AN INTXN WHICH WAS NOT IN THE STORED FLT PLAN WAS INSERTED IN THE INS ROUTING; BUT INSERTED IN THE WRONG PLACE. THEN; WHEN THE ACFT PASSED A CHK POINT; IT STARTED TO TURN BACK TO THE INTXN INSTEAD OF THE CORRECT FIX. FLC CONTACTED CTLR WHO AIDED THEM GETTING BACK ON COURSE.

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WE WERE FLYING OUT OF NEWARK ON A SID AND EVENTUALLY CLRED DIRECT ON COURSE. THE COURSE WAS PARKE J6 TO CHARLESTON. WHEN THE INS WAS PROGRAMMED AND THE STORED FLT PLAN WAS PULLED UP; THE LISTED POINTS WERE PARKE; LANDCASTER; MARTINSBURG; ETC. THE FLIRT INTXN WAS NOT ON THE STORED FLT PLAN AND THE FO WHO WAS FLYING DECIDED HE WANTED IT IN HIS FLT PLAN. HE INSERTED FLIRT CHKED THE COORDS AND WE WENT FLYING. NOW AS WE WERE FLYING FROM PARKE ON THE AIRWAY WE DIDN'T REALIZE HE INSERTED THE INTXN FLIRT BTWN PARKE AND LANDCASTER INSTEAD OF BTWN LANDCASTER AND MARTINSBURG. SO WHEN WE PASSED FLIRT THE ACFT STARTED A L TURN BACK TO LANDCASTER. WE KNEW THE ACFT SHOULDN'T BE TURNING AT THIS TIME BECAUSE MRB VOR WAS STILL IN FRONT OF US. WE TOOK THE ACFT OFF INS NAV WENT TO HDG; TALKED TO CTR AND GOT BACK ON COURSE. WE WERE ABOUT 4 MI L OF CTRLINE MAX DEV. THERE WERE NO OTHER ACFT IN THE AREA. THE CTLR SAID THERE WAS NO PROB AS FAR AS THEY WERE CONCERNED. IT TOOK US A COUPLE OF MINS TO FIGURE OUT WHY THE INS WAS TRYING TO TURN THE ACFT. THE MORAL OF THIS IS NOT ONLY CHK THE COORDS OF A WAYPOINT BUT BE SURE YOU INSERT IT IN THE PROPER PLACE IN THE FLT PLAN.