AN ACR ASSIGNED A VISUAL APCH AT SEATTLE IS HANDED OFF TO TWR WHO ASSIGNS A 360 DEG TURN TO ACCOMMODATE TFC THAT HAS BEEN CLRED ON TO THE ASSIGNED RWY. UNABLE TO COMPLETE THE TURN BECAUSE OF TFC CONFLICT THE ACR CROSSES THE FINAL APCH COURSE OF TWO RWYS.

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WE WERE CLRED FOR A VISUAL APCH TO RWY 34R BY SEATTLE APCH. SINCE WE WERE ONLY A FEW MI FROM THE THRESHOLD THIS REQUIRED AN EARLY TURN. WE WERE ALSO INSTRUCTED NOT TO PASS A B757; ACFT Y; THAT WAS ESTABLISHED FOR RWY 34L. I CONFIGURED THE ACFT FOR LNDG AND JOINED THE EXTENDED CTRLINE FOR RWY 34R. I SAW WHAT APPEARED TO BE A DC10; ACFT Z; APCHING THE HOLD LINES OF THE SAME RWY. INSTEAD OF HOLDING SHORT HE WAS GIVEN POS-AND-HOLD. SEATTLE TWR INSTRUCTED US TO EXECUTE A R 360 DEGS. AS WE CAME ABOUT THE 270 DEG POINT WE REALIZED; ACFT W; A B737; WAS NOW ESTABLISHED FOR RWY 34L AND WOULD THUS PREVENT US FROM FINISHING OUR TURN. I ROLLED OUT OF THE TURN EARLY; CROSSED BOTH EXTENDED CTRLINES AND COMPLETED THE TURN AS SOON AS POSSIBLE; BUT NOT BEFORE ALARMING ANOTHER ACR FLT; ACFT V; FOR RWY 34L. I SUSPECT SEATTLE APCH AND TWR WERE NOT COORDINATING CLOSELY ENOUGH. HAD THEY HELD ACFT Z THIS WOULD NOT HAVE OCCURRED. I SHOULD HAVE EXECUTED A GAR RATHER THAN THE R 360 DEGS WHICH PUT MY FLT INTO FURTHER PERIL.