MD S80 ENCOUNTERS BOEING 747 WAKE AT 7 MI IN TRAIL AND 170 KTS.

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WE WERE CLRED FOR A VISUAL TO RWY 17L AND WERE 7 MI BEHIND A 747. AT APPROX 3500 FT WE EXPERIENCED MODERATE TURB AND WERE FORCED TO CLB 200 FT TO GET ABOVE HIS WAKE. ONCE CLR OF THE WAKE THE REST OF THE APCH WAS NORMAL. WE REQUESTED AND GOT RWY 17R WHICH ALSO HELPED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR WAS SURPRISED AT HOW FAR IN TRAIL THE WAKE FROM A 747 COULD LINGER. THIS ANALYST FEELS THE WAKE ENERGY DETERIORATES AS A FUNCTION OF TIME AND NOT DISTANCE. THE RPTR STATED HE HAD FLAPS 15 DEGS AND GEAR UP. THE DISTANCE 7 MI BEHIND THE 747 WAS OBSERVED ON THE TCASII AND CONFIRMED BY ATC. THE RPTR WAS THE PNF. HE SAID HE WAS IN THE WAKE ABOUT 6 SECONDS AND ENCOUNTERED A L ROLL OF ABOUT 20-30 DEGS BACK TO WINGS LEVEL AND A R ROLL 20- 30 DEGS AND BACK TO LEVEL AND THEN THE WHOLE SEQUENCE REPEATED BEFORE CLBING ABOVE AND ESCAPING THE WAKE. THE RPTR FELT HE WAS PITCHED ABOUT 100 OF THE 200 FT NEEDED TO ESCAPE THE WAKE. HE SAID HE EXPERIENCED LESS THAN 1 NEGATIVE 'G' AND MORE THAN 1 POSITIVE 'G' DURING THE ENCOUNTER.