A B727-200 CHANGES ALT BECAUSE OF TCASII RA. FLC THEN SEES AN ACR ACFT FLY UNDERNEATH THEM AT THE ALT THEY WERE CRUISING. ARTCC SEEMS TO HAVE MADE AN ERROR IN SEPARATION BUT DOES NOTHING TO ACKNOWLEDGE THE ERROR.

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WE DEPARTED ACA FOR LAX AS FLT YY ON APR/XX/97. WE WERE CLRED AS FILED AT AN INITIAL ALT OF FL310 (SEE ATTACHED FLT PLAN). AFTER DEP WE WERE CLRED TO FL310 WITH A RESTR TO BE ABOVE FL180 BY 50 DME FROM ACA. WE CLBED THROUGH FL180 AS WE PASSED 46 DME WNW OF ACA; AND WERE TOLD TO CONTACT MEXICO CTR PASSING ZIHUATANEJO. WE WERE LEVEL AT FL310 WHEN WE CHKED IN WITH MEXICO CTR OVER ZIHUATANEJO. THE CTLR SAID 'RADAR CONTACT; MAINTAIN FL310.' WE ACKNOWLEDGED AND PROCEEDED ALONG J1 TOWARDS MANZANILLO. ALTHOUGH WE HAD OMEGA ON BOARD; I WAS NAVING THE ACFT BY USING VORS. WHEN WE WERE 33 DME WNW OF MANZANILLO (116.8); THE TIME WAS XX30Z; THE TCASII WHICH WAS SELECTED TO THE 25 MI RANGE GAVE A 'TFC' WARNING. I SWITCHED TO THE 10 MI RANGE TO GET A BETTER PICTURE OF THE APCHING TARGET AND SAW THAT WE HAD A RELATIVE ALT DIFFERENCE OF ONLY 100 FT. AT A CLOSURE RATE I WOULD ESTIMATE WAS IN EXCESS OF 900 KTS PER HR; THE TA IMMEDIATELY TURNED INTO AN RA; INDICATING A CLB WAS NECESSARY TO MISS THE ONCOMING ACFT. SINCE WE WERE IN THE CLR ON A BEAUTIFUL; ALMOST CLOUDLESS DAY AND THE TCASII DID NOT INDICATE THAT ANY OTHER ACFT WERE IN THE AREA; I DISCONNECTED THE AUTOPLT AND STARTED A CLB OF APPROX 2500 FPM. ALTHOUGH ALL 3 OF US WERE DOING OUR BEST TO LOCATE THE ONCOMING TFC VISUALLY WE WERE UNABLE TO DO SO. AT APPROX FL320 THE TCASII GAVE A 'MONITOR VERT SPD' AND I STARTED TO LEVEL THE ACFT. I LEVELED THE ACFT AT FL325 AND FO RPTED TO MEXICO CTR THE SIT AND THE EVASIVE ACTION THAT WE WERE TAKING. AS WE LEVELED AT FL325 WE GOT A VISUAL CONTACT WITH AN ACR JET (DC9 OR MD-SOMETHING OR OTHER; I CAN'T TELL THE DIFFERENCE) PASSING BELOW FL310 JUST OFF TO OUR R SIDE. I IMMEDIATELY STARTED A DSCNT BACK TO FL310 WHILE FO INFORMED MEXICO CTR OF OUR ACTIONS AND ASKED FOR AN EXPLANATION. WE WERE IMMEDIATELY SWITCHED TO MAZATLAN CTR WHEN WE AGAIN ASKED FOR AN EXPLANATION. THEY SAID IT WAS A CTR PROB AND ASKED US TO CONTACT MEXICO CTR ON THE PREVIOUS FREQ. WE WENT BACK TO MEXICO CTR BUT THEY DIDN'T WANT TO TALK TO US AND HAD US IMMEDIATELY SWITCH BACK TO MAZATLAN CTR. WHEN WE RETURNED TO MAZATLAN CTR HE ASKED US IN VERY BROKEN ENGLISH AND A ROUND ABOUT WAY IF WE HAD RECEIVED AN 'EXPLANATION.' WE SAID NO AND THAT WAS THE END OF IT AS FAR AS MEXICAN ATC WAS CONCERNED. THE CLB TO FL325 WAS QUICK BUT SMOOTH; AND NEITHER THE FLT ATTENDANTS NOR THE PAX WERE EVER AWARE OF OUR MANEUVER TO AVOID THE OTHER ACFT. IT IS THE OPINION OF ALL 3 OF US THAT THE ACR ACFT NEVER DEVIATED FROM HIS FLT PATH OR ALT. THAT WOULD LEAD ME TO BELIEVE THAT THEY EITHER HAD NO TCASII; IT WASN'T WORKING; OR IT WASN'T TURNED ON. WE ALSO ALL AGREED THAT HAD THE TCASII NOT GIVEN US THE WARNING; WE MORE THAN LIKELY WOULD HAVE MISSED THE OTHER ACFT HORIZLY; BUT IT WOULD HAVE BEEN VERY CLOSE! I AM GOING TO FILE A NASA RPT ON THIS INCIDENT NOT BECAUSE I AM CONCERNED ABOUT THIS CREW'S POSSIBLE ERROR; BUT BECAUSE I THINK THAT IT IS IMPORTANT THAT THIS INCIDENT BECOME PART OF THE STATISTICAL RECORD. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR WAS DISTURBED AND FRUSTRATED THAT ARTCC DID NOT ACKNOWLEDGE ONCOMING TFC. WHEN THE TFC WAS RPTED AS SIGHTED; ARTCC QUICKLY HAD THEM CHANGE CTR FREQS. WHEN THE FLC QUERIED THE NEW CTLR; HE HAD THE RPTR SWITCH BACK TO THE OLD FREQ; BUT EVEN THERE; NO ONE WANTED TO TALK TO THEM. RPTR STATED THAT THEY WERE BEING SWITCHED BACK AND FORTH BECAUSE NO GND CTLR WAS GOING TO BE RESPONSIBLE FOR WHAT HAD HAPPENED. FLC IS GRATEFUL FOR TCASII. HAD THEY NOT HAD IT; THEY BELIEVE THEY WOULD HAVE MISSED THE ONCOMING TFC; BUT NOT BY MUCH. THE OTHER ACFT TOOK NO EVASIVE ACTION AND PROBABLY NEVER SAW RPTR'S ACFT. NOT ALL ACR ACFT IN MEXICO HAVE TCASII.