A B737 CREW ENCOUNTERS WAKE OF A PRECEDING B757 AND SLOWS WITHOUT CLEARANCE TO INCREASE SEPARATION DURING AN ARRIVAL TO LAX.

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WE WERE ABOUT 18-20 MILES FROM THE AIRPORT ON APPROACH TO RWY 25L FLYING AT ABOUT 240 KTS. THERE WAS AN AIRCRAFT IN SEQUENCE IN FRONT OF US ABOUT 7 MILES; VISUAL. WE WERE ON GLIDESLOPE; THE PLANE WE WERE FOLLOWING APPEARED ALSO TO BE ON GLIDESLOPE FROM TCAS AND VISUALLY. THE WINDS WERE CALM. APPROACH ASKED US IF WE HAD THE PLANE IN SIGHT. HE THEN SAID FOLLOW THE B757 HEAVY CLEARED VISUAL APPROACH. AT THIS TIME WE BEGAN TO GET INTO THE HEAVY'S WAKE AND WE WERE CLOSING AT 6 MILES. THE CAPTAIN BEGAN TO SLOW FROM 240 KTS AND RAISE THE NOSE TO GET HIGH ON GLIDESLOPE TO AVOID THE TURBULENCE. AT THIS POINT; APPROACH TOLD US TO MAINTAIN 250 KTS. WE WERE AT ABOUT 15-17 MILES FROM LANDING. THE CAPTAIN SAID TO ME WE CAN'T DO THAT. AT THIS TIME HE WAS AT ABOUT 220 KTS. STILL AT ABOUT 6 MILES FROM THE B757; APPROACH CALLED US NOW AND SAID; 'WHAT SPEED ARE YOU AT; COMPANY?' THE CAPTAIN RESPONDED TO HIM; 'WE CAN'T FLY THIS CLOSE TO THE HEAVY.' HE SAID; 'WHAT IS YOUR SPEED?' NOW WE WERE AT ABOUT 11-12 MILES FROM LANDING; HIGH; AT 210 KTS AND 5.8 MILES FROM THE HEAVY TRYING TO STILL AVOID HIS WAKE TURBULENCE. I STATED OUR SPEED TO HIM. HE SAID; 'YOU CAN'T SLOW AND NOT TELL ME ABOUT IT; THIS IS NOT ALLOWED! CALL ME FOR A POSSIBLE ATC SPEED DEVIATION.' THERE WAS A PAUSE; AND HE THEN SAID; 'SLOW TO 190 KTS AND CONTACT TOWER AT LIMMA.' UPON LANDING MY CAPTAIN CALLED THE APPROACH SUPERVISOR AND TOLD HIM WHAT HAPPENED. THE SUPERVISOR SAID APPROACH HAD US TOO CLOSE TO THE HEAVY AND NOT TO WORRY ABOUT GETTING VIOLATED. TWO THINGS DONE WOULD HAVE PREVENTED THIS SITUATION. FIRST; APPROACH SHOULD NOT HAVE HAD US APPROACHING 5 MILES BEHIND A HEAVY ON FINAL WITH CALM WINDS. THERE IS NO WAY THIS WILL WORK. SECOND; AS PILOT MONITORING; I SHOULD HAVE TOLD APPROACH; 'ENCOUNTERING WAKE FROM THE HEAVY; CLEARANCE UNABLE;' IMMEDIATELY WHEN HE TOLD US TO MAINTAIN 250 KTS BEHIND THE HEAVY.