Narrative:

First officer flying, all aircraft system appeared normal, dtw landing 21R. We received a 200 degree heading to intercept 21R ILS and cleared for the approach. Descending to 3000 ft, approximately 5-6 mi outside marker. Both flight directors set to approach automatic modes and radios to 110.7 (215 degrees). It appeared that we were set up for a normal intercept, but the flight directors were calling for more intercept to the left. We intercepted 21R but the flight director still called for more intercept. The first officer flew according to what the instruments were telling him to do thinking the computer must know more about the wind than we do. We drifted towards 21L ILS when the tower asked us if we had the right frequencys set in. I quickly checked, we did. I asked if there was a traffic conflict and could we have 21L ILS. No conflict and tower cleared us for 21L ILS approach. I set 21L frequencys in both sides and gave the first officer the new mins for the approach. Again the first officer flew the approach automatic mode in which the instruments were telling him to make more of an intercept. I was performing checklist and checking instruments. We again drifted left of course. At this time I instructed the first officer to make a go around before the situation deteriorated any further and let's find out why our instruments are lying to us. We asked approach to set us up on 21L ILS and tell us when we were on the localizer. At this point we determined that the automatic approach mode was defective so we proceeded with a manual approach. I wrote the system up on log. Thanks to approach and tower for monitoring and having the flexibility to assist in giving us an alternate approach. At no time did a flight safety problem exist since tower advised us of no traffic conflict. All company procedures of briefing, tuning and identing were followed. FAA guidelines for discontinuing an approach when deviations have occurred were followed. Supplemental information from acn 198096: I think the biggest problem was my reliance on the flight director V bars which caused me to deviate so far from the desired course. Had I attempted to complete the first approach we could have had a serious accident. On the second approach the captain helped me with the intercept and after disregarding the V bars I flew the approach using raw data to an uneventful landing.

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Original NASA ASRS Text

Title: ACR HAS MALFUNCTION OF FLT DIRECTOR, FORCING GAR.

Narrative: FO FLYING, ALL ACFT SYS APPEARED NORMAL, DTW LNDG 21R. WE RECEIVED A 200 DEG HDG TO INTERCEPT 21R ILS AND CLRED FOR THE APCH. DSNDING TO 3000 FT, APPROX 5-6 MI OUTSIDE MARKER. BOTH FLT DIRECTORS SET TO APCH AUTO MODES AND RADIOS TO 110.7 (215 DEGS). IT APPEARED THAT WE WERE SET UP FOR A NORMAL INTERCEPT, BUT THE FLT DIRECTORS WERE CALLING FOR MORE INTERCEPT TO THE L. WE INTERCEPTED 21R BUT THE FLT DIRECTOR STILL CALLED FOR MORE INTERCEPT. THE FO FLEW ACCORDING TO WHAT THE INSTS WERE TELLING HIM TO DO THINKING THE COMPUTER MUST KNOW MORE ABOUT THE WIND THAN WE DO. WE DRIFTED TOWARDS 21L ILS WHEN THE TWR ASKED US IF WE HAD THE RIGHT FREQS SET IN. I QUICKLY CHKED, WE DID. I ASKED IF THERE WAS A TFC CONFLICT AND COULD WE HAVE 21L ILS. NO CONFLICT AND TWR CLRED US FOR 21L ILS APCH. I SET 21L FREQS IN BOTH SIDES AND GAVE THE FO THE NEW MINS FOR THE APCH. AGAIN THE FO FLEW THE APCH AUTO MODE IN WHICH THE INSTS WERE TELLING HIM TO MAKE MORE OF AN INTERCEPT. I WAS PERFORMING CHKLIST AND CHKING INSTS. WE AGAIN DRIFTED L OF COURSE. AT THIS TIME I INSTRUCTED THE FO TO MAKE A GAR BEFORE THE SITUATION DETERIORATED ANY FURTHER AND LET'S FIND OUT WHY OUR INSTS ARE LYING TO US. WE ASKED APCH TO SET US UP ON 21L ILS AND TELL US WHEN WE WERE ON THE LOC. AT THIS POINT WE DETERMINED THAT THE AUTO APCH MODE WAS DEFECTIVE SO WE PROCEEDED WITH A MANUAL APCH. I WROTE THE SYS UP ON LOG. THANKS TO APCH AND TWR FOR MONITORING AND HAVING THE FLEXIBILITY TO ASSIST IN GIVING US AN ALTERNATE APCH. AT NO TIME DID A FLT SAFETY PROBLEM EXIST SINCE TWR ADVISED US OF NO TFC CONFLICT. ALL COMPANY PROCS OF BRIEFING, TUNING AND IDENTING WERE FOLLOWED. FAA GUIDELINES FOR DISCONTINUING AN APCH WHEN DEVS HAVE OCCURRED WERE FOLLOWED. SUPPLEMENTAL INFO FROM ACN 198096: I THINK THE BIGGEST PROBLEM WAS MY RELIANCE ON THE FLT DIRECTOR V BARS WHICH CAUSED ME TO DEVIATE SO FAR FROM THE DESIRED COURSE. HAD I ATTEMPTED TO COMPLETE THE FIRST APCH WE COULD HAVE HAD A SERIOUS ACCIDENT. ON THE SECOND APCH THE CAPT HELPED ME WITH THE INTERCEPT AND AFTER DISREGARDING THE V BARS I FLEW THE APCH USING RAW DATA TO AN UNEVENTFUL LNDG.

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.