Narrative:

I was cleared for an ILS runway 28R at pit behind a B757. Controller gave warning about wake turbulence behind 757. I was 4 mi in- trail according to TCASII, but the 757 was about 1500 ft above us. First officer slowed to 170 KIAS to maintain 4 mi separation, but 757 must have slowed substantially. Controller canceled our approach clearance and turned us left to a 250 degree heading, then back to a 300 degree heading for additional spacing. We then encountered a fairly strong roll rate and turbulence from the 757. This was because 757 was given a much higher GS intercept than we were. 4 mi separation is ok only if aircraft fly the same descent path. I also highly recommend 4 mi separation from 757's on departure. Callback conversation with reporter revealed the following information: the reporter 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 problem the controller vectored an s-turn which was south or left of his courser. The wind was very light but was from the northwest and blowing the wake to the left of course. The reporter feels if the controller had given vectors to the right or north of course the wake encounter would not have been as bad or maybe not have happened at all. The secondary cause, the reporter stated, was the 4 mi separation in-trail will still cause control problems if the 757 wake is encountered. He believes the separation criterion for in- trail of 757 aircraft should be 5 mi on departures and approachs the same as for the heavy aircraft.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.