A B727-200 FREIGHTER RECEIVES A GPWS ALERT WHILE INSIDE THE 17 DME ARC OF TLC VOR DURING A NIGHT APCH.

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AFTER THE SECOND ATTEMPT AT A HDOF FROM MEXICO CTR TO TOLUCA APCH CTL; WE RECEIVED A GARBLED; NOT UNDERSTOOD CLRNC IN THE TERMINAL AREA TO LEAD TO A LNDG ON RWY 15. I ATTEMPTED TO GET CLARIFICATION AND WAS NOT RESPONDED TO BY TOLUCA APCH. AS A CREW; WE HAD BRIEFED THE PUBLISHED DME ARC (GAVIA) TRANSITION TO THE RWY 15 APCH AND THIS WAS BEGUN. A LOSS OF SITUATIONAL AWARENESS AT THIS POINT PUT US INSIDE THE ARC DISTANCE AND ALT. AFTER ROLLING WINGS LEVEL WE GOT A TERRAIN ALERT AND EXECUTED THE ESCAPE MANEUVER. AFTER A FEW MOMENTS THE ALERT ENDED AND TOLUCA APCH THEN VECTORED US TO INTERCEPT THE LOC 15 AND THE RWY WAS IMMEDIATELY IN SIGHT. I REQUESTED AND RECEIVED A VISUAL APCH CLRNC. EXCEPT FOR THOSE CRITICAL 4 MINS; THE FLT WAS NORMAL BEFORE AND AFTER. A SAFETY RPT WAS FILED PER COMPANY PROC AND A MEDIATED DEBRIEF WAS ACCOMPLISHED. MITIGATING FACTORS INCLUDED FATIGUE (A RESERVE CREW); PF (FO) WAS UNDER HIS 100 HR EXPERIENCE LEVEL; AND THE FE WAS HIRED 6 MONTHS AGO. WHILE THEY PERFORMED VERY WELL; INDIVIDUALLY; AND AS A CREW; THE OVERALL EXPERIENCE LEVEL WAS LOW. 2 OTHER CRITICAL FACTORS WERE: 1) EXTREMELY POOR COM WITH BOTH MEXICO CTR AND TOLUCA APCH CTL. 2) THE INITIAL CLRNC TO DIRECT TLC VOR BY MEXICO CTR FROM N OF MEXICO CITY (30N) PLACED OUR RTE OF FLT OVER HIGH TERRAIN NE OF TOLUCA. OTHER CREWS HAVE RPTED TO ME THAT THEY HAD GOTTEN GPWS WARNINGS ON THIS APCH BUT WERE VFR DAY DUE TO A PREVIOUS STOP IN MONTEREY AND APCHED TLC LATER IN THE MORNING. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE ACFT WAS OFF THE PUBLISHED RTE DUE TO THE DELAY IN TURNING R AND FOLLOWING THE 17 MI DME ARC DURING THE EXCHANGE WITH TLC APCH AND THE SWITCHING BACK AND FORTH TO MMEX. THE FLT HAD BEEN N OF MEXICO CITY AND THEN RECEIVED THE DIRECT TLC VOR. THAT ROUTING PLACED THE ACFT OVER THE HIGHER TERRAIN NE OF TLC. THIS WAS THE FIRST TRIP INTO TLC FOR THE FO; WHO HAD 50 HRS OF FLT TIME. THE SO WAS ON HIS SECOND TRIP INTO TLC. THE CAPT SAID THAT THE APCH CTLR HAD A 'BAD ATTITUDE' AND WAS VERY DIFFICULT TO UNDERSTAND DURING HIS 'LONG WINDED CLRNC.' AFTER THE INITIAL READBACK; THERE WAS SILENCE FROM THE CTLR; NECESSITATING A CALL BACK TO MMEX CTR. AFTER GETTING BACK TO TLC APCH CTL; THE PIC TOOK OVER THE CTLS AND WAS IN THE TURN DURING THE GPWS. A 90 DEG R TURN FOLLOWED BY A VECTORED L TURN PLACED THEM ON BASE FOR RWY 14. A COMPANY REVIEW WAS HELD. THE NEW PAGE FOR THE MMEX/TLC AREA IS NOW A SHADED CHART; SHOWING ALTS AROUND THE AREA WHICH THE CAPT SAYS IS A BIG HELP. HE FEELS THAT A MORE STRUCTURED ARR FROM MMEX CTR TO TLC IS REQUIRED. THE CAPT RECEIVED A LINE CHK FOR HIS NEXT TRIP INTO TLC. THE COMPANY IS ADVISING THEIR PLTS TO 'STAY ON THE PUBLISHED RTES.' THE COMPANY ALSO EXPLAINED THAT 'S OF THE BORDER' RADAR VECTORS BY CTLRS DO NOT PROVIDE TERRAIN CLRNC. THAT IS THE FLC'S RESPONSIBILITY.