TRIP INTO MEX WITHOUT ANY NAV EVENTS OCCURRING. ON THE INITIAL INCIDENT THE CREW RAN A GOOD COCKPIT; XCHKING WITH ALL NAV EQUIP POSSIBLE. THE FO STATED THAT ENRTE; FMC UPDATES WERE CHKED UPON PASSING THE VORS TO CHK THE ACCURACY OF THE ACFT SYS. ON THE APCH THE CAPT SAID THERE WAS NO DOUBT THAT THEY WERE ON THE CORRECT VOR FREQ; WITH THE DME SHOWING AN INCREASE ON THE OUTBOUND LEG. AS THEY WERE INBOUND MEX APCH CTLR VOICED HIS CONCERN; WITH THE CREW ADVISING HIM THAT THEY HAD TURNED INBOUND AT THE CORRECT DME INDICATION. THIS ELICITED NO COMMENT FROM THE CTLR; EXCEPT FOR HIM TO SAY THEY HAD GONE TOO FAR. THE CAPT ESTIMATES THAT THEY WERE WITHIN 2500 FT OF THE GND; WITH GOOD RADAR DISPLAY OF THE HILLS AND PARTIAL VISUAL. THERE WAS NO TERRAIN WARNING THROUGHOUT THE APCH. SUPPLEMENTAL INFO FROM ACN 281783: LATE TURN TO FINAL ON VASOS 3 ARR (COMMERCIAL CHART 10-2F) AT MEXICO CITY. OUTBOUND LEG FROM MEX VOR WAS FLOWN WITH RAW DATA VOR DISPLAYING COURSE AND DME (BOTH PLTS DISPLAYING RAW DATA). BOTH PLTS AGREE OUR VOR DME READ 6.8 WHEN L TURN STARTED TOWARD VASOS. AFTER 7 DME INDICATED; A DSCNT WAS MADE TO THE VASOS XING ALT OF 8800 FT. MEXICO CITY APCH CTL SAID OUR EXTENDED POS FROM THE MEX VOR WAS 13 1/2 MI AS SHOWN ON THEIR RADAR. IT IS MY OPINION THAT WE DID MAKE A LATE TURN TO FINAL. FURTHER; IN MY OPINION; THE LATE TURN WAS DUE TO A MALFUNCTION IN THE MEX VOR DME EQUIP (UNRELIABLE VOR DME). WE (BOTH PLTS) WERE WELL AWARE OF THE MOUNTAINOUS TERRAIN IN THE AREA. THE MOUNTAINS WERE CONSTANTLY DISPLAYED AND MONITORED ON OUR RADAR. I WOULD ESTIMATE OUR MAX DISTANCE FROM THE MEX VOR WAS BTWN 10 AND 11 MI BASED ON THE TIME REQUIRED TO FLY BACK TO VASOS. I HAVE NO FAITH OR TRUST IN THE NAVAIDS AT MEXICO CITY. PLTS BETTER BACK EVERYTHING UP WITH WHATEVER THEY'VE GOT.