Narrative:

We were cleared for a visual approach to runway 16L sea after reporting bay and airport in sight. Commuter traffic was in the area and subsequently cleared for runway 16R. I was captain and PF of an MD83. We rolled out on final just east of centerline about 9-10 mi out. Between 8 and 7 mi out on centerline of runway 16L and about 2100 ft, we got a TA, immediately followed by an RA which told us to climb. I had the autoplt off by this time and immediately added power to climb. I remember seeing -03 on the ivsi, indicating traffic 300 ft below. The red warning square was also practically centered on the small aircraft depicted on the ivsi. I called for a go around, instructed the first officer to notify the tower, and asked the copilot if he could visually see the traffic. He spotted the traffic and indicated it was very close, possibly 200 ft, consistent with the RA warning. The tower 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 degrees/25 KTS at 3000 ft). This could have caused the parallel traffic to be pushed east of runway 16R centerline.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.