C421 PLT BECAME CONFUSED ON DEP WHEN ON AN ATC HDG VECTOR. RPTR ALTERED HDG OFF THE VECTOR THINKING IT WAS TO INTERCEPT A RTE; BUT CTLR INTERVENED AND REMINDED HIM HE WAS STILL ON A VECTOR.

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AROUND XA50 DISPATCH ASKED TO HAVE THE PLANE MOVED FROM PASCO; WA; TO ROSWELL; NM. SLC WAS CHOSEN FOR A LATE NIGHT REFUELING STOP BECAUSE OF 24 HR SVC. THE FUELING STOP TOOK ABOUT 40 MINS WITH A DEP FROM SLC ABOUT XD50. AN IFR FLT PLAN WAS FILED FROM SLC TO ROSWELL AS SLC FFU J12 JNC J15 CNX ROW AT FL180 (SHOULD HAVE BEEN FL190 -- ADD FOR EASTERLY DIRECTION). THE CLRNC RECEIVED WAS CLRED ROW SLC 5 DEP; SLC 095 DEG RADIAL TO MTU 303 DEG RADIAL JNC AS FILED MAINTAIN 11000 FT. THE SLC 5 DEP HAS A MINIMUM CLB GRADIENT OF 380 FT PER NM TO 8000 FT AND A HDG OF 340 DEGS (VECTOR) TO ASSIGNED ALT. THE HDG WAS GIVEN AS EXPECTED AND A CLRNC TO CLB TO 16000 FT WAS GIVEN ON CLBOUT FROM RWY 35. 4-5 MI N OF THE RWY; I CROSSED THE 095 DEG RADIAL OF SLC AND SOMEHOW GOT IT IN MY HEAD THAT I HAD A VECTOR TO AN ASSIGNED RTE AND I SHOULD HAVE ANTICIPATED IT TO LEAD THE TURN EASTWARD. (I HAD LOTS OF CITY LIGHTS IN THAT DIRECTION.) THANKFULLY THE CTLR CAUGHT THE DEV QUICKLY AND TOLD ME TO RETURN TO MY ORIGINAL HDG (ACTUALLY 330 DEGS NOW). HE ASKED IF I'D BEEN ASSIGNED 340 DEGS. I ACKNOWLEDGED THAT I HAD; BUT THOUGHT IT WAS TO AN ASSIGNED RTE. IN RETROSPECT; THAT WAS CORRECT. HOWEVER; A LOOK AT THE SPECIAL INSTRUCTIONS SHOULD HAVE MADE CLR TO ME THAT I WOULD CLB A LONG WAYS NBOUND BEFORE BEING TURNED AROUND AND BROUGHT BACK TO THAT RTE. I KNEW THERE WERE MOUNTAINS E OF ME THAT I COULDN'T OUTCLB HEAD-ON IN A PISTON TWIN AND THAT I SHOULD QUESTION ATC (BEFORE MAKING) ABOUT SEEMINGLY UNWISE MOVES. SOMETHING ABOUT THE 7.5 DME ARC AT 8000 FT (GOV CHART) COMFORTED ME INTO THINKING I WAS SAFE INSIDE THAT ARC CLBING TO ALT. USUALLY A PHRASE STATING; 'THIS IS A VECTOR TO...' SETTLES APPREHENSIONS FOR ME; BUT PROBABLY WOULD HAVE BEEN CONFUSING HERE. I'LL TRY TO STICK WITH THE VECTORS I'M GIVEN. MAYBE; 'CONTINUE HDG 340 DEGS; MAINTAIN 16000 FT' WOULD BE ENOUGH OF A HINT.