Narrative:

I was cleared for the ILS runway 21L approach and proceeded to turn onto the localizer. After I thought I was established, ATC asked me if I was, and I answered accordingly. ATC then advised me that I had gone through the localizer and told me to go around. As soon as I was told this, I noticed that I had 'flags' on both instruments left and right. At this point, I should have told them about this condition, but I did not. I consequently came back and shot the ILS to runway 21L with no further problems. Upon reaching the ground, dtw ground control advised me to call TRACON, which I did upon reaching the FBO. This flight originated in ttf and due to the short time of the trip, I set up all radios for the approach for runway 21L (as I was given a clearance on the phone prior to departure). Before initiating the approach, I verified and idented on both radios the ILS frequency and both checked good. I think what happened was on the final stages of the approach the flags must have come up right before I intercepted the localizer, which would cause the needle to center, hence I thought I was established and in reality I was right of course. The total event must have taken approximately 30 seconds. My belief is that an aircraft was beyond the ILS hold short line causing deviation in instrumentation.

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Original NASA ASRS Text

Title: AN ATX SMT PLT FLIES THROUGH THE LOC COURSE OF RWY 21L AT DTW. APCH CTLR MAKES HIM PERFORM A MISSED APCH. POSSIBLE TFC CONFLICT WITH ACFT ON ILS FOR RWY 21R.

Narrative: I WAS CLRED FOR THE ILS RWY 21L APCH AND PROCEEDED TO TURN ONTO THE LOC. AFTER I THOUGHT I WAS ESTABLISHED, ATC ASKED ME IF I WAS, AND I ANSWERED ACCORDINGLY. ATC THEN ADVISED ME THAT I HAD GONE THROUGH THE LOC AND TOLD ME TO GO AROUND. AS SOON AS I WAS TOLD THIS, I NOTICED THAT I HAD 'FLAGS' ON BOTH INSTS L AND R. AT THIS POINT, I SHOULD HAVE TOLD THEM ABOUT THIS CONDITION, BUT I DID NOT. I CONSEQUENTLY CAME BACK AND SHOT THE ILS TO RWY 21L WITH NO FURTHER PROBS. UPON REACHING THE GND, DTW GND CTL ADVISED ME TO CALL TRACON, WHICH I DID UPON REACHING THE FBO. THIS FLT ORIGINATED IN TTF AND DUE TO THE SHORT TIME OF THE TRIP, I SET UP ALL RADIOS FOR THE APCH FOR RWY 21L (AS I WAS GIVEN A CLRNC ON THE PHONE PRIOR TO DEP). BEFORE INITIATING THE APCH, I VERIFIED AND IDENTED ON BOTH RADIOS THE ILS FREQ AND BOTH CHKED GOOD. I THINK WHAT HAPPENED WAS ON THE FINAL STAGES OF THE APCH THE FLAGS MUST HAVE COME UP RIGHT BEFORE I INTERCEPTED THE LOC, WHICH WOULD CAUSE THE NEEDLE TO CTR, HENCE I THOUGHT I WAS ESTABLISHED AND IN REALITY I WAS R OF COURSE. THE TOTAL EVENT MUST HAVE TAKEN APPROX 30 SECONDS. MY BELIEF IS THAT AN ACFT WAS BEYOND THE ILS HOLD SHORT LINE CAUSING DEV IN INSTRUMENTATION.

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.