Narrative:

We were being vectored with the autoplt engaged for an ILS approach to runway 21R at dtw. On base leg, turning right to final, we were given heading 120 degrees and while turning to that heading, given heading 160 degrees to intercept the localizer. I selected heading 160 degrees and pushed the 'localizer' push button to arm the FMGC for the intercept. The flight director then immediately commanded a left 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 heading knob and turned it to the right, which should have caused the guidance system to exit whatever mode it had gone into and turn to the selected heading. This did not work, however, and the aircraft continued in the left turn. I then disengaged the autoplt, disregarded the flight director and turned the aircraft back to the right. 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 flight guidance system went into that caused the left turn. When we got things sorted out with the controller and were on an intercept heading from the other side of the localizer, the flight director continued to command a left turn, in spite of repeated attempts by the first officer to select a heading to the right. The selected heading showed in the window but the 'managed navigation' DOT would not go out. We armed for the approach by pushing the 'appr' push button and the flight director then successfully captured the localizer and seemed to return to normal operation. After landing we wrote up the flight guidance system, but as I write this, I am at a loss to explain what happened. The left turn should not have been commanded, and the managed navigation mode should have been disengaged by selecting a heading to the right. The heading selected was only about 40 degrees or 50 degrees right of the present heading, so it was not a case of selecting a heading change of over 180 degrees and having the aircraft turn the shortest direction. With only 100 hours in the aircraft, I do not consider myself an authority/authorized, so I am going to bring this situation up to the 'experts.'

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Original NASA ASRS Text

Title: 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.

Narrative: 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.'

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.