Narrative:

Air carrier X vectored on downwind at 180 K, cleared to 4000 ft by approach control. In the descent we were directed to expedite through 5000 ft for traffic crossing left to right at 11:30, 6000 ft, a B-757. We called the traffic insight as we verified altitude and position on the TCASII approach directed us to maintain visual separation and advised we would follow the 757 to runway 26R. The 757 was past our 12:00 position and on a dog-leg to the final approach course as we were vectored to heading 180 and slowed to 170 KTS. The preceding 757 showed 2 mi from our aircraft when approach turned us right to 220 and cleared us for the visual approach. I immediately advised approach that we were too close and asked for traffic behind us so that we could slow down. Approach did not comment on traffic but said we could turn to 200 degree for intercept and slow to 160 KTS. We crossed the localizer heading 200 degree at 160 KTS but were forced to correct back to course due to parallel traffic on the visual approach to runway 27L. We had started descent, holding about 1 1/2 dots high on the GS intentionally, when we encountered the 757's wake turbulence. The first officer, who was flying the aircraft, responded to the brief encounter with timely roll control inputs, added power, and lowered the nose to maintain airspeed and recover to stable flight. Maximum uncommanded bank was 15 to 20 degree and airspeed loss was noted but its extent was not noted. We continued straight ahead under tower direction and were returned to approach control for vectors in the pattern to an uneventful landing. During this approach, we were close to 1 3/4 mi to the preceding traffic (during the go around) and never spaced more than 2 1/2 mi. This separation was inadequate for following commuter traffic and totally unacceptable behind a B757 with its special wake turbulence hazard. I feel that the controller set up a situation that produced inadequate separation in his desire to keep traffic moving efficiently. We realized as the situation developed that we were technically responsible for separation once we called the traffic in sight on downwind but were boxed in by the constraints of following assigned vectors and speeds for separation with other traffic, frequency congestion that prevented timely communication with controllers, and not knowing the separation `game plan' until being turned to final where we exercise our only available option (the go around). The TCASII system was invaluable in providing specific separation distance measurements to us to supplement the uncalibrated eye-ball. Even with the numerous targets presented during high density parallel approachs we were able to see the situation developing and were ready when safe separation was lost. Only the wake turbulence from the 757 descending ahead of us on what later became our flight path proved unpredictable though not unexpected. My feeling is that a request for 4 to 5 mi behind a B757 is in order on all approachs and departures and should be mandatory.

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Original NASA ASRS Text

Title: ACR X LOS OF ACFT CTL FROM B757 WAKE TURB WHILE ON VISUAL APCH.

Narrative: ACR X VECTORED ON DOWNWIND AT 180 K, CLRED TO 4000 FT BY APCH CTL. IN THE DSCNT WE WERE DIRECTED TO EXPEDITE THROUGH 5000 FT FOR TFC XING L TO R AT 11:30, 6000 FT, A B-757. WE CALLED THE TFC INSIGHT AS WE VERIFIED ALT AND POS ON THE TCASII APCH DIRECTED US TO MAINTAIN VISUAL SEPARATION AND ADVISED WE WOULD FOLLOW THE 757 TO RWY 26R. THE 757 WAS PAST OUR 12:00 POS AND ON A DOG-LEG TO THE FINAL APCH COURSE AS WE WERE VECTORED TO HDG 180 AND SLOWED TO 170 KTS. THE PRECEDING 757 SHOWED 2 MI FROM OUR ACFT WHEN APCH TURNED US R TO 220 AND CLRED US FOR THE VISUAL APCH. I IMMEDIATELY ADVISED APCH THAT WE WERE TOO CLOSE AND ASKED FOR TFC BEHIND US SO THAT WE COULD SLOW DOWN. APCH DID NOT COMMENT ON TFC BUT SAID WE COULD TURN TO 200 DEG FOR INTERCEPT AND SLOW TO 160 KTS. WE CROSSED THE LOC HDG 200 DEG AT 160 KTS BUT WERE FORCED TO CORRECT BACK TO COURSE DUE TO PARALLEL TFC ON THE VISUAL APCH TO RWY 27L. WE HAD STARTED DSCNT, HOLDING ABOUT 1 1/2 DOTS HIGH ON THE GS INTENTIONALLY, WHEN WE ENCOUNTERED THE 757'S WAKE TURB. THE FO, WHO WAS FLYING THE ACFT, RESPONDED TO THE BRIEF ENCOUNTER WITH TIMELY ROLL CTL INPUTS, ADDED PWR, AND LOWERED THE NOSE TO MAINTAIN AIRSPD AND RECOVER TO STABLE FLT. MAX UNCOMMANDED BANK WAS 15 TO 20 DEG AND AIRSPD LOSS WAS NOTED BUT ITS EXTENT WAS NOT NOTED. WE CONTINUED STRAIGHT AHEAD UNDER TWR DIRECTION AND WERE RETURNED TO APCH CTL FOR VECTORS IN THE PATTERN TO AN UNEVENTFUL LNDG. DURING THIS APCH, WE WERE CLOSE TO 1 3/4 MI TO THE PRECEDING TFC (DURING THE GAR) AND NEVER SPACED MORE THAN 2 1/2 MI. THIS SEPARATION WAS INADEQUATE FOR FOLLOWING COMMUTER TFC AND TOTALLY UNACCEPTABLE BEHIND A B757 WITH ITS SPECIAL WAKE TURB HAZARD. I FEEL THAT THE CTLR SET UP A SIT THAT PRODUCED INADEQUATE SEPARATION IN HIS DESIRE TO KEEP TFC MOVING EFFICIENTLY. WE REALIZED AS THE SIT DEVELOPED THAT WE WERE TECHNICALLY RESPONSIBLE FOR SEPARATION ONCE WE CALLED THE TFC IN SIGHT ON DOWNWIND BUT WERE BOXED IN BY THE CONSTRAINTS OF FOLLOWING ASSIGNED VECTORS AND SPDS FOR SEPARATION WITH OTHER TFC, FREQ CONGESTION THAT PREVENTED TIMELY COM WITH CTLRS, AND NOT KNOWING THE SEPARATION `GAME PLAN' UNTIL BEING TURNED TO FINAL WHERE WE EXERCISE OUR ONLY AVAILABLE OPTION (THE GAR). THE TCASII SYS WAS INVALUABLE IN PROVIDING SPECIFIC SEPARATION DISTANCE MEASUREMENTS TO US TO SUPPLEMENT THE UNCALIBRATED EYE-BALL. EVEN WITH THE NUMEROUS TARGETS PRESENTED DURING HIGH DENSITY PARALLEL APCHS WE WERE ABLE TO SEE THE SIT DEVELOPING AND WERE READY WHEN SAFE SEPARATION WAS LOST. ONLY THE WAKE TURB FROM THE 757 DSNDING AHEAD OF US ON WHAT LATER BECAME OUR FLT PATH PROVED UNPREDICTABLE THOUGH NOT UNEXPECTED. MY FEELING IS THAT A REQUEST FOR 4 TO 5 MI BEHIND A B757 IS IN ORDER ON ALL APCHS AND DEPS AND SHOULD BE MANDATORY.

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.