AN A320 ACFT ON VECTORS FOR ILS APCH. FLC HAS DIFFICULTY GETTING ACFT OUT OF 'MANAGED' MODE AND ACFT TURNS OPPOSITE DIRECTION FROM HDG DESIRED. FLC WAS ABLE TO CORRECT THE PROB. LOW TIME CAPT MAY HAVE CONTRIBUTED TO LACK OF UNDERSTANDING MANAGED VERSUS MANUAL MODES.

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WE WERE BEING VECTORED WITH THE AUTOPLT ENGAGED FOR AN ILS APCH TO RWY 21R AT DTW. ON BASE LEG; TURNING R TO FINAL; WE WERE GIVEN HDG 120 DEGS AND WHILE TURNING TO THAT HDG; GIVEN HDG 160 DEGS TO INTERCEPT THE LOC. I SELECTED HDG 160 DEGS AND PUSHED THE 'LOC' PUSH BUTTON TO ARM THE FMGC FOR THE INTERCEPT. THE FLT DIRECTOR THEN IMMEDIATELY COMMANDED A L TURN AND THE AUTOPLT FOLLOWED IT. AFTER A SLIGHT 'REACTION TIME' DELAY WHILE I WENT THROUGH THE GLASS COCKPIT TROUBLESHOOTING SEQUENCE (WHAT'S IT DONE? WHY'S IT DOING THAT? HOW DO I MAKE IT STOP?); I PULLED THE HDG KNOB AND TURNED IT TO THE R; WHICH SHOULD HAVE CAUSED THE GUIDANCE SYS TO EXIT WHATEVER MODE IT HAD GONE INTO AND TURN TO THE SELECTED HDG. THIS DID NOT WORK; HOWEVER; AND THE ACFT CONTINUED IN THE L TURN. I THEN DISENGAGED THE AUTOPLT; DISREGARDED THE FLT DIRECTOR AND TURNED THE ACFT BACK TO THE R. THIS ALL TOOK PLACE IN JUST A FEW SECONDS; BUT IT WAS ENOUGH TO CAUSE AN OVERSHOOT ON WHAT WAS ALREADY A TIGHT INTERCEPT. ALSO; BECAUSE OF THE COMPRESSED SEQUENCE OF EVENTS; I DID NOT OBSERVE THE ANNUNCIATIONS ON THE FMA PANEL AND DON'T KNOW WHAT MODE THE FLT GUIDANCE SYS WENT INTO THAT CAUSED THE L TURN. WHEN WE GOT THINGS SORTED OUT WITH THE CTLR AND WERE ON AN INTERCEPT HDG FROM THE OTHER SIDE OF THE LOC; THE FLT DIRECTOR CONTINUED TO COMMAND A L TURN; IN SPITE OF REPEATED ATTEMPTS BY THE FO TO SELECT A HDG TO THE R. THE SELECTED HDG SHOWED IN THE WINDOW BUT THE 'MANAGED NAV' DOT WOULD NOT GO OUT. WE ARMED FOR THE APCH BY PUSHING THE 'APPR' PUSH BUTTON AND THE FLT DIRECTOR THEN SUCCESSFULLY CAPTURED THE LOC AND SEEMED TO RETURN TO NORMAL OP. AFTER LNDG WE WROTE UP THE FLT GUIDANCE SYS; BUT AS I WRITE THIS; I AM AT A LOSS TO EXPLAIN WHAT HAPPENED. THE L TURN SHOULD NOT HAVE BEEN COMMANDED; AND THE MANAGED NAV MODE SHOULD HAVE BEEN DISENGAGED BY SELECTING A HDG TO THE R. THE HDG SELECTED WAS ONLY ABOUT 40 DEGS OR 50 DEGS R OF THE PRESENT HDG; SO IT WAS NOT A CASE OF SELECTING A HDG CHANGE OF OVER 180 DEGS AND HAVING THE ACFT TURN THE SHORTEST DIRECTION. WITH ONLY 100 HRS IN THE ACFT; I DO NOT CONSIDER MYSELF AN AUTH; SO I AM GOING TO BRING THIS SIT UP TO THE 'EXPERTS.'