MD80 FO MISINTERPS CLRNC AND TURNS EARLY WHILE CAPT WAS BUSY PROGRAMMING FMS.

Narrative

ON VANDALIA 4 ARR INTO STL; PROCEEDING DIRECT TO THE P INTXN AND DSNDING TO 7000 FT MSL; CAPT/PNF RECEIVED CLRNC FROM STL APCH CTL TO DEPART S INTXN ON A HDG OF 290 DEGS. FO/PF HEARD ONLY THE CAPT READ BACK THE CLRNC TO ATC -- FO/PF UNDERSTOOD THE CLRNC TO BE 'TURN R TO 290 DEGS AND THEN PROCEED TO S INTXN.' FO/PF TURNED IMMEDIATELY TO THE NEW HDG OF 290 DEGS/HDG SELECT MODE; WHILE CONTINUING THE DSCNT TO 7000 FT. PRIOR TO THE HDG CHANGE; THE ACFT WAS IN THE NAV TRACK MODE DIRECT TO THE P INTXN. DURING THE TURN TO 290 DEGS; THE CAPT/PNF WAS LOADING THE CORRECT CLRNC INTO THE FMS AND DID NOT NOTICE THAT THE FO/PF HAD SELECTED THE NEW HDG MANUALLY. THE CAPT/PNF FELT THE ACFT IN THE R-HAND TURN; BUT UNDERSTOOD THIS TO BE THE ACFT INTERCEPTING THE OUTBOUND RADIAL FROM VANDALIA VOR/P INTXN TO S INTXN. THE TURN PUT THE ACFT N OF THIS OUTBOUND RADIAL; 250 DEG RADIAL. AFTER A FEW MINS; STL APCH CTL ASKED US TO TURN DIRECT STL VOR. I DO NOT BELIEVE ANY LOSS OF SEPARATION OCCURRED DURING THIS DEV PREMATURE TURN.