WAKE TURB ENCOUNTER.

Narrative

I WAS CLRED FOR AN ILS RWY 28R AT PIT BEHIND A B757. CTLR GAVE WARNING ABOUT WAKE TURB BEHIND 757. I WAS 4 MI IN- TRAIL ACCORDING TO TCASII; BUT THE 757 WAS ABOUT 1500 FT ABOVE US. FO SLOWED TO 170 KIAS TO MAINTAIN 4 MI SEPARATION; BUT 757 MUST HAVE SLOWED SUBSTANTIALLY. CTLR CANCELED OUR APCH CLRNC AND TURNED US L TO A 250 DEG HDG; THEN BACK TO A 300 DEG HDG FOR ADDITIONAL SPACING. WE THEN ENCOUNTERED A FAIRLY STRONG ROLL RATE AND TURB FROM THE 757. THIS WAS BECAUSE 757 WAS GIVEN A MUCH HIGHER GS INTERCEPT THAN WE WERE. 4 MI SEPARATION IS OK ONLY IF ACFT FLY THE SAME DSCNT PATH. I ALSO HIGHLY RECOMMEND 4 MI SEPARATION FROM 757'S ON DEP. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE PRIMARY CAUSE OF THE WAKE ENCOUNTER WAS THE BOEING 757 WAS HIGH ON PROFILE. IN ORDER TO GET DOWN THE 757 SLOWED UP TO EXTEND ALL HIS FLAPS CAUSING A LOSS IN SPACING. TO CORRECT THE SPACING PROB THE CTLR VECTORED AN S-TURN WHICH WAS S OR L OF HIS COURSER. THE WIND WAS VERY LIGHT BUT WAS FROM THE NW AND BLOWING THE WAKE TO THE L OF COURSE. THE RPTR FEELS IF THE CTLR HAD GIVEN VECTORS TO THE R OR N OF COURSE THE WAKE ENCOUNTER WOULD NOT HAVE BEEN AS BAD OR MAYBE NOT HAVE HAPPENED AT ALL. THE SECONDARY CAUSE; THE RPTR STATED; WAS THE 4 MI SEPARATION IN-TRAIL WILL STILL CAUSE CTL PROBS IF THE 757 WAKE IS ENCOUNTERED. HE BELIEVES THE SEPARATION CRITERION FOR IN- TRAIL OF 757 ACFT SHOULD BE 5 MI ON DEPS AND APCHS THE SAME AS FOR THE HVY ACFT.