WHILE FLYING A VISUAL APCH INTO SFO; A B757 ENCOUNTERS WAKE TURB FROM A PRECEDING B747 ON THE ADJACENT RWY. THIS CAUSES THE B757 TO EXECUTE A GAR.

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XA15Z; SFO TWR; 300 FT. WHILE MAKING PARALLEL VISUAL APCHS TO RWY 28R/L AT SFO; WE MADE AN FMS VISUAL APCH TO RWY 28L. PARALLEL TO US AND ABOUT 1/2 MI IN FRONT OF US AND GOING TO RWY 28L WAS A B747-400. WE HAD A L QUARTERING HEADWIND OF ABOUT 14 KTS AT 1000 FT. AT ABOUT 300 FT WE ENCOUNTERED TURB AND THE ACFT STARTED TO ROLL. THE FO HAD TO USE ABOUT 1/2 OF THE AVAILABLE AILERON TO COUNTERACT. AT ABOUT 150 FT THE ROLL GOT WORSE AND THE TURB INCREASED SO WE DECIDED TO MISS THE APCH. THIS APCH POSES A GREAT RISK FOR WAKE TURB ENCOUNTERS EVEN IF ATC USES THE PROPER CRITERIA FOR SPACING. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: PLT PARTICIPATED IN THE WAKE TURB STRUCTURED CALLBACK STUDY. PLT WAS UNSURE WHAT MODEL OF B747 WAS FLYING UPWIND FROM HIM. HE WAS FLYING ON THE GS WHEN THE WAKE STARTED TO AFFECT HIM. WHEN THE ACFT ROLLED INTO A 40 DEG BANK AT 300 FT AGL; THE FLC RIGHTED THE ACFT AND PERFORMED A GAR. THE PLT STATED THAT THE CAUSE OF THE ENCOUNTER WAS THE CLOSE PROX OF THE RWYS AT SFO; AND THE TRAILING DISTANCE BEHIND THE B747 WAS ONLY 1/2 - 1 MI. IF WE WERE TO PERFORM THE SAME APCH AGAIN; HE WOULD FLY ABOVE THE GS IN ORDER TO AVOID ANY POSSIBLE WAKE TURB.