B727 EXPERIENCES WAKE ENCOUNTER TURB BEHIND A B757 ON FINAL APCH AT ORD. WAKE TURB SEPARATION CRITERION VIOLATED.

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FLT XXX SCHEDULED CVG-ORD. VECTORED FROM KOKOMO 1 ARR SE OF ORD VOR TO DOWNWIND. INSTRUCTED TO SLOW TO 180 KTS DOWNWIND. VECTORED TO FINAL AT 180 KTS. TCASII INDICATED TFC AHEAD THAT WE WERE FOLLOWING WAS 3.5 MI AHEAD AS WE INTERCEPTED FINAL 10 MI OUT AT ABOUT 3000 FT AGL. WE WERE NEVER INFORMED OF THE TYPE OF ACFT THAT WE WERE FOLLOWING UNTIL AFTER APCH CTL SLOWED US TO '170 KTS AS LONG AS FEASIBLE;' AND WE ENCOUNTERED MODERATE WAKE TURB. I SAID TO THE CREW; 'I HOPE THAT'S NOT A 757.' THE FO THEN ASKED THE TWR (LCL -- WE HAD BEEN TURNED OVER TO TWR JUST PRIOR TO THE WAKE ENCOUNTER) THE TYPE OF ACFT WE WERE FOLLOWING; WE WERE TOLD '757.' I IMMEDIATELY PUT OUR ACFT 1/2 - 1 DOT ABOVE THE GS; SLOWED AS MUCH AS POSSIBLE TO VREF PLUS 10 WHICH WAS 142 KTS AND CONTINUED. AT APPROX 150 FT AGL WE ENCOUNTERED CONSIDERABLE WAKE CONTINUOUSLY TO TOUCHDOWN WITH WING ROCKING OF PLUS OR MINUS 10 DEGS; AND RAPID AND CONTINUOUS CTL INPUTS REQUIRED TO MAINTAIN FLT PATH. WIND AT THE TIME WAS 140 DEGS AT 5 KTS; AND THE ATMOSPHERE WAS DEAD CALM. HAD I BEEN INFORMED; AS REQUIRED BY 7110.65 THAT WE WERE FOLLOWING A 757; I WOULD HAVE ASKED FOR AT LEAST 5 MI; AND PREFERABLY 6 MI SEPARATION; ESPECIALLY WITH THOSE ATMOSPHERIC CONDITIONS. AFTER BLOCKING IN; I ASKED TO SPEAK WITH THE TWR SUPVR; AND WE SPOKE ON 121.15 AND I SUGGESTED SEVERAL THINGS FOR HIM TO CONSIDER; IRRESPECTIVE OF WHAT 7110.65 SAYS. HE WAS MOST RECEPTIVE TO MY COMMENTS. I WOULD URGE NASA TO REEMPHASIZE THE HAZARDS OF 757 WAKE ENCOUNTERS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR COMPLAINED OF THE STANDARD PRACTICE OF ORD APCH CTL TO COMPRESS TFC; PLANNING A '2.5' MI SEPARATION IN-TRAIL ON FINAL IN HIS B727. THIS 'DOESN'T WORK WITHIN IN-TRAIL BEHIND THE B757.' THERE HAD BEEN NO ADVISORY AS TO BEING BEHIND A 757 ON THIS APCH. THE DISTANCE (NOTED ON TCASII) DECREASED FROM 4 MI TO 3.5 MI DURING THIS PARTICULAR APCH; WITH AN INSTRUCTION TO 'KEEP YOUR SPD UP.' THEIR CONCERN MOUNTED AS TO ACFT TYPE THEY WERE FOLLOWING ON ACCOUNT OF THE WAKE VORTEX PROB THEY STARTED TO ENCOUNTER. THE FO ASKED THE TWR FOR AN ACFT IDENT ON PRECEDING ACFT AND THAT WAS THE FIRST THEY KNEW OF IT BEING A 757. BY THIS TIME THEY WERE FAIRLY CLOSE IN TO THE RWY. CAPT ADDED SPD AND LANDED LONG; ENCOUNTERING ROLLS TO 5-10 DEGS NEAR THE GND. CAPT SPOKE WITH ATC SUPVR LATER AND REMARKED THAT A 5 MI FINAL WOULD BE APPRECIATED BEHIND THESE 757'S; PARTICULARLY WHEN THERE IS A CLAM WIND SIT AS HE HAD IN THIS INCIDENT. HE ASKED THE SUPVR TO 'TALK WITH APCH CTL REGARDING THIS.' HE ALSO ADVOCATES THE FLC GETTING INTO A DETAILED CREW BRIEFING PRIOR TO TKOF REGARDING ANY WAKE VORTEX ENCOUNTER PROCS; FOR TKOF OR LNDG EVENTS. HE FINDS THAT VERY FEW CREWS DO THIS (AS RPTED BY HIS FO).