ACR X TCASII TA RA NON ADHERENCE TO ATC CLRNC HAD POSSIBLE LTSS FROM ACR Y. EVASIVE ACTION DSND.

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ACR X APCHING SAN JUAN; PR; FROM SAALR INTXN. 12 DME NW OF ARPT HDG 180 DEGS; DSNDING FROM 3000 FT TO ASSIGNED ALT 2200 FT. ACR Y WAS AT OUR 2 O'CLOCK; ABOUT 2 MI W OF US (ESTIMATED) AND ABOUT SAME ALT. ATC TOLD Y TO TURN TO HDG 050 DEGS. FOLLOWED BY AN INSTRUCTION TO US TO TURN HDG 280 DEGS. IT WAS VFR AND WE COULD SEE IMMEDIATE CONFLICT IF WE COMPLIED; SO WE DID NOT; WE MAINTAINED HDG 180 DEGS. Y MADE TURN TO 050 DEGS AND PASSED ABEAM OUR R SIDE; CLBING. OF COURSE; Y COULD NOT SEE US TILL HIS TURN NEARLY COMPLETED. TCASII TA/RA WORKED FOR DSCNT THEN MONITOR VERT SPD; WE COMPLIED. WE CONTINUED IN TO LOC RWY 10; CALLED FIELD IN SIGHT AND LANDED RWY 10. I DID NOT PERSONALLY CONTACT ATC UPON LNDG. DO NOT KNOW REASONS FOR TURN INSTRUCTIONS GIVEN US. SUPPLEMENTAL INFO FROM ACN 296473: NNW OF SJU WE WERE HDG S DSNDING TO 2200 FT FOR THE LAGOON VISUAL APCH TO RWY 8. WE NOTICED A 727 ABOUT 1 O'CLOCK WHICH APPEARED TO BE HDG E ON THE RWY 10 LOC. THAT ACFT TURNED OUT TO BE ACR Y. AS WE APCHED 3000 FT AND BEGAN TO SLOW DOWN; ATC GAVE Y A L TURN TO 050 DEGS; PUTTING HIM ON A RAPIDLY CONVERGING COURSE WITH US; ALTHOUGH HE WAS STILL BANKING TO THE R. ATC THEN GAVE US AN IMMEDIATE R TURN TO 280 DEGS. I STARTED BUT JUDGED THAT WE MAY CROSS PATHS AND BE BELLY UP TO Y IF I CONTINUED THE TURN. I ROLLED BACK L AND SAID TO THE CAPT I'M NOT TURNING R. BEFORE I COMPLETED THE SENTENCE WE GOT A TCASII 'DSND; DSND NOW' COMMAND. I STOPPED OUR TURN AND FOLLOWED THE TCASII VSI COMMANDS DOWN WHILE THE CAPT INFORMED ATC. Y PASSED OFF TO OUR R ABOVE US AND CLBING.