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.

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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.