Date: 201707
Local time of day: 1801-2400
Location: BFI.Airport, WA
Flight conditions: VMC
Light: Daylight
Aircraft: Medium Transport
Flight plan: IFR
Flight phase: Final Approach
Airspace: Class E S46
ATC / advisory: TRACON S46
Reporter organization: Government
Reporter function: Approach
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness
Anomaly: ATC Issue All Types; Conflict Airborne Conflict
Detector: Person Air Traffic Control
Result: Air Traffic Control Separated Traffic; Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance
Contributing factors: Aircraft; Airspace Structure; Procedure
Primary problem: Airspace Structure

Narrative: Aircraft X on vectors to BFI. I turned Aircraft X to an intercept heading to join the localizer for Runway 13R. As is my habit; particularly on days with lots of VFR traffic like today; I scanned ahead of Aircraft X to see if any VFR aircraft were maneuvering in the vicinity of the localizer. I immediately identified traffic northeast of ISOGE at 2;500 feet; tracking southwest. I issued traffic to aircraft X; but he did not report traffic in sight. I subsequently issued Aircraft X a vector to the southwest and informed him that the VFR was likely going to cross his localizer at ISOGE and to expect vectors across for a teardrop back after traffic cleared. As expected; the VFR continued to the southwest and crossed the 13R final approach course at ISOGE. Aircraft X was vectored back onto the ILS and subsequently landed without further incident.I made the supervisor aware of the VFR and contacted nearby ATCT; in an attempt to determine if we could talk to the pilot and raise their awareness of the airspace and potential conflicts where they were maneuvering. The aircraft ultimately dropped off radar in the vicinity of an uncontrolled field.While there was no TCAS-RA involved in this report; it is still an example of an extremely unsafe situation. A FALCON replay illustrated that when I turned Aircraft X away from the VFR to cross the localizer they were separated by 2.6 NM and 300 feet. The replay also illustrated that the VFR crossed within .25NM of ISOGE at 2;500 feet. Had I been too busy; or not seen the traffic; at a minimum I would have expected a TCAS-RA as the targets merged in the vicinity of ISOGE. In the replay; you can also see a collision alert (CA) between this VFR and another VFR outbound off BFI heading northwest.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 having to vector an aircraft off their approach to avoid VFR traffic not in communication with ATC.