Narrative:

Air carrier X on a visual approach to runway 16R sea. We had a near miss with a PA31 piper navajo, atx Y. We were at approximately 1000 ft AGL/1400 ft MSL. The first officer was flying and coupled to the autoplt on the runway 16R ILS. ATC called traffic which was a commuter on short final to runway 16R. Both pilots saw the traffic. When passing over boeing field we received a TCASII TA for traffic with no altitude reported. This is common and we did not see that traffic. The approach continued and we received another TA for traffic 5 mi at 11 O'clock and closing. This second TA was rapidly followed by an RA to descend. The first officer increased descent while both pilots looked for the traffic. The captain spotted the traffic visually. He took control of the aircraft and banked sharply left while continuing to follow the RA to descend. Both pilots then spotted the PA31 traffic initiating a sharp left bank between 500 ft and 1000 ft in front of us. After the conflict had been avoided, we advised sea tower and the captain maneuvered back to the approach path. We made a normal landing on runway 16R sea. Upon arrival to the terminal, we discussed the incident with supervisor of the sea tower. The WX was VFR. The TCASII warning was followed and saved us from a possible fatal crash. We were told the PA31 was on a visual to boeing field.

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Original NASA ASRS Text

Title: ACR X TCASII TA RA. HAD NMAC, LTSS WITH ATX Y ON VISUAL APCH TO ANOTHER ARPT. SYS ERROR.

Narrative: ACR X ON A VISUAL APCH TO RWY 16R SEA. WE HAD A NEAR MISS WITH A PA31 PIPER NAVAJO, ATX Y. WE WERE AT APPROX 1000 FT AGL/1400 FT MSL. THE FO WAS FLYING AND COUPLED TO THE AUTOPLT ON THE RWY 16R ILS. ATC CALLED TFC WHICH WAS A COMMUTER ON SHORT FINAL TO RWY 16R. BOTH PLTS SAW THE TFC. WHEN PASSING OVER BOEING FIELD WE RECEIVED A TCASII TA FOR TFC WITH NO ALT RPTED. THIS IS COMMON AND WE DID NOT SEE THAT TFC. THE APCH CONTINUED AND WE RECEIVED ANOTHER TA FOR TFC 5 MI AT 11 O'CLOCK AND CLOSING. THIS SECOND TA WAS RAPIDLY FOLLOWED BY AN RA TO DSND. THE FO INCREASED DSCNT WHILE BOTH PLTS LOOKED FOR THE TFC. THE CAPT SPOTTED THE TFC VISUALLY. HE TOOK CTL OF THE ACFT AND BANKED SHARPLY L WHILE CONTINUING TO FOLLOW THE RA TO DSND. BOTH PLTS THEN SPOTTED THE PA31 TFC INITIATING A SHARP L BANK BTWN 500 FT AND 1000 FT IN FRONT OF US. AFTER THE CONFLICT HAD BEEN AVOIDED, WE ADVISED SEA TWR AND THE CAPT MANEUVERED BACK TO THE APCH PATH. WE MADE A NORMAL LNDG ON RWY 16R SEA. UPON ARR TO THE TERMINAL, WE DISCUSSED THE INCIDENT WITH SUPVR OF THE SEA TWR. THE WX WAS VFR. THE TCASII WARNING WAS FOLLOWED AND SAVED US FROM A POSSIBLE FATAL CRASH. WE WERE TOLD THE PA31 WAS ON A VISUAL TO BOEING FIELD.

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.