Narrative:

While on final to bfi on the ILS runway 13R approach around the OM, we received a TCASII alert for traffic 200-300 ft below us and just off our left front. We then visually acquired a single engine airplane, opposite direction, in level flight. Immediately thereafter we received an RA to climb, which I did not do, since I was easily able to see and avoid. The copilot then informed the tower about the traffic, and they replied to the effect that yes, there is traffic out there. We then proceeded to a normal landing. Looking at the 10-1A and 11-1 charts for seattle, as can see that class B airspace on final for runway 13R extends from 1800-10000 ft MSL. Therefore, our traffic was probably operating legally outside the class B airspace in VFR, and was not being handled by a radar controller. The traffic was most likely in the class D airspace of bfi, and we got the impression that he was communicating with the tower, but they never called the traffic for us until after the fact. It would be interesting to know if bfi tower has a repeater scope. In theory, approach control probably could spot a conflict and have tower warn us, but this isn't very timely information. Compounding the problem is terrain in the area of bfi, which probably limits departure paths for light aircraft. This is not the first traffic conflict pilots in our department have had going into bfi. Given the large amount of traffic operating in the area, both VFR and IFR, it seems that this area has real potential for a midair collision. Our TCASII helped protect us in this situation, but it seems like something needs to be done about VFR aircraft flying outbound on or near the localizer course. Callback conversation with TRACON specialist revealed the following: TRACON specialist reports that the seatac runway 16 final approach course crosses directly over the bfi airport. The runway 16 OM is located on the bfi airport. The class B airspace and ILS approach was designed to accommodate bfi airport traffic pattern restrs. Bfi traffic pattern altitude is 800 ft. Bfi traffic is requested to departure due west, under the class B areas, prior to climbing. Bfi and seatac atcts have the same BRITE radar display. The two towers and the TRACON have direct access voice lines to coordination non standard operational procedure requests and to inform each other when aircraft execute non standard/non coordinated actions. Each facility will exchange observed and known traffic. Aircraft intruding class B airspace are coordination with each other when observed and as timely as practicable. Because of the proximity of the traffic pattern and crossing traffic, aircraft executing a runway 16 approach will be exposed to possible TCASII alerts from bfi aircraft operating within the traffic pattern.

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Original NASA ASRS Text

Title: CPR JET ENCOUNTERS NMAC ON APCH TO BFI.

Narrative: WHILE ON FINAL TO BFI ON THE ILS RWY 13R APCH AROUND THE OM, WE RECEIVED A TCASII ALERT FOR TFC 200-300 FT BELOW US AND JUST OFF OUR L FRONT. WE THEN VISUALLY ACQUIRED A SINGLE ENG AIRPLANE, OPPOSITE DIRECTION, IN LEVEL FLT. IMMEDIATELY THEREAFTER WE RECEIVED AN RA TO CLB, WHICH I DID NOT DO, SINCE I WAS EASILY ABLE TO SEE AND AVOID. THE COPLT THEN INFORMED THE TWR ABOUT THE TFC, AND THEY REPLIED TO THE EFFECT THAT YES, THERE IS TFC OUT THERE. WE THEN PROCEEDED TO A NORMAL LNDG. LOOKING AT THE 10-1A AND 11-1 CHARTS FOR SEATTLE, AS CAN SEE THAT CLASS B AIRSPACE ON FINAL FOR RWY 13R EXTENDS FROM 1800-10000 FT MSL. THEREFORE, OUR TFC WAS PROBABLY OPERATING LEGALLY OUTSIDE THE CLASS B AIRSPACE IN VFR, AND WAS NOT BEING HANDLED BY A RADAR CTLR. THE TFC WAS MOST LIKELY IN THE CLASS D AIRSPACE OF BFI, AND WE GOT THE IMPRESSION THAT HE WAS COMMUNICATING WITH THE TWR, BUT THEY NEVER CALLED THE TFC FOR US UNTIL AFTER THE FACT. IT WOULD BE INTERESTING TO KNOW IF BFI TWR HAS A REPEATER SCOPE. IN THEORY, APCH CTL PROBABLY COULD SPOT A CONFLICT AND HAVE TWR WARN US, BUT THIS ISN'T VERY TIMELY INFO. COMPOUNDING THE PROB IS TERRAIN IN THE AREA OF BFI, WHICH PROBABLY LIMITS DEP PATHS FOR LIGHT ACFT. THIS IS NOT THE FIRST TFC CONFLICT PLTS IN OUR DEPT HAVE HAD GOING INTO BFI. GIVEN THE LARGE AMOUNT OF TFC OPERATING IN THE AREA, BOTH VFR AND IFR, IT SEEMS THAT THIS AREA HAS REAL POTENTIAL FOR A MIDAIR COLLISION. OUR TCASII HELPED PROTECT US IN THIS SIT, BUT IT SEEMS LIKE SOMETHING NEEDS TO BE DONE ABOUT VFR ACFT FLYING OUTBOUND ON OR NEAR THE LOC COURSE. CALLBACK CONVERSATION WITH TRACON SPECIALIST REVEALED THE FOLLOWING: TRACON SPECIALIST REPORTS THAT THE SEATAC RWY 16 FINAL APCH COURSE CROSSES DIRECTLY OVER THE BFI ARPT. THE RWY 16 OM IS LOCATED ON THE BFI ARPT. THE CLASS B AIRSPACE AND ILS APCH WAS DESIGNED TO ACCOMMODATE BFI ARPT TFC PATTERN RESTRS. BFI TFC PATTERN ALT IS 800 FT. BFI TFC IS REQUESTED TO DEP DUE W, UNDER THE CLASS B AREAS, PRIOR TO CLBING. BFI AND SEATAC ATCTS HAVE THE SAME BRITE RADAR DISPLAY. THE TWO TWRS AND THE TRACON HAVE DIRECT ACCESS VOICE LINES TO COORD NON STANDARD OPERATIONAL PROC REQUESTS AND TO INFORM EACH OTHER WHEN ACFT EXECUTE NON STANDARD/NON COORDINATED ACTIONS. EACH FACILITY WILL EXCHANGE OBSERVED AND KNOWN TFC. ACFT INTRUDING CLASS B AIRSPACE ARE COORD WITH EACH OTHER WHEN OBSERVED AND AS TIMELY AS PRACTICABLE. BECAUSE OF THE PROX OF THE TFC PATTERN AND CROSSING TFC, ACFT EXECUTING A RWY 16 APCH WILL BE EXPOSED TO POSSIBLE TCASII ALERTS FROM BFI ACFT OPERATING WITHIN THE TFC PATTERN.

Data retrieved from NASA's ASRS site as of July 2007 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.