Narrative:

We were on the ILS32L at bfi cleared for the straight in visual approach for that runway. We were handed off to tower at about a 10 mile final and seattle approach asked us to maintain 170 knots or greater to a 5 mile final. Upon initial contact with tower; they cleared us to land 32L. Shortly after that; tower advised us of outbound traffic he was working at 1000 feet that was to remain east of the 32L final. A little bit later; we heard tower ask the cessna if he had us in sight and to turn left (away from us and the localizer course) 10 degrees and again instructed the cessna to remain clear of our path. Tower then updated the aircraft's position to us and right as a TA sounded; we both visually acquired the aircraft at our 2 o'clock position. We noted the aircraft was continuing to fly very close to the final and localizer course; and subsequently got a 'level off' RA. We could tell we were not on a collision course; that we would pass clear of the traffic; however it was close laterally. We did begin to level off though not to the extent of the RA due to visual contact and not wanting to destabilize the approach. In hindsight; we should have fully flown the RA as the cessna apparently felt they gave enough room; but the TCAS did not and the proximity was closer than I would have preferred. We did get a subsequent change to a climb RA but before we could act on that; the clear of conflict was announced by TCAS. We continued to land and advised tower after stopped clear of the runway that the cessna caused an RA and then a momentary increased RA direction.as a side note; this is far from my first RA I have gotten while in sea/bfi approach/tower airspace. We even briefed the possibility for ras on our approach brief.one of the biggest factors in my opinion to this event is the airspace structure around bfi due to the sea class B configuration. I am certain the cessna was trying to fly either through or depart from bfi while remaining clear and below the B airspace. Given the hilly terrain below; the cessna likely did not want to turn much more east while at 1000 MSL. It is a hazardous setup for opposite direction traffic. Tower saw the encroachment of the cessna to the localizer course; but in my opinion did not issue a strong enough turn away from the course to the cessna. In our case; with the RA; climbing while on the ILS32L course is not a great idea given sea was also departing north; potentially causing an RA issue with traffic departing there. We should have at least reduced our descent more aggressively initially; perhaps slightly turned to our left (while not allowing full course deflection) to allow greater lateral separation with a cessna clearly not giving enough way and if it was required; overfly the runway and enter left traffic; advising tower of such.

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Original NASA ASRS Text

Title: Phenom 505 flight crew reported a TCAS RA while on approach to BFI from a departing Cessna.

Narrative: We were on the ILS32L at BFI cleared for the straight in visual approach for that runway. We were handed off to Tower at about a 10 mile final and Seattle Approach asked us to maintain 170 knots or greater to a 5 mile final. Upon initial contact with Tower; they cleared us to land 32L. Shortly after that; Tower advised us of outbound traffic he was working at 1000 feet that was to remain east of the 32L final. A little bit later; we heard Tower ask the Cessna if he had us in sight and to turn left (away from us and the localizer course) 10 degrees and again instructed the Cessna to remain clear of our path. Tower then updated the aircraft's position to us and right as a TA sounded; we both visually acquired the aircraft at our 2 o'clock position. We noted the aircraft was continuing to fly very close to the final and localizer course; and subsequently got a 'Level Off' RA. We could tell we were not on a collision course; that we would pass clear of the traffic; however it was close laterally. We did begin to level off though not to the extent of the RA due to visual contact and not wanting to destabilize the approach. In hindsight; we should have fully flown the RA as the Cessna apparently felt they gave enough room; but the TCAS did not and the proximity was closer than I would have preferred. We did get a subsequent change to a climb RA but before we could act on that; the clear of conflict was announced by TCAS. We continued to land and advised Tower after stopped clear of the runway that the Cessna caused an RA and then a momentary increased RA direction.As a side note; this is far from my first RA I have gotten while in SEA/BFI Approach/Tower airspace. We even briefed the possibility for RAs on our approach brief.One of the biggest factors in my opinion to this event is the airspace structure around BFI due to the SEA Class B configuration. I am certain the Cessna was trying to fly either through or depart from BFI while remaining clear and below the B airspace. Given the hilly terrain below; the Cessna likely did not want to turn much more east while at 1000 MSL. It is a hazardous setup for opposite direction traffic. Tower saw the encroachment of the Cessna to the localizer course; but in my opinion did not issue a strong enough turn away from the course to the Cessna. In our case; with the RA; climbing while on the ILS32L course is not a great idea given SEA was also departing north; potentially causing an RA issue with traffic departing there. We should have at least reduced our descent more aggressively initially; perhaps slightly turned to our left (while not allowing full course deflection) to allow greater lateral separation with a Cessna clearly not giving enough way and if it was required; overfly the runway and enter left traffic; advising Tower of such.

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.