Date: 201812
Local time of day: 1201-1800
Location: BFI.Airport, WA
Flight conditions: VMC
Light: Daylight
Aircraft: EMB-505 / Phenom 300
Operator: Fractional
Operating under: Part 91
Flight phase: Initial Approach
Airspace: Class B SEA
ATC / advisory: Tower BFI
Second aircraft: Cessna Aircraft Undifferentiated or Other Model
Reporter organization: Fractional
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: First Officer; Pilot Flying
Anomaly: Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft RA; Automation Aircraft TA; Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Took Evasive Action
Contributing factors: Airspace Structure; Human Factors
Primary problem: Airspace Structure

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.


Second narrative: This is a commonplace occurrence at BFI in VMC conditions; unfortunately. I am based on the East Coast and do not frequent BFI yet this has happened to me on my last 3 visits to Boeing Field. This is so common (in my experience) that I included the possibility of a traffic conflict on my approach briefing prior this particular event! I don't disagree with the Captain that I should have probably initiated a go-around; but this is a very difficult location and situation in which to feel comfortable about going around. We could clearly see and avoid the single-engine airplane below and to our right but could not see and avoid any larger airplanes climbing above and behind us out of SEA. The tight confines; terrain constraints; airspace and traffic complexity AND closely spaced parallel runways at BFI seem uniquely combined to make this situation more likely than not for operations at this particular airport.


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