FLC OF MD87 ENCOUNTERS WAKE FROM B747-400 ON APCH WHEN THE B747 IS INSTRUCTED TO MAKE A GAR BY TWR FOR SPACING.

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ON APCH 20 MI OUT WE WERE TOLD TO SLOW; THAT WE WOULD BE FOLLOWING A HVY. WE COMPLIED; WERE DSNDED AND WERE TOLD TO CAPTURE THE LOC; THEN CLRED FOR THE APCH. WE WERE 6 MI IN TRAIL OF A HVY. WX CONDITIONS AT THE TIME WERE A BROKEN CEILING AT 800 FT; 6 MI. WE WERE SWITCHED TO TWR AND THEN CLRED TO LAND. WE COULD SEE THE PRECEDING TFC AT ALL TIMES; AND THE RWY INTERMITTENTLY. AT ABOUT 1500 FT; WE SAW THE HVY IN FRONT OF US START A GAR. WE HIT HIS WAKE AT ABOUT 1200 FT AGL AS WE WERE ENTERING THE CLOUD TOPS. WE LATER FOUND OUT THAT IT WAS A B747-400 AND IT ROLLED THE ACFT 20 DEGS IN EITHER DIRECTION AS WE PASSED THROUGH ITS WAKE. TWR NEVER SAID A WORD ABOUT THE PRECEDING TFC MAKING A MISS. AFTER WE LANDED; I CALLED TWR; TALKED TO A SUPVR WHO TOLD ME THAT THE B747 WAS UNABLE TO MAINTAIN THE SPACING REQUIRED ON THE B757 IN FRONT OF IT; AND WAS INSTRUCTED BY ATC TO GAR. WHEN I ASKED WHY WE WEREN'T TOLD OF THE GAR; THE SUPVR SAID THAT THE CTLRS WERE 'TOO BUSY' WITH THE SPACING ISSUE TO ISSUE ANY FURTHER ADVISORIES. I WOULD BET THAT ANY CTLR THAT HAS SAT IN A COCKPIT OF AN ACFT THAT HAS BEEN HIT WITH B747-400 WAKE TURB WOULD UNDERSTAND THE IMPORTANCE OF THESE ADVISORIES. THE AIM DEVOTES 7 PAGES TO WAKE TURB. NOWHERE IN THERE DOES IT REF THIS SIT. IF THERE IS ANY SIT GUARANTEED TO ESTABLISH A WAKE TURB ENCOUNTER; THIS WOULD BE IT; AND MAYBE SOME GUIDELINES COULD BE ESTABLISHED; OR THOUGHT GIVEN TO SOME. THE NEW 'HVY' TRANSPORTS HAVE PARTICULARLY NASTY WAKE; AND I DON'T SEE THAT IMPROVING ANY TIME SOON. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT HE FEELS ATC WAS REMISS IN NOT ADVISING HIM OF THE GAR OF THE B747. HE COULD HAVE AT LEAST BEEN AWARE OF THE CHANGE OF FLT PATH AND ADJUSTED HIS OWN APCH ACCORDINGLY. AT LEAST HE MIGHT HAVE BEEN PREPARED FOR THE RESULTING WAKE ACTIVITY. RPTR HAS REVIEWED THE AIM AND THERE IS NO REQUIREMENT TO ADVISE THE TRAILING ACFT IN SUCH A SIT. HE FEELS THERE SHOULD BE SUCH A REQUIREMENT. RPTR ALSO FEELS THE NEWER BOEING WINGS DEVELOP MUCH GREATER WAKE THAN PREVIOUS ONES.