FLC OF A CARGO DC8 FAILED TO FOLLOW THEIR FLT PLAN ACTUAL CLRNC DURING CLB RESULTING IN ARTCC CTLR INTERVENTION AND VECTOR TO ASSIGNED COURSE.

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ON OUR SCHEDULED FLT FROM ILN TO FLL; THE INITIAL PART OF OUR CLRNC WAS ILN-CVG-IIU-VXV; THENCE ON TO FLL VIA VARIOUS FIXES. WHEN SETTING UP THE GPS; BOTH MYSELF AND FO MISSED NOTICING THAT ILN.FLL1 (OUR PRECOMPUTED FLT PLAN) HAD IIU FIX OMITTED AND HAD US GOING DIRECT VXV (VOLUNTEER) AFTER CVG. WHILE CLBING OUT OF FL220 TOWARDS OUR THEN ASSIGNED ALT OF FL310; FLYING A COURSE OF 174 DEGS CVG-VXV (HDG 177 DEGS); ZID ASKED US IF WE WERE PROCEEDING DIRECT TO IIU. I WAS PF; FO RESPONDED 'NEGATIVE; WE'RE DIRECT VOLUNTEER.' I BELIEVE THAT ATC ASKED US TO GO DIRECT LOUISVILLE; FO TOLD ATC THAT OUR FLT PLAN SHOWED US DIRECT VXV AFTER CVG; CTLR THEN CLRED US TO FLY A 190 DEG HDG (ROUGHLY 1/3 OF THE WAY BTWN WHERE WE HAD BEEN GOING AND WHERE WE SHOULD HAVE BEEN GOING). FO THEN LOOKED AT THE CLRNC PART OF OUR FLT RELEASE AND FOUND THAT INDEED IIU VORTAC WAS AFTER CVG. THERE WAS NO APPARENT CONFLICT WITH TFC; ALTHOUGH WE MAY HAVE GONE INTO ANOTHER CTLR'S SECTOR. ZID SHORTLY THEREAFTER CLRED US DIRECT TO ALMA; GA; THEN AS FILED. FO ACKNOWLEDGED CLRNC; THEN ADDED THAT IIU WAS ON OUR FLT PLAN AFTER ALL.