ACR X HAD NMAC POSSIBLE LTSS WITH FOREIGN ACR Y. EVASIVE ACTION DSCNT POSSIBLE SYS ERROR. FOREIGN CTLR MAY HAVE BEEN USING VISUAL SEPARATION. SEE AND AVOID CONCEPT.

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ACR X ABOUT TO TURN BASE FOR VISUAL APCH 5 MI FINAL. TWR CALLED AND SAID CONTINUE ON DOWNWIND AND THAT TWR WOULD CALL THE TURN TO BASE. I COMMENTED TO THE CREW THAT THERE WERE MOUNTAINS/HIGH TERRAIN STRAIGHT AHEAD AND TOLD THE FO TO CALL FOR A BASE TURN. TWR REPLIED TO CONTINUE ON DOWNWIND THAT WE COULD EXPECT A BASE TURN IN 1 MI. BY NOW WE WERE ABOUT 7 MI AND HDG INTO HIGHER TERRAIN. AT 8000 FT I TOLD CREW MAINTAIN 8500 FT. WE WERE FINALLY CLRED FOR A BASE TURN. WE WERE 15-17 DME OFF GDL VOR; IN THE TURN TO BASE; WHEN FO SAW AN ACFT HDG ABOUT 100 DEGS CLBING DIRECTLY AT US AND CALLED FOR ME TO IMMEDIATELY DSND BUT WE WERE LIMITED AS TO HDG AND DSCNT RATE BECAUSE OF THE HIGH TERRAIN AROUND AND DIRECTLY BELOW US. WE DSNDED TO ABOUT 8100 FT. NEITHER THE SO NOR I SAW THE OTHER ACFT UNTIL IT PASSED DIRECTLY OVERHEAD. WE ESTIMATED HIS ALT TO BE 500-900 FT DIRECTLY OVERHEAD. THE OTHER ACFT WAS ACR MLG. ADDITIONALLY; I BELIEVE A COM PROB EXISTED. I THINK TWR WANTED US TO TURN BASE LEG THE FIRST TIME HE MENTIONED A TURN IN 1 MI BUT HE NEVER PROPERLY COMMUNICATED THIS WHEN WE QUERIED HIM AFTER PROCEEDING ON DOWNWIND AT LEAST A MI. HE THEN FURTHER CONFUSED AND COMPLICATED THE PROB BY TELLING US BASICALLY THE SAME THING; I.E.; TO CONTINUE BASE LEG FOR 1 MORE MI FOR A TURN TO BASE BUT HE NEVER CLRED US UNTIL NUMEROUS CALLS ON OUR PART BECAUSE OF OUR CONCERN ABOUT THE RISING TERRAIN; WHICH FURTHER COMPLICATED THINGS AND WAS A CONTRIBUTING FACTOR TO OUR MANEUVERING. THE HIGH TERRAIN WAS ALSO A LIMITING FACTOR AS TO OUR CORRECTIVE ACTION TO AVOID THE OTHER ACFT.