Narrative:

Aircraft X was on final for runway 16R. I noticed a VFR aircraft flying along our class B border. It appears the VFR aircraft aircraft Y; we found out the call sign by calling bfi; was inbound to bfi entering a right downwind for runway 13 at bfi. The aircraft was heading right for aircraft X. I did not have time to call bfi about their VFR traffic and immediately issued traffic to aircraft X. At this point they were similar altitudes 1.5-2 miles apart. Aircraft Y continued on their course which caused them to enter the class B airspace. I was able to see both aircraft out the window and determined it was not a factor. I issued traffic again and aircraft X had aircraft Y in sight. After watching replay they were .87 miles and 200 feet at their closest point; it may have been even closer; but after that point; courses diverged. Aircraft Y was at 1200 feet in class B. We own 1100-1800 feet in that area. There was no point out from bfi.this is a very common problem at sea. The VFR aircraft that operate at bfi on the downwind just southwest of bfi are constantly causing unsafe situations. There have been many RA's and go-arounds due to this operation. There is a very narrow margin for error and not enough time to react if action is needed. I feel that those VFR operations southwest of bfi should be terminated. A normal operation barely gives you the required 500 feet vertical from IFR to VFR. Any time a pilot flies a wide downwind or is slightly off on altitude it causes an extremely unsafe situation. I also feel at times weather can play a huge part as well. Bfi is class 'D' which requires VFR pilots to be a minimum of 500 feet below clouds. When IFR arriving aircraft are on instrument approaches in the clouds they cannot see the VFR traffic that we controllers are scrambling to advise them of. Most of the time these aircraft are being switched to tower frequency right over bfi and the first thing the controller tells them is traffic which they can't see. For IFR aircraft to be descending out of clouds and VFR aircraft to be flying just below the clouds is a very bad mix. I am surprised that this operation has existed as long as it has. I personally cannot count how many ras or go-arounds that I have had because of it. There are many other controllers that have encountered it as well. This operation needs to be terminated immediately.

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Original NASA ASRS Text

Title: SEA Local Controller reported an aircraft enter their Class B airspace on a conflicting course with an air carrier arrival.

Narrative: Aircraft X was on final for Runway 16R. I noticed a VFR aircraft flying along our Class B border. It appears the VFR aircraft Aircraft Y; we found out the call sign by calling BFI; was inbound to BFI entering a right downwind for Runway 13 at BFI. The aircraft was heading right for Aircraft X. I did not have time to call BFI about their VFR traffic and immediately issued traffic to Aircraft X. At this point they were similar altitudes 1.5-2 miles apart. Aircraft Y continued on their course which caused them to enter the Class B airspace. I was able to see both aircraft out the window and determined it was not a factor. I issued traffic again and Aircraft X had Aircraft Y in sight. After watching replay they were .87 miles and 200 feet at their closest point; it may have been even closer; but after that point; courses diverged. Aircraft Y was at 1200 feet in Class B. We own 1100-1800 feet in that area. There was no point out from BFI.This is a very common problem at SEA. The VFR aircraft that operate at BFI on the downwind just southwest of BFI are constantly causing unsafe situations. There have been many RA's and Go-Arounds due to this operation. There is a very narrow margin for error and not enough time to react if action is needed. I feel that those VFR operations southwest of BFI should be terminated. A normal operation barely gives you the required 500 feet vertical from IFR to VFR. Any time a pilot flies a wide downwind or is slightly off on altitude it causes an extremely unsafe situation. I also feel at times weather can play a huge part as well. BFI is class 'D' which requires VFR pilots to be a minimum of 500 feet below clouds. When IFR arriving aircraft are on Instrument approaches in the clouds they cannot see the VFR traffic that we controllers are scrambling to advise them of. Most of the time these aircraft are being switched to tower frequency right over BFI and the first thing the controller tells them is traffic which they can't see. For IFR aircraft to be descending out of clouds and VFR aircraft to be flying just below the clouds is a very bad mix. I am surprised that this operation has existed as long as it has. I personally cannot count how many RAs or Go-arounds that I have had because of it. There are many other controllers that have encountered it as well. This operation needs to be terminated immediately.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.