A DC9-10 ON SHORT FINAL HAS TO EXECUTE A GAR WHEN ENCOUNTERING WAKE TURB FROM THE PRECEDING LNDG B747 ON RWY 21R AT DTW.

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POSITIONED 6 MI BEHIND B747-400. WIND; DIRECT XWIND AT 5 KTS. BASED ON RPTED WIND; ELECTED TO FLY THE VISUAL APCH 1 DOT HIGH ON THE GS. BRIEFED THIS AND A TOUCHDOWN BEYOND B747 TOUCHDOWN. GOT GOOD VISUAL ON B747 TIRE SMOKE AND HAD AIM POINT APPROX 1500 FT DOWN THE 1200 FT RWY. AT 200 FT AGL; ENCOUNTERED VIOLENT ROLL; WHICH REQUIRED ALMOST FULL AILERON TO CTL; FOLLOWED IMMEDIATELY BY EQUALLY VIOLENT ROLL IN OPPOSITE DIRECTION. ELECTED TO DISCONTINUE LNDG. EXECUTED MISSED APCH AND RETURNED FOR NORMAL LNDG. SEPARATION AND EXTRA CARE WON'T ALWAYS AVOID THE TURB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CAPT STATED THAT THERE MAY HAVE BEEN A QUARTERING TAILWIND. THAT; HE THOUGHT; MIGHT HAVE RAISED THE WAKE TURB UP INTO THE ACFT'S APCH PATH INSTEAD OF SIMPLY DISSIPATING. HE HAS FOLLOWED B747-400'S BEFORE AND ALWAYS EXERCISED DUE CAUTION. THIS INCIDENT WAS MORE OF A RANDOM EVENT; PERHAPS DUE TO THE SURFACE WIND. SUPPLEMENTAL INFO FROM ACN 416859: TWR RPTS 5 MI IN TRAIL OF B747 AFTER WE ASKED WHEN BEING HANDED OFF FROM APCH. TCASII SHOWS 6+ MI IN TRAIL AS B747 IS LNDG. WIND WAS 290 DEGS AT 4 KTS. AT ABOUT 150 FT; ACFT ROLLED ABOUT 20 DEGS ONE DIRECTION; THEN OPPOSITE. SUSPECT UPWIND VORTEX DRIFTED INTO LNDG AREA.