Narrative:

While flying the ILS 16C at sea; traffic from bfi crossed our flight path on a northwest heading and well below us by more than 1000 feet. The high-winged cessna then circled to the left and again crossed our flight path near sodoe; this time causing a TCAS TA followed by an RA. The first officer visually acquired the aircraft and complied with the TCAS RA (monitor vertical speed; which was commanding a maximum descent rate nearly identical to the GS descent rate). The aircraft passed slightly below and to the right and then behind our A319. TCAS showed a vertical separation of 500 to 200 feet during the RA. After the TCAS 'clear of contact'; we asked the seatac tower about the traffic and they responded that is was 'behind us and no factor.' I contacted the seattle TRACON to ask about the traffic.

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Original NASA ASRS Text

Title: An Air Carrier crew received a TCAS RA from an high winged aircraft near BFI as they were on approach to SEA Runway 16C at 1;900 FT.

Narrative: While flying the ILS 16C at SEA; traffic from BFI crossed our flight path on a NW heading and well below us by more than 1000 feet. The high-winged Cessna then circled to the left and again crossed our flight path near SODOE; this time causing a TCAS TA followed by an RA. The First Officer visually acquired the aircraft and complied with the TCAS RA (monitor vertical speed; which was commanding a maximum descent rate nearly identical to the GS descent rate). The aircraft passed slightly below and to the right and then behind our A319. TCAS showed a vertical separation of 500 to 200 feet during the RA. After the TCAS 'CLEAR OF CONTACT'; we asked the SEATAC tower about the traffic and they responded that is was 'Behind us and no factor.' I contacted the Seattle TRACON to ask about the traffic.

Data retrieved from NASA's ASRS site as of April 2012 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.