Narrative:

Executing an ILS approach to runway 16R at seatac international airport under IFR, my aircraft was on localizer and GS from about 3000 ft MSL. Both boeing field (bfi) and sea were intermittently visible through a scattered to broken cloud deck 200-300 ft thick, with a base at 1100 ft AGL (1500 ft MSL). Approaching overhead bfi, the TCASII indicated lower traffic (many aircraft), non-conflicting, apparently in the bfi traffic pattern below. As we passed bfi, another TCASII target appeared, low at our 12 O'clock position and climbing through about 1000 ft below our altitude, about 1 mi ahead. Neither I nor my copilot could acquire the traffic visually. The TCASII warned, 'traffic.' the copilot observed the target change color from olive to amber to red rapidly, and the TCASII gave a RA, 'climb... Climb...' while commanding a minimum climb rate of 1500 FPM. The copilot saw the target close to about 200 ft directly below us just after I had added power and started climbing. After we had climbed about 700 ft, the TCASII announced 'clear of traffic,' and I did my best to transition smoothly back to the GS for a normal landing on runway 16R. At no time before or after the TCASII warning could we acquire the other aircraft visually, due to a combination of circumstances, e.g., the nose high approach attitude of my aircraft, partial visibility loss due to the broken cloud deck, and the visual clutter of trees, roads, cars, houses, etc., in the background. About 45 min later in a conference phone call between me, my airlines' dispatch and safety staff, and sea tower supervisor (ATC), the latter said that the traffic 'popped up' without warning, and was still unident. He also indicated that the 'class B' airspace boundary is only 100 ft below the ILS GS. Notwithstanding this fact, it seems that any aircraft in that area is required to be idented, cleared and in radio contact with ATC. This is my second near miss in 3 months with an aircraft (unknown) encroaching on approach airspace (the first was an emergency engine out approach to runway 8L at hnl). Apparently, the system needs work. In this case, I believe that the TCASII may well have prevented a midair collision.

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Original NASA ASRS Text

Title: ACFT 1 ON AN ILS APCH IS ALERTED BY TCASII OF CLBING OPPOSITE DIRECTION TFC (ACFT 2). ACFT 1 DID NOT SIGHT THE TFC AND THEN TAKES EVASIVE ACTION AS DIRECTED BY TCASII. AFTER THE CLR ADVISORY FROM TCASII, ACFT 1 RESUMES THE ILS. ACFT 2 WAS NEVER SIGHTED.

Narrative: EXECUTING AN ILS APCH TO RWY 16R AT SEATAC INTL ARPT UNDER IFR, MY ACFT WAS ON LOC AND GS FROM ABOUT 3000 FT MSL. BOTH BOEING FIELD (BFI) AND SEA WERE INTERMITTENTLY VISIBLE THROUGH A SCATTERED TO BROKEN CLOUD DECK 200-300 FT THICK, WITH A BASE AT 1100 FT AGL (1500 FT MSL). APCHING OVERHEAD BFI, THE TCASII INDICATED LOWER TFC (MANY ACFT), NON-CONFLICTING, APPARENTLY IN THE BFI TFC PATTERN BELOW. AS WE PASSED BFI, ANOTHER TCASII TARGET APPEARED, LOW AT OUR 12 O'CLOCK POS AND CLBING THROUGH ABOUT 1000 FT BELOW OUR ALT, ABOUT 1 MI AHEAD. NEITHER I NOR MY COPLT COULD ACQUIRE THE TFC VISUALLY. THE TCASII WARNED, 'TFC.' THE COPLT OBSERVED THE TARGET CHANGE COLOR FROM OLIVE TO AMBER TO RED RAPIDLY, AND THE TCASII GAVE A RA, 'CLB... CLB...' WHILE COMMANDING A MINIMUM CLB RATE OF 1500 FPM. THE COPLT SAW THE TARGET CLOSE TO ABOUT 200 FT DIRECTLY BELOW US JUST AFTER I HAD ADDED PWR AND STARTED CLBING. AFTER WE HAD CLBED ABOUT 700 FT, THE TCASII ANNOUNCED 'CLR OF TFC,' AND I DID MY BEST TO TRANSITION SMOOTHLY BACK TO THE GS FOR A NORMAL LNDG ON RWY 16R. AT NO TIME BEFORE OR AFTER THE TCASII WARNING COULD WE ACQUIRE THE OTHER ACFT VISUALLY, DUE TO A COMBINATION OF CIRCUMSTANCES, E.G., THE NOSE HIGH APCH ATTITUDE OF MY ACFT, PARTIAL VISIBILITY LOSS DUE TO THE BROKEN CLOUD DECK, AND THE VISUAL CLUTTER OF TREES, ROADS, CARS, HOUSES, ETC., IN THE BACKGROUND. ABOUT 45 MIN LATER IN A CONFERENCE PHONE CALL BTWN ME, MY AIRLINES' DISPATCH AND SAFETY STAFF, AND SEA TWR SUPVR (ATC), THE LATTER SAID THAT THE TFC 'POPPED UP' WITHOUT WARNING, AND WAS STILL UNIDENT. HE ALSO INDICATED THAT THE 'CLASS B' AIRSPACE BOUNDARY IS ONLY 100 FT BELOW THE ILS GS. NOTWITHSTANDING THIS FACT, IT SEEMS THAT ANY ACFT IN THAT AREA IS REQUIRED TO BE IDENTED, CLRED AND IN RADIO CONTACT WITH ATC. THIS IS MY SECOND NEAR MISS IN 3 MONTHS WITH AN ACFT (UNKNOWN) ENCROACHING ON APCH AIRSPACE (THE FIRST WAS AN EMER ENG OUT APCH TO RWY 8L AT HNL). APPARENTLY, THE SYS NEEDS WORK. IN THIS CASE, I BELIEVE THAT THE TCASII MAY WELL HAVE PREVENTED A MIDAIR COLLISION.

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.