MLG HAS HDG DEV DUE TO MALFUNCTION OF THE FMS.

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FLT DTW-ORD ON JUN/XX/94. FLT WAS HOLD ON THE GND AT DTW FOR ABOUT 2 HRS DUE TO WX AT ORD. AFTER AIRBORNE AND HAVING BEEN TURNED OVER TO ZAU FROM ZOB WE WERE CLRED 'DIRECT' TO PAPPI INTXN WHICH IS ON THE PULLMAN STAR. I PUSHED THE 'DIRECT TO' BUTTON AND SELECTED PAPPI. THE AIRPLANE STARTED A TURN ON AUTOPLT AND I THOUGHT IT WAS MAKING AN EXCESSIVE TURN. IN FACT IT TURNED TOO FAR TO GO DIRECT TO PAPPI AND JUST AS I STARTED TO 'DESELECT' THE MANAGED NAV FEATURE AND GO TO A SELECTED HDG TO FLY DIRECT PAPPI WE ROLLED OUT OF THE TURN AND I THOUGHT THE AIRPLANE WAS MAKING ITS OWN CORRECTION. AT THAT POINT I WAS DISTRACTED BY SOMETHING OR SOMEONE AND DIDN'T 'FOLLOW UP' ON WHAT THE AIRPLANE WAS DOING. THE NEXT THING I KNOW; ATC CALLS US AND ASKS US IF WE ARE GOING DIRECT PAPPI AS PREVIOUSLY CLRED. I LOOKED AND THE AIRPLANE WAS ON ABOUT A 230 DEG HDG AS THOUGH IT WERE TRYING TO INTERCEPT A COURSE; NOT ON A 280 DEG HDG DIRECT TO PAPPI. ATC GAVE US A 310 DEG HDG AND SAID WE WERE 'GETTING TOO CLOSE' TO THE CHICAGO DEP CORRIDOR. I DON'T KNOW WHY THE FMS DID NOT TAKE US DIRECT PAPPI. WHEN I PUSHED THE 'DIRECT TO' PUSH BUTTON IT GAVE ME A SOLID LINE FROM PRESENT POS TO PAPPI BUT THEN IT MADE TOO LARGE A TURN AND WHEN I THOUGHT IT WAS MAKING ITS OWN CORRECTION IT OBVIOUSLY WASN'T. I'VE NEVER SEEN THAT OR A SIMILAR SIT OCCUR BEFORE. MY ERROR WAS TO ALLOW MYSELF TO BE DISTRACTED BEFORE I FINISHED FOLLOWING UP ON WHAT THE AIRPLANE WAS DOING WHEN IT ROLLED OUT OF ITS TURN AFTER TURNING TOO FAR FOR A DIRECT PAPPI.