FLC OF A DO328 FAILED TO FOLLOW THEIR FPR DUE TO AN ERROR IN THEIR FMC STORED FLT PLAN RESULTING IN ATC INTERVENTION TO BRING THEM BACK ON COURSE.

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I WAS THE CAPT OF FLT FROM GJT TO COS. OUR CLRNC WAS AS FILED WHICH WAS GJT-JNC-HBU J28 DIRECT COS 168 DEG RADIAL AT 38 DME TO COS. WE PROGRAMMED THE FMS WITH A STORED FLT PLAN WHICH WE THOUGHT WAS CORRECT; BUT THE STORED FLT PLAN WAS IN ERROR FROM THE POINT AFTER HBU VOR. THE STORED FLT PLAN WENT FROM HBU DIRECT COS AND NOT J28 TO THE COS 168 DEG RADIAL AT 38 DME. AS WE APCHED THE HBU VOR; I BEGAN TO LOOK UP SOME PERFORMANCE DATA IN THE ACFT'S POH. THE FO WAS THE PF AND HE WAS TRACKING THE FMS COURSE. I WAS BACKING HIM UP USING VOR NAV. AFTER PASSING THE HBU VOR; THE FO TRACKED A COURSE DIRECT TO COS WHICH WAS APPROX THE 050 DEG RADIAL FROM HBU AND NOT THE 080 DEG RADIAL; WHICH WOULD HAVE BEEN J28. I HAD NEGLECTED TO MONITOR THIS BECAUSE I WAS LOOKING UP SOME PERFORMANCE DATA. AFTER ABOUT 30 MI; CTR CAME ON FREQ AND SAID; FLT XX TURN R 20 DEGS -- YOU ARE NOW CLRED J28 TO THE COS 168 DEG RADIAL AT 38 DME COS; WHICH WAS HOW WE WERE CLRED BEFORE. I THEN REALIZED WHAT WAS WRONG AND REPROGRAMMED THE FMS CORRECTLY. ANOTHER CONTRIBUTING FACTOR WAS THAT THE FO WAS NOT MONITORING HIS MOVING MAP DISPLAY CORRECTLY BECAUSE HE HAD THE SCALE DOWN TO LOW SO THAT IT DID NOT SHOW HIS NEXT WAYPOINT WHICH IF HE HAD SEEN IT; HE WOULD HAVE REALIZED HE WAS PROCEEDING DIRECT TO COS AND NOT THE 168 DEG RADIAL AT 38 DME OFF COS FIX.