Date: 201708
Local time of day: 0001-0600
Location: S46.TRACON, WA
Flight conditions: VMC
Light: Daylight
Aircraft: Small Transport; Low Wing; 2 Turboprop Eng
Operator: Air Taxi
Operating under: Part 91
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Cruise
Airspace: Class D BFI
ATC / advisory: TRACON S46
Second aircraft: Small Aircraft; High Wing; 1 Eng; Fixed Gear
Reporter organization: Government
Reporter function: Approach
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness; Communication Breakdown
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic; General Flight Cancelled / Delayed
Contributing factors: Airspace Structure
Primary problem: Airspace Structure

Narrative: I was working Arrival sectors combined during evening arrival push. It was very busy with multiple IFR inbounds as well as VFR advisory services. I had just finished resolving conflicts with a preceding aircraft landing BFI and the VFR targets in the area. When I turned Aircraft X towards the localizer; I noticed one of the previous VFR targets had turned slightly more towards the North and climbed. Since it appeared the VFR would directly conflict with Aircraft X; I chose to turn Aircraft X to the left away from the VFR. Through a few vectors I basically gave Aircraft X a left 360 to join the ILS approach.Approach later called for a point-out on a VFR target that I believed was involved in this occurrence; so I asked the controller for the callsign and they identified the aircraft as Aircraft Y. I asked the approach controller to pass along that the aircraft had flown outbound along an active approach course for BFI and that such actions are not necessarily the safest way to operate.Even with the turn; FALCON indicated that the closest proximity between the aircraft was 1 mile laterally and 200 feet vertically. Once again; the timing of this event immediately following my previous report illustrates perfectly the continuous nature of this problem. Aircraft Y was close enough to four IFR arrivals to cause the Conflict Alarm to sound as they flew the localizer and then finished it off with a proximity to the fifth aircraft which required evasive maneuvering to avoid a potential collision. While Aircraft X did not mention a TCAS-RA; I have no doubt one would have resulted had I not issued traffic and a vector away from the VFR.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 solutions 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 having to vector an aircraft off of their final approach course to avoid an unidentified VFR target.