ACR X POSSIBLE LTSS FROM ACR Y. POSSIBLE SYS ERROR.

Narrative

WE TOOK OFF RWY 34L SEA AND CLRED SEA 2 DEP TO VICTORIA (YYJ) FLT PLAN RTE; MAINTAIN 9000 250 KTS OR LESS UNTIL FURTHER ADVISED. TWR ADVISED US TO MAINTAIN VISUAL SEPARATION ON A DEPARTING ACFT. APPROX 8-10 NM N OF SEA; WE GOT A VECTOR TO ABOUT 300 DEG HDG; THEN 250 DEG HDG; THEN 240 DEG HDG TO INTERCEPT THE AIRWAY (J-502; SEA 307 DEG RADIAL). ALSO; WE GOT CLB CLRNC TO 15000. CLBING THROUGH ABOUT 11000; DEP CTLR TOLD US TO DSND IMMEDIATELY TO 10000 AND TURN BACK TO A 250 DEG HDG TO AVOID ACR Y AT 1-2 O'CLOCK AND 5 MI AT 11000. I DISCONNECTED THE AUTOPLT AND AUTO THROTTLES (LNAV AND VNAV HAD ENGAGED TO CAPTURE THE AIRWAY AND CLB) TO EXPEDITE THE TURN AND DSCNT; ALSO TCASII WAS GIVING US A 'CLB' SOLUTION TO AVOID ACR Y BUT THE FO HAD THE TFC IN SIGHT AND WE COMPLIED WITH DEP CTLR INSTRUCTIONS. TFC PASSED OFF THE R AT 4-5 MI. WE HAD CLBED UP TO ABOUT 11000 BEFORE DSNDING BACK TO 10000. AFTER GIVING US THE TURN AND DSCNT SEA DEP STATED THAT OUR CLRNC WAS TO INTERCEPT THE SEA 278 DEG RADIAL. AFTER WE CHANGED TO SEA CENTER; WE ASKED WHAT OUR CLRNC WAS AFTER THE SEA 278 DEG RADIAL. SEA CLRED US PRESENT POS DIRECT ANC. SOME OF THE FLT ATTENDANTS AND PAX WERE ALARMED BY THE TURN; DSCNT; AND PWR REDUCTION TO AVOID THE TFC. THERE WAS OBVIOUSLY A DIFFERENCE BTWN WHAT WE THOUGHT OUR CLRNC WAS: I.E.; J-502; 307 DEG RADIAL TO YYJ AND DEP CTL SEA 278 DEG RADIAL. I FEEL THAT SEA DEP SHOULD HAVE USED HDG AND ALT SEPARATION; RATHER THAN RADIAL SEPARATION TO CLR TFC. I.E.; DIFFERENT IN MI BTWN SEA 278 DEG AND 307 DEG AT 10-15 NM IS NOT VERY GREAT. FIRST AND ONLY TFC CALL WAS TO TURN AND DSND IMMEDIATELY.