AN MD83 FLT CREW PERFORMS A GAR WHEN TCASII RA ALERTS THEM TO AN NMAC CONDITION WITH UNRPTED TFC ON A PARALLEL APCH AT SEA; WA.

Narrative

WE WERE CLRED FOR A VISUAL APCH TO RWY 16L SEA AFTER RPTING BAY AND ARPT IN SIGHT. COMMUTER TFC WAS IN THE AREA AND SUBSEQUENTLY CLRED FOR RWY 16R. I WAS CAPT AND PF OF AN MD83. WE ROLLED OUT ON FINAL JUST E OF CTRLINE ABOUT 9-10 MI OUT. BTWN 8 AND 7 MI OUT ON CTRLINE OF RWY 16L AND ABOUT 2100 FT; WE GOT A TA; IMMEDIATELY FOLLOWED BY AN RA WHICH TOLD US TO CLB. I HAD THE AUTOPLT OFF BY THIS TIME AND IMMEDIATELY ADDED PWR TO CLB. I REMEMBER SEEING -03 ON THE IVSI; INDICATING TFC 300 FT BELOW. THE RED WARNING SQUARE WAS ALSO PRACTICALLY CTRED ON THE SMALL ACFT DEPICTED ON THE IVSI. I CALLED FOR A GAR; INSTRUCTED THE FO TO NOTIFY THE TWR; AND ASKED THE COPLT IF HE COULD VISUALLY SEE THE TFC. HE SPOTTED THE TFC AND INDICATED IT WAS VERY CLOSE; POSSIBLY 200 FT; CONSISTENT WITH THE RA WARNING. THE TWR ASKED US WHY WE WERE GOING AROUND. WE TOLD THEM WE WERE RESPONDING TO AN RA. WE THEN WERE VECTORED AROUND AGAIN TO LAND AT SEATAC. A POSSIBLE CONTRIBUTING FACTOR WAS THE WIND (ABOUT 240 DEGS/25 KTS AT 3000 FT). THIS COULD HAVE CAUSED THE PARALLEL TFC TO BE PUSHED E OF RWY 16R CTRLINE.