Date: 201605
Local time of day: 1801-2400
Location: BFI.Airport, WA
Light: Night
Aircraft: Citation X (C750)
Operator: Fractional
Operating under: Part 135
Flight plan: IFR
Flight phase: Final Approach
Airspace: Class B SEA
ATC / advisory: TRACON S46
Second aircraft: Any Unknown or Unlisted Aircraft Manufacturer
Reporter organization: Government
Reporter function: Approach
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Distraction; Situational Awareness
Anomaly: Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Separated Traffic
Contributing factors: Airspace Structure; Human Factors; Procedure
Primary problem: Airspace Structure

Narrative: Working Boeing satellite sector; light traffic; marginal VFR conditions. Aircraft X on a vector to the ILS rwy 13R approach at 2200 feet. As I turned Aircraft X to a base leg heading of 040 I scanned and saw VFR traffic NE of the localizer moving westbound at 2000 feet and the target appeared to be climbing. Anticipating that this traffic would become a factor for Aircraft X I continued their turn to a 090 heading and called the traffic. Aircraft X did not have the traffic in sight; so I also advised them of the field for the visual approach. Aircraft X reported the field in sight; so I instructed them to continue the right turn towards the field and cleared them for the visual approach. By this time; the VFR target's Mode-C indicated the same altitude as Aircraft X. After Aircraft X cleared the traffic the pilot advised me that they had responded to a TCAS-RA reference the VFR traffic I had issued as they were in the turn on the visual approach. I informed the supervisor on duty and after determining that no more information was required for the daily log I transferred communication with Aircraft X to BFI ATCT and the aircraft landed without further incident.The FALCON replay demonstrated the targets were .89 NM apart at the same altitude and closest laterally at .71 NM by which point Aircraft X had descended 200 feet below the VFR. While I believe both of those measurements occurred after I had taken positive control action to separate Aircraft X from the VFR target; had I been busier or not caught the VFR target in my scan it could have been much worse. FALCON also illustrates that the VFR target crossed the BFI runway 13R localizer less than 2NM North of ISOGE at 2100 feet climbing; which is exactly where an IFR aircraft would be positioned; both vertically and laterally; to execute the ILS approach to BFI.This is another report of a continued problem within our airspace. Something needs to change. The VFR aircraft are transiting a very narrow; busy corridor of airspace and are doing so without any communication with ATC. It is simply unsafe. The VFR aircraft in this area at the very least need to be in communication with ATC so that we can assign; as necessary; altitude restrictions ensuring the safety of all the aircraft involved. The solution(s) are not hard and while they are potentially more restrictive to VFR aircraft the bottom line is that what happens day in and day out in that airspace as it exists and operates now will eventually result in a very bad accident.




Synopsis: S46 TRACON Controller reported of a conflict between and IFR arrival and a VFR aircraft. Controller reported this is an ongoing problem with VFR aircraft using a corridor that conflicts with IFR aircraft arrivals at the same altitudes.