Narrative:

After departing boeing field, I was leveling off at 2900', just below the TCA floor, when I sighted a light twin just ahead of the leading edge of my left wing. I would estimate it was 300' away at my altitude or slightly higher on a collision course. It appeared to be nose down in a descent. I pulled up and it went underneath. It was an small aircraft Y. It appeared to be headed for renton airport. The small aircraft Y, which I think was descending from above me, was blocked from my view by the high wing of my small aircraft X until the last second, I believe. I would have thought the small aircraft Y pilot would have had a good view of my aircraft well in advance. However, having several hundred hours in that type as well, visibility it not a strong point for that type. The mercer island area is a congested area, what with the close proximity of boeing and renton airports. There have been other conflicts there. Perhaps a more structured procedure for departing and arriving aircraft at both airports through one controling agency would help. For myself, I'll peek under a high wing or drop a low wing for a better look more often, especially while flying in the more congested areas. Also, the small aircraft Y apparently was descending out of the bottom of the TCA. I wonder if my plane was pointed out to the other pilot by ATC. In the future, I'll give myself more room below the TCA as I flew up into it as part of my avoidance maneuver.

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Original NASA ASRS Text

Title: NMAC BETWEEN 2 SMA'S AT BOUNDARY OF SEA TCA.

Narrative: AFTER DEPARTING BOEING FIELD, I WAS LEVELING OFF AT 2900', JUST BELOW THE TCA FLOOR, WHEN I SIGHTED A LIGHT TWIN JUST AHEAD OF THE LEADING EDGE OF MY LEFT WING. I WOULD ESTIMATE IT WAS 300' AWAY AT MY ALT OR SLIGHTLY HIGHER ON A COLLISION COURSE. IT APPEARED TO BE NOSE DOWN IN A DSCNT. I PULLED UP AND IT WENT UNDERNEATH. IT WAS AN SMA Y. IT APPEARED TO BE HEADED FOR RENTON ARPT. THE SMA Y, WHICH I THINK WAS DSNDING FROM ABOVE ME, WAS BLOCKED FROM MY VIEW BY THE HIGH WING OF MY SMA X UNTIL THE LAST SECOND, I BELIEVE. I WOULD HAVE THOUGHT THE SMA Y PLT WOULD HAVE HAD A GOOD VIEW OF MY ACFT WELL IN ADVANCE. HOWEVER, HAVING SEVERAL HUNDRED HRS IN THAT TYPE AS WELL, VISIBILITY IT NOT A STRONG POINT FOR THAT TYPE. THE MERCER ISLAND AREA IS A CONGESTED AREA, WHAT WITH THE CLOSE PROX OF BOEING AND RENTON ARPTS. THERE HAVE BEEN OTHER CONFLICTS THERE. PERHAPS A MORE STRUCTURED PROC FOR DEPARTING AND ARRIVING ACFT AT BOTH ARPTS THROUGH ONE CTLING AGENCY WOULD HELP. FOR MYSELF, I'LL PEEK UNDER A HIGH WING OR DROP A LOW WING FOR A BETTER LOOK MORE OFTEN, ESPECIALLY WHILE FLYING IN THE MORE CONGESTED AREAS. ALSO, THE SMA Y APPARENTLY WAS DSNDING OUT OF THE BOTTOM OF THE TCA. I WONDER IF MY PLANE WAS POINTED OUT TO THE OTHER PLT BY ATC. IN THE FUTURE, I'LL GIVE MYSELF MORE ROOM BELOW THE TCA AS I FLEW UP INTO IT AS PART OF MY AVOIDANCE MANEUVER.

Data retrieved from NASA's ASRS site as of August 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.