INSTRUCTOR WITH INST STUDENT TURNS TOWARD VOR INSTEAD OF MAINTAINING RWY HDG ON DEP IN IMC.

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DEPARTED RENTON WITH CLRNC AS FOLLOWS AND CLRED ARLINGTON (AWO) AS FILED: RWY HDG; MAINTAIN 3000 FT EXPECT 4000 FT 5 MINS AFTER DEP. I HAD FILED RENTON DIRECT SEATTLE VOR DIRECT PAINE VOR DIRECT ARLINGTON. THE INST DEP PROC FROM RENTON REQUIRES CLBING RWY HDG TO 700 FT AND THEN GOING DIRECT TO THE SEATTLE VOR. SINCE THE CLRNC RECEIVED PROVIDED NO INFO AS TO WHAT WAS EXPECTED SUCH AS: EXPECT RADAR VECTORS; EXPECT L/R TURN TO/AT; OR SOMETHING OTHER THAN 'AS FILED' IN THE 'DETAILED' DEP INSTRUCTIONS; I EXPECTED TO TURN DIRECT SEATTLE VOR IN ACCORDANCE WITH THE CLRNC 'AS FILED' AND THE PUBLISHED INST DEP PROC. I PROCEEDED TO ALLOW THE STUDENT TO TURN TOWARD THE SEATTLE VOR AS WE CONTACTED DEP CTL. DEP QUESTIONED OUR TURN TOWARD THE VOR AND TOLD US TO CONTINUE AN EXPEDITED R TURN TO A HDG OF 20 DEGS DUE TO POTENTIAL TFC CONFLICT WITH ARRIVING SEATTLE TFC (WE HAD DEPARTED RENTON RWY 15). IT WAS NOW OBVIOUS WE WERE EXPECTED TO MAINTAIN RWY HDG UNTIL GIVEN VECTORS BY SEATTLE DEP. SINCE I WAS NOT FAMILIAR WITH THE UNPUBLISHED LCL PROCS I DID WHAT I THOUGHT WAS EXPECTED; BY THE CLRNC WE HAD RECEIVED AND THE PRECONDITIONING ENGENDERED BY THE PUBLISHED INST DEP. IN RETROSPECT I COULD HAVE ASKED FOR CLRNC CLARIFICATION OF RENTON GND CTL OR DEP CTL IF I HAD ANY SUSPICION THAT WE WERE TO 'MAINTAIN' RWY HDG UNTIL GIVEN FURTHER CLRNC. EVEN SO; SOME ADDITIONAL CLRNC DETAIL WAS IN ORDER FOR THE POSSIBILITY OF LOST COMS. I BELIEVE MY ACTIONS IN THIS SIT WERE NOT INAPPROPRIATE GIVEN THE FLT PLAN FILED AND THE CLRNC RECEIVED; BUT I WILL CERTAINLY QUESTION ANY CLRNC THAT I SUSPECT CONTAINS THE KIND OF AMBIGUITY DISPLAYED BY THIS ONE IN THE FUTURE. SUBSEQUENTLY WE WERE HANDED OFF TO ZLC AND GIVEN A VECTOR. WHEN ASKED WHERE THE VECTOR WAS TO; THE CTLR ACTED SURPRISED. WE RESPONDED TO HIS QUESTION AS TO OUR CLRNC THAT WE WERE CLRED DIRECT PAINE VOR DIRECT ARLINGTON. HE WAS SURPRISED WE HADN'T BEEN TOLD WE WERE BEING VECTORED TO ARLINGTON.