Narrative:

I took off on a local VFR flight from bfi runway 13L and headed 15 mi northeast via the mercer departure to practice maneuvers. Ceiling was 16000 ft broken; winds light and variable and greater than 10 mi visibility. I eventually made my way northwest to pae; performed 3 touch-and-go lndgs on runway 34R then headed southwest toward vashon island to set up for the vashon arrival into bfi. As I flew over bainbridge island; I decided to turn east to set up for a straight-in approach to runway 13L instead of flying further south for the vashon arrival. All the while I had been mindful to monitor my altitude and position so as to stay out of the seattle TCA. While transiting eastbound across puget sound towards the west point reporting point; I received a TCAS alert for traffic at my altitude at less than 1 mi south of my position heading in a northerly direction. I looked out to my right and they had the right-of-way and also to put vertical separation between us. When I was able to identify the traffic; it was a small high-wing aircraft; probably a C172; and appeared to be approximately 100-200 ft above me and passed less than 200 ft behind. At this point I was already over the west point reporting point and called bfi tower and advised them of my position and my intentions to land. The tower controller advised me to make straight-in runway 13L. As I turned southwest for the straight-in approach I was at 2500 ft MSL and still outside of seattle's class B airspace. However; I became distraction as I fixated on how close I had come to the C172 and how disconcerting it was that it had been indistinguishable from the lights off in the distance until the very last moment. I did not realize that I was quickly approaching the lateral boundary of the seattle TCA extending from 1800 ft MSL to 10000 ft MSL. After a few moments; the bfi tower controller asked me to say my altitude and I reported my altitude as 2500 ft. She quickly responded and instructed me to immediately descend below 1800 ft because I had just entered into seattle's class B airspace. I began an emergency descent while making a turn to the north. As I neared the floor of the class B shelf the bfi controller gave me a TA of an aircraft (I believe she said it was a dash-8) at 3700 ft. I do not remember what the controller said regarding the traffic's location to my current position. I informed the controller that I had descended out of class B airspace and proceeded to turn south to continue my approach to runway 13L and landed safely without further incident. The controller did not request that I call the tower. Focusing my thoughts on the approach rather than dwelling on the encounter with the C172 over puget sound would likely have prevented the incursion into the TCA.

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Original NASA ASRS Text

Title: DA20 WAS INVOLVED IN A NMAC. THE PLT WAS DISTRACTED BY THE EVENT; AND INADVERTENTLY ENTERED CLASS B AIRSPACE.

Narrative: I TOOK OFF ON A LCL VFR FLT FROM BFI RWY 13L AND HEADED 15 MI NE VIA THE MERCER DEP TO PRACTICE MANEUVERS. CEILING WAS 16000 FT BROKEN; WINDS LIGHT AND VARIABLE AND GREATER THAN 10 MI VISIBILITY. I EVENTUALLY MADE MY WAY NW TO PAE; PERFORMED 3 TOUCH-AND-GO LNDGS ON RWY 34R THEN HEADED SW TOWARD VASHON ISLAND TO SET UP FOR THE VASHON ARR INTO BFI. AS I FLEW OVER BAINBRIDGE ISLAND; I DECIDED TO TURN E TO SET UP FOR A STRAIGHT-IN APCH TO RWY 13L INSTEAD OF FLYING FURTHER S FOR THE VASHON ARR. ALL THE WHILE I HAD BEEN MINDFUL TO MONITOR MY ALT AND POS SO AS TO STAY OUT OF THE SEATTLE TCA. WHILE TRANSITING EBOUND ACROSS PUGET SOUND TOWARDS THE W POINT RPTING POINT; I RECEIVED A TCAS ALERT FOR TFC AT MY ALT AT LESS THAN 1 MI S OF MY POS HEADING IN A NORTHERLY DIRECTION. I LOOKED OUT TO MY R AND THEY HAD THE RIGHT-OF-WAY AND ALSO TO PUT VERT SEPARATION BTWN US. WHEN I WAS ABLE TO IDENT THE TFC; IT WAS A SMALL HIGH-WING ACFT; PROBABLY A C172; AND APPEARED TO BE APPROX 100-200 FT ABOVE ME AND PASSED LESS THAN 200 FT BEHIND. AT THIS POINT I WAS ALREADY OVER THE W POINT RPTING POINT AND CALLED BFI TWR AND ADVISED THEM OF MY POS AND MY INTENTIONS TO LAND. THE TWR CTLR ADVISED ME TO MAKE STRAIGHT-IN RWY 13L. AS I TURNED SW FOR THE STRAIGHT-IN APCH I WAS AT 2500 FT MSL AND STILL OUTSIDE OF SEATTLE'S CLASS B AIRSPACE. HOWEVER; I BECAME DISTR AS I FIXATED ON HOW CLOSE I HAD COME TO THE C172 AND HOW DISCONCERTING IT WAS THAT IT HAD BEEN INDISTINGUISHABLE FROM THE LIGHTS OFF IN THE DISTANCE UNTIL THE VERY LAST MOMENT. I DID NOT REALIZE THAT I WAS QUICKLY APCHING THE LATERAL BOUNDARY OF THE SEATTLE TCA EXTENDING FROM 1800 FT MSL TO 10000 FT MSL. AFTER A FEW MOMENTS; THE BFI TWR CTLR ASKED ME TO SAY MY ALT AND I RPTED MY ALT AS 2500 FT. SHE QUICKLY RESPONDED AND INSTRUCTED ME TO IMMEDIATELY DSND BELOW 1800 FT BECAUSE I HAD JUST ENTERED INTO SEATTLE'S CLASS B AIRSPACE. I BEGAN AN EMER DSCNT WHILE MAKING A TURN TO THE N. AS I NEARED THE FLOOR OF THE CLASS B SHELF THE BFI CTLR GAVE ME A TA OF AN ACFT (I BELIEVE SHE SAID IT WAS A DASH-8) AT 3700 FT. I DO NOT REMEMBER WHAT THE CTLR SAID REGARDING THE TFC'S LOCATION TO MY CURRENT POS. I INFORMED THE CTLR THAT I HAD DSNDED OUT OF CLASS B AIRSPACE AND PROCEEDED TO TURN S TO CONTINUE MY APCH TO RWY 13L AND LANDED SAFELY WITHOUT FURTHER INCIDENT. THE CTLR DID NOT REQUEST THAT I CALL THE TWR. FOCUSING MY THOUGHTS ON THE APCH RATHER THAN DWELLING ON THE ENCOUNTER WITH THE C172 OVER PUGET SOUND WOULD LIKELY HAVE PREVENTED THE INCURSION INTO THE TCA.

Data retrieved from NASA's ASRS site as of May 2009 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.