A B737-300 ON APCH TO LAX ENCOUNTERS WAKE TURB FROM A B757 ON A PARALLEL RWY.

Narrative

LAX APCH CTL ASSIGNED ANOTHER CARRIER B757 AHEAD TO RWY 24L AND ASSIGNED US TO RWY 24R. THE B757 WAS APPROX 7 MI AHEAD; HOWEVER; WINDS WERE LIGHT QUARTERING TAILWIND FROM L TO R. THIS COMBINATION OF RELATIVE POS OF ACFT AND WINDS CAUSED WAKE VORTICES FROM B757 TO DRIFT FROM THEIR FLT PATH TO OURS (L TO R XWIND) AND TO DISSIPATE LESS RAPIDLY THAN NORMAL (LIGHT TAILWIND). WE ENCOUNTERED THE B757'S WAKE AT APPROX 9 MI WITH A SIGNIFICANT ROLL TO THE R (ABOUT 30+ DEGS). ACFT CTL WAS IMMEDIATELY REGAINED AND WE RESUMED APCH ALONG A HIGHER FLT PATH WITH SOME ENSUING MINOR ENCOUNTERS WITH THE B757 WAKE. LISTENING TO ATC ASSIGNMENTS TO OTHER ACFT PRIOR TO THE ENCOUNTER AND ALL THE WAY THROUGH LNDG ROLLOUT GAVE ME THE IMPRESSION THAT LCL ATC AT THAT TIME WAS ROUTINELY ASSIGNING HVY ACFT TO RWY 24L AND NON-HVY LNDG TFC TO RWY 24R. I QUERIED THE TWR CTLR ABOUT THE PRACTICE AND HE SUGGESTED THAT I WRITE A RPT SINCE IT WAS NOT UP TO HIM HOW THE ACFT WERE ASSIGNED TO THE RWYS. I FEEL THAT SAFETY AND PAX COMFORT AND PEACE OF MIND COULD ALL BE ENHANCED IF ATC MIGHT BE MORE AWARE OF WIND EFFECTS ON WAKE TURB AND TAKE THIS INTO ACCOUNT WHEN MAKING RWY ASSIGNMENTS WITH HVY ACFT. WHILE I UNDERSTAND THAT THE LAX OP THAT DAY WAS ABNORMAL (RWY 25L WAS SHUT DOWN) SENDING THE HEAVIES TO RWY 24R WOULD HAVE LIKELY PREVENTED MY WAKE ENCOUNTER AND POSSIBLY OTHERS THAT WENT UNRPTED THAT EVENING. WHEN OPERATING HVY ACFT IN LATERAL PROX TO OTHER ACFT (CLOSE PARALLEL OPS) DURING APCH AND LNDG; ATC SHOULD EVALUATE THE RPTED WIND CONDITIONS ON SOME REASONABLE INTERVAL. IF WINDS CREATE AN INCREASED RISK OF WAKE ENCOUNTERS (LIGHT QUARTERING TAILWINDS); ATC SHOULD TAKE STEPS TO REDUCE THIS RISK (CHANGE RELATIVE POS; INCREASE SEPARATION LATERALLY; INCREASE FLT PATH SEPARATION).