Narrative:

Air carrier X was flying on autoplt with the altitude hold engaged, heading 160 degree, heading and altitude assigned by seatac approach control. Seatac approach advised us of VFR traffic at 12 O'clock at 10500 ft. Seatac was talking to the VFR traffic on a different frequency because he was on top of the TCA. I don't remember the DME in the TA, but it was too late to take evasive action. Small aircraft Y passed under us at less than 200 ft and slightly to the right 200 ft. After landing I called approach control and found out the following information. The small aircraft was canadian. He had been tagged by approach control and mode C was transmitting. The pilot saw us and thought he was at the same altitude as we were. He took evasive action and left 10500 ft descending into the TCA without clearance. Seatac said they would serve a traffic violation to him. I believe approach control should have prevented the situation by assigning us a different altitude or heading. Approach control was the only one in radio and radar contact with both airplanes. Just because the small aircraft was 500 ft above the TCA does not mean there will be no conflict of traffic, as we just experienced. Approach seemed to be doing a standard procedure and was not flexible in altitude or heading. I have no way of knowing if training was in progress with the controller, I should have that information available when possible.

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Original NASA ASRS Text

Title: SMA Y UNAUTHORIZED DSCNT THROUGH OCCUPIED ALT UNAUTHORIZED TCA ENTRY HAD NMAC WITH ACR X EVASIVE ACTION TAKEN BY SMA Y. PLTDEV.

Narrative: ACR X WAS FLYING ON AUTOPLT WITH THE ALT HOLD ENGAGED, HDG 160 DEG, HDG AND ALT ASSIGNED BY SEATAC APCH CTL. SEATAC APCH ADVISED US OF VFR TFC AT 12 O'CLOCK AT 10500 FT. SEATAC WAS TALKING TO THE VFR TFC ON A DIFFERENT FREQ BECAUSE HE WAS ON TOP OF THE TCA. I DON'T REMEMBER THE DME IN THE TA, BUT IT WAS TOO LATE TO TAKE EVASIVE ACTION. SMA Y PASSED UNDER US AT LESS THAN 200 FT AND SLIGHTLY TO THE R 200 FT. AFTER LNDG I CALLED APCH CTL AND FOUND OUT THE FOLLOWING INFO. THE SMA WAS CANADIAN. HE HAD BEEN TAGGED BY APCH CTL AND MODE C WAS XMITTING. THE PLT SAW US AND THOUGHT HE WAS AT THE SAME ALT AS WE WERE. HE TOOK EVASIVE ACTION AND LEFT 10500 FT DSNDING INTO THE TCA WITHOUT CLRNC. SEATAC SAID THEY WOULD SERVE A TFC VIOLATION TO HIM. I BELIEVE APCH CTL SHOULD HAVE PREVENTED THE SITUATION BY ASSIGNING US A DIFFERENT ALT OR HDG. APCH CTL WAS THE ONLY ONE IN RADIO AND RADAR CONTACT WITH BOTH AIRPLANES. JUST BECAUSE THE SMA WAS 500 FT ABOVE THE TCA DOES NOT MEAN THERE WILL BE NO CONFLICT OF TFC, AS WE JUST EXPERIENCED. APCH SEEMED TO BE DOING A STANDARD PROC AND WAS NOT FLEXIBLE IN ALT OR HDG. I HAVE NO WAY OF KNOWING IF TRAINING WAS IN PROGRESS WITH THE CTLR, I SHOULD HAVE THAT INFO AVAILABLE WHEN POSSIBLE.

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.