Date: 201709
Local time of day: 1801-2400
Location: S46.TRACON, WA
Flight conditions: VMC
Aircraft: Light Transport; Low Wing; 2 Turbojet Eng
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Ambulance
Flight phase: Final Approach
Airspace: Class D BFI
ATC / advisory: TRACON S46
Second aircraft: Small Aircraft
Reporter organization: Government
Reporter function: Approach
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Situational Awareness
Anomaly: ATC Issue All Types; Conflict Airborne Conflict
Detector: Automation Aircraft RA; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic; Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented
Contributing factors: Procedure; Company Policy; Airspace Structure
Primary problem: Airspace Structure

Narrative: I was working sectors combined. Aircraft X was inbound on the visual approach. I was preoccupied with a preceding VFR aircraft who I had been attempting to contact for about 10-15 miles. As such; I was late in issuing traffic to Aircraft X on a VFR target northbound crossing the final approach course at an indicated altitude of 1800 feet. Aircraft X got the traffic in sight as I cancelled their visual approach clearance and issued a climb. I subsequently re-cleared Aircraft X for the visual approach and they landed without further incident.While the pilot made no specific mention of it; I believe they responded to a TCAS-RA. A review of the FALCON radar replay illustrated that Aircraft X climbed 200 feet (from 2100 to 2300 feet) between my initial traffic advisory and my traffic alert. Thankfully; in this instance; there was no conflicting traffic above Aircraft X landing at an adjacent airport. Had there been conflicting traffic; the resulting climb of Aircraft X would have likely resulted in a loss of separation whether the climb resulted from my instruction or a TCAS-RA.My preoccupation with the preceding traffic who was no longer responding to me is an important illustration of how close the encounters in this portion of our airspace can get with very little time; or airspace; to react and recover. Had my landline coordination with Tower been a couple of seconds later; or taken a couple of seconds longer; I may have missed the traffic advisory entirely and only reacted to the aural alert from the CA.Another important aspect of this scenario is that Aircraft X was already cleared for the Visual Approach; but had retained altitude and not started a descent yet. I would not have considered it unexpected if Aircraft X had descended more by this point on the Visual Approach. That said; it was fortunate that the pilot did not because if Aircraft X had chosen to descend sooner; or more swiftly; it may have been a more significant event.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 canceling a Visual Approach clearance to avoid VFR traffic that was not in communication with ATC.