MD11 IN SLLF; FO; AIRSPACE; INITIATES EVASIVE MANEUVER WHEN SLLF CTLR CLRS DC10 TO DSND; OPPOSITE DIRECTION; THROUGH THE MD11'S ALT. MD11 PLT STATES TCASII SAVED THE DAY.

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PROCEEDING NBOUND ON UA301; LA PAZ ATC ASKED US (ACFT X) OUR RADIAL AND DISTANCE FROM VIR VOR. AT THE TIME WE WERE ABOUT 25 MI NW OF VIR ON THE 336 DEG RADIAL. LA PAZ ATC THEN ASKED ACR Y (A DC10) HIS RADIAL AND DISTANCE FROM VIR VOR. HE WAS LNDG AT SANTA CRUZ; SAWU; AND HAD REQUESTED A DSCNT CLRNC OR ATC WANTED TO GIVE HIM A DSCNT CLRNC. ATC TOLD ACR Y TO INTERCEPT THE 320 DEG RADIAL INBOUND TO VIR FOR HIS DSCNT CLRNC; THEN REVISED IT TO THE 316 DEG RADIAL. ACR Y WAS 20 MI IN FRONT OF US AT ABOUT OUR 12 O'CLOCK POS WHEN HE RPTED SEEING US VISUALLY AS OUR LNDG LIGHTS WERE ON. ATC CLRED HIM TO BEGIN HIS DSCNT; THINKING THAT HE WAS ON A DIFFERENT RADIAL; WHEN IN FACT HE WAS VERY CLOSE TO OUR RADIAL. LESS THAN 20 MI IN FRONT OF US; ACR Y BEGAN HIS DSCNT. WE SAW THIS ON TCASII. WE THEN GOT THE TCASII TA FOLLOWED IMMEDIATELY BY AN RA TO DSND; THEN INCREASE DSCNT. WE DSNDED TO FL302 WITH THE FO TURNING R TO AVOID A COLLISION AT THE SAME TIME. NUMEROUS CALLS TO ACR Y TO STOP HIS DSCNT WERE MADE. ACR Y FINALLY STOPPED HIS DSCNT AND PULLED UP AND TO THE R AS HE PASSED US. VERT SEPARATION WAS APPROX 700 FT. LATERAL SEPARATION WAS 1 MI OR LESS AS HE WENT BY OUR WINDOW. IF TCASII HAD NOT BEEN OPERATIONAL; WE WOULD HAVE COLLIDED. TCASII SAVED US. AT FIRST; I FELT THAT ATC HAD MADE THE ERROR; BUT AFTER TALKING TO ACR Y; HE DID HAVE US ON TCASII AND STARTED HIS DSCNT ANYWAY BASED ON HIS VISUAL SIGHTING OF US. I FEEL THAT WAS THE BIGGEST CONTRIBUTOR TOWARD THIS NMAC -- THAT OF ACR Y AND HE PROVIDING HIS OWN SEPARATION AND DSCNT CLRNC BASED ON A VISUAL. TCASII IS TOO VALUABLE TO BE LEFT BEHIND IN SOUTH AMERICA. I WILL NO LONGER OPERATE IN SOUTH AMERICA WITHOUT THE BENEFIT OF AN OPERATIONAL TCASII UNIT. IT IS IRONIC THAT THE TCASII TEST PRODUCED A FAILURE TWICE BEFORE WE DEPARTED. MAINT WAS ABLE TO GET IT TO FUNCTION PROPERLY BUT THE MEL ALLOWED US TO DEPART WITH IT INOP. IF WE HAD BEEN FLYING WITH AN INOP TCASII; A COLLISION WOULD HAVE OCCURRED. SAFETY IS PARAMOUNT IN THIS BUSINESS -- TCASII IS AN IMPORTANT PART OF IT.