AN SA227 CREW OVERFLIES THE CTRLINE OF RWY 34R WHEN CLRED FOR THE VISUAL TO RWY 34L. CTLR HAD THEM TOO CLOSE FOR A TURN ONTO RWY 34L. CREW CITES ATC AND THEMSELVES IN THE ERROR. THEIR TCASI PICKED UP TFC ON APCH TO RWY 35R.

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WE WERE ON L DOWNWIND FOR SEATAC'S RWY 34; ABOUT 7 MI AWAY FROM THE ARPT. VISIBILITY WAS RPTED AT 10 MI WITH RAIN SHOWERS. WE WERE IN THE RAIN ALL OF DOWNWIND. ABOUT ABEAM THE NUMBERS; WE WERE GIVEN VECTOR OF 130 DEGS FROM 150 DEGS. THIS PUT US ON A PATH THAT WOULD INTERCEPT FINAL JUST S OF THE MALL (SEE DIAGRAM). WE WERE GIVEN THE MALL VISUAL AND WE IMMEDIATELY STARTED A L TURN. AT THAT POINT WE HAD NOT CALLED (INSIGHT) ANY TFC; NOR BEEN INFORMED OF ANY TFC. AS WE STARTED TO CROSS THE EXTENDED CTRLINE WE SAW TFC ON OUR TCASII JUST UNDER 3 MI AND ABOUT 400 FT LOWER; AT 3 O'CLOCK. WE SLOWED OUR DSCNT AND CONTINUED THE TURN. AS WE ROLLED LEVEL THE AIRBUS WAS ABOUT 700 FT LOWER AND SLIGHTLY BEHIND US; AT ABOUT 1-2 MI. LOOKING BACK; MAYBE WE SHOULDN'T HAVE ACCEPTED THE MALL VISUAL OR WE COULD HAVE TOLD THE CTLR WE WOULD CROSS CTRLINE. HOWEVER; WE ASSUMED HE KNEW OUR HEADING AND POS AND KNEW THERE WAS NO WAY TO MAKE A 150 DEG COURSE CHANGE FROM OUR POS AND NOT CROSS CTRLINE. ALSO; WE WERE MORE CONCERNED ABOUT AVOIDING THE TFC THAN TALKING ON THE RADIO. I BELIEVE THE LATENESS OF THE VISUAL CLRNC COMBINED WITH OUR HEADING AND US NOT HAVING TFC IN SIGHT CAUSED THIS. SUPPLEMENTAL INFO FROM ACN 366872: VECTOR TOO CLOSE IN TO COMPLETE TURN ON FINAL WITHOUT XING RWY 24R CTRLINE. LATE APCH CLRNC. DID NOT CALL TFC TO FOLLOW OR ADVISE ANY TFC. SPACING TOO CLOSE FOR SAFE FLOW. SHOULD NOT HAVE ACCEPTED CLRNC. QUESTIONED ATC ABOUT TFC; SHOULD HAVE QUESTIONED TFC. ADVISE ATC OF XING RWY 34R DUE TO LATE TURN.