THE RPTR ON CLBOUT OF SEATAC ON THE FMS BLUIT2 DEP WAS ADVISED BY DEP CTL TO DISCONTINUE THE FMS DEP AND FLY AN ASSIGNED HDG. THE CTR'S COMMENTS INDICATED HE EXPECTED THE FLT TO BE ON THE MOUNTAIN 4 DEP 158 DEG RADIAL AND THE CTLR SAID IF SEA WERE ON N DEPS THE RPTR COULD GET KILLED DOING THAT. THE PDC WAS 'SEA J70 MLP AS FILED PIT; BLUIT2 FMS DEP MAINTAIN 4000 FT.'

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ON CLBOUT WHILE FLYING FMS BLUIT2 DEP; DEP CTL ADVISED DISCONTINUE FMS DEP AND ASSIGNED A HDG. HIS COMMENTS TO US MAKE ME BELIEVE THAT HE EXPECTED US TO BE ON THE MOUNTAIN 4 DEP (158 DEG RADIAL SEA). OUR PDC SEA J70 MLP/PIT BLUIT2 DEP FMS MAINTAIN 8000 FT. THERE APPEARS TO BE A COM PROB BTWN FACILITY ISSUING ATC CLRNC AND DEP CTLR SEA. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THIS RPTR WAS AN INSTRUCTOR AND CHK AIRMAN FOR THE AIRLINE WHEN THE FMS WAS BEING DEVELOPED. THE FLC AT THE APPROPRIATE TIME AFTER DEP SELECTED LNAV AND AUTOPLT ON TO CAPTURE THE BLUIT2 FMS DEP. THE FMS FOLLOWS A COURSE AND NOT THE RADIAL. THE RPTR WAS AWARE THE COURSE WAS E OF THE RADIAL. THE RPTR THOUGHT THE CTLR WAS CONCERNED ABOUT TFC; NOT TERRAIN; WHEN THE CTLR SAID IF SEA WERE ON A N FLOW; FOLLOWING THE FMS COULD GET THEM KILLED. THE RPTR THOUGHT PERHAPS THE CTLR WAS ALLUDING TO AN FMS ARR TO AN ILS LNDG PROC TO THE N AT SEA. THERE IS NO PLACE IN THE FMS ARR TO THE RWY 34R ILS THAT SPECIFICALLY SAYS WHEN TO SELECT ILS AND HAVE THE ILS TRACK THE LOC RATHER THAN THE LNAV. BY COMPANY POLICY WHEN THE ILS LOC IS ALIVE THAT IS THE CUE TO SELECT THE ILS APCH MODE. THE RPTR SAID HE HAS HEARD THERE HAVE BEEN PROBS WITH THOSE APCHS THAT ARE DONE ALL THE WAY TO THE LNDG ON LNAV. DEPENDING ON THE ALIGNMENT OF THE IRS THE LNAV IS NOT AS PRECISE AS THE ILS. THE RPTR SAID THERE WAS NO MOUNTAIN 4 MENTIONED IN THE PDC. HE SAID THAT ON DEPS PRIOR TO THIS DEP HE HAD TO OCCASIONALLY ASK FOR THE BLUIT2 FMS DEP. HOWEVER; IN THIS CASE THE CLRNC DELIVERY PROVIDED IT WITHOUT ASKING. SUPPLEMENTAL INFO FROM ACN 342856: WAS CLRED VIA PDC AT SEA 'BLUIT2 FMS DEP.' AFTER TKOF; DEP CTL SHOWED US OFF COURSE FROM THE SEA 158 DEG RADIAL. I STATED THAT WE WERE CLRED VIA THE FMS DEP AND WERE FOLLOWING THE COURSE IN FMS LNAV AND VNAV. CTLR THEN CANCELED OUR FMS DEP AND ISSUED RADAR VECTORS TO INTERCEPT THE AIRWAY. CTLR THEN STATED; 'DON'T USE THAT FMS DEP; IF WE WERE ON N DEPS YOU COULD GET KILLED DOING THAT.' IN CONFUSION; I THEN RADIOED THAT WE WERE CLRED VIA THE FMS AND WERE FOLLOWING THE COURSE. I BELIEVE THIS PARTICULAR CTLR DOES NOT UNDERSTAND FMS/RNAV PROCS. AS THERE IS SOME IRS DRIFT; FLYING AN FMS DEP DOES NOT ALWAYS LIE EXACTLY OVER A PARTICULAR NAV RADIAL/FIX THAT HE/SHE MAY BE USING ON RADAR. THE DEP CHART ONLY STATES TO FOLLOW THE FMS RTE (WHICH WAS VERIFIED BEFORE LEAVING THE GATE) NOT A PARTICULAR VOR RADIAL. I BELIEVE WE WERE FULLY COMPLIANT WITH ALL PROCS ASSOCIATED WITH AN RNAV DEP. CALLBACK CONVERSATION WITH RPTR ACN 342856 REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT PRIOR TO DEP HE DID NOT NOTICE IF THE IRS WAS ALIGNED. IF THE CREW HAS TIME AND HAPPENS TO SEE AN OUT OF ALIGNMENT CONDITION EXISTS THE CREW WILL DO A QUICK ALIGNMENT WHICH REQUIRES THE AIRPLANE REMAIN PERFECTLY STATIONARY FOR 30 SECONDS. HE DOES KNOW THE LNAV DID TAKE THE FLT E OF THE 158 DEG RADIAL. HE DID NOT REALIZE THE GAP FOR THE DEP AT 3 MI WAS THAT NARROW TO REMAIN IN THE CLASS B AIRSPACE. THE FMS DEP TO THE N IS A CLB OVER ELLIOTT BAY WITH A CLBING TURN TO THE W AND THE S AND BACK TO THE E OVER SEATAC AT ALT FOR THE EBOUND FLTS THAT TAKE OFF TO THE N. THE RPTR HAD NO IDEA WHAT THE CTLR MEANT WHEN HE SAID IF SEATTLE WERE ON A N FLOW THE FMS DEP COULD GET THEM KILLED.