ACR MDT VECTORED TO INTERCEPT THE AIRWAY THEN CHANGED TO MAINTAIN VECTOR HEADING FOR TFC. CAPT; PF; LEFT THE FLT CTL PANEL IN THE ARMED MODE AND AT DECEE INTXN THE ACFT TURNED TO THE AIRWAY HEADING. CTLR QUESTIONED HEADING WHEN THE ERROR WAS CAUGHT BY THE FLC. NO CONFLICT.

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WE WERE PLACED ON A HEADING TO INTERCEPT THE AIRWAY FROM CVG TO VHP. DECEE INTXN LIES BTWN; WITH A TURN OCCURRING THERE. UPON SWITCHING FROM CINTI DEP TO ZID; WE WERE PLACED ON A HEADING; VECTOR FOR TFC; WHICH WOULD PRECLUDE TURNING AT DECEE FOR INDY (VHP). AFTER SEVERAL MINS WE WERE AGAIN SWITCHED TO ANOTHER FREQ (ZID). THIS CTLR THEN ASKED IF WE WERE ON A HEADING; TO WHICH I RESPONDED; 'YES.' HE ASKED WHAT THAT WAS; TO WHICH I REPLIED WITH OUR CURRENT HEADING. AT THIS TIME; I REALIZED WHAT HAD HAPPENED. THE CAPT HAD SOMEHOW LEFT THE FLT CTL PANEL IN AN ARMED MODE -- HEADING; WITH NAV ARMED TO CAPTURE THE AIRWAY BEND AT DECEE. THIS ALLOWED THE ACFT TO TURN AND INTERCEPT THE AIRWAY TO VHP; INSTEAD OF REMAINING ON THE PREVIOUSLY ASSIGNED HEADING. I APOLOGIZED AND ASKED IF THERE WAS ANY PROB; TO WHICH THE CTLR RESPONDED THAT THERE WAS AN ACFT AT 6 MI OFF OUR R SIDE. CAPT ACKNOWLEDGED MISTAKE; APOLOGIZED TO ME. I ACKNOWLEDGED MY INATTENTION CONTRIBUTED TO THIS MISTAKE. THE CTLR ALSO STATED THAT IT WAS NOT A PROB.