ORD APCH CTLR RPT ON LOSS OF SEPARATION BTWN A LEADING B737 AND A TRAILING B767 THAT WAS INITIATED WHEN THE B737 TURNED TOO EARLY FOR A DIRECT ORD; IL.

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BOTH ACFT WERE IN TRAIL APCHING ORD FROM THE NE WITH THE B737 IN THE LEAD. THE B737 ON INITIAL CONTACT WAS INSTRUCTED TO DEPART THE PAPPI INTXN DIRECT ORD. THE XMISSION WAS ACKNOWLEDGED BUT THE B737 TURNED DIRECT 15 MI EARLY. THE B737 WAS DSNDED TO 7000 FT AND THAT XMISSION WAS ALSO ACKNOWLEDGED. I NOTICED THE B737 HAD TURNED DIRECT ORD TOO SOON AND ISSUED A HDG TO PUT THE B737 BACK INTO THE AREA IT SHOULD HAVE BEEN IN AND TO ALSO LET THE PLT KNOW HE HAD DEVIATED FROM THE ORIGINAL CLRNC. THE PLT READ BACK THE HDG ONLY. HOLDING THE ASSIGNED HDG FOR ABOUT 4 MI; THE PLT BEGAN A R TURN BACK TO THE PAPPI INTXN; THAT WAS MORE THAN 90 DEGS OFF HIS R WING. ALSO; BECAUSE OF A NON STANDARD DSCNT RATE; THE B737 HAD ONLY LEFT 9800 FT IN ABOUT 12-15 MI OF FLT. THE HARD R TURN PUT THE B737 DIRECTLY INTO THE PATH OF THE B767 THAT WAS GOING PAPPI DIRECT O'HARE. WHEN I SAW WHAT THE B737 WAS DOING; I ISSUED INSTRUCTIONS TO BOTH ACFT TO GET THEM BACK APART; BUT THE B737 REACTED TOO SLOWLY AND SEPARATION WAS LOST. 1) IF THE B737 INITIALLY FOLLOWS THE INSTRUCTIONS THE INCIDENT NEVER OCCURS. 2) IF THE B737 MAKES A NORMAL DSCNT; THE INCIDENT NEVER OCCURS BECAUSE IT WOULD HAVE BEEN BELOW THE B767. 3) IF THE PLT QUESTIONS THE 260 DEG HDG PRIOR TO STARTING A 90 DEG TURN AWAY FROM THE ARPT; THEN THE INCIDENT NEVER OCCURS.