Narrative:

In VMC while on an IFR approach; level at 2200 feet assigned; we were being vectored on base leg preparing to join localizer course for bfi runway 13R; when we received a TCAS traffic alert. Target aircraft was straight ahead on our course temporarily level 500ft below. As we closed; the target aircraft began to climb; causing an RA commanded climb for us. Simultaneously the approach controller announced the target aircraft climbing and assigned us a heading to deconflict. As required; I had already initiated the commanded RA climb; and my pilot monitoring bugged my new heading and informed the controller we had an RA. The pilot monitoring announced the RA twice. When clear of conflict; I slowly descended back down to our assigned altitude. ATC then gave us a new heading back to intercept the localizer. The remaining approach and landing were uneventful.

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Original NASA ASRS Text

Title: CE-560XL flight crew reported climbing in response to a RA on approach to BFI.

Narrative: In VMC while on an IFR approach; level at 2200 feet assigned; we were being vectored on base leg preparing to join localizer course for BFI Runway 13R; when we received a TCAS traffic alert. Target aircraft was straight ahead on our course temporarily level 500ft below. As we closed; the target aircraft began to climb; causing an RA commanded climb for us. Simultaneously the Approach Controller announced the target aircraft climbing and assigned us a heading to deconflict. As required; I had already initiated the commanded RA climb; and my pilot monitoring bugged my new heading and informed the controller we had an RA. The pilot monitoring announced the RA twice. When clear of conflict; I slowly descended back down to our assigned altitude. ATC then gave us a new heading back to intercept the localizer. The remaining approach and landing were uneventful.

Data retrieved from NASA's ASRS site 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.