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.