WAKE TURB -- 757 VERSES DC-9.

Narrative

APCHING DTW FROM NE ON RADAR VECTORS TO ILS RWY 21L (VMC); WE WERE FLYING AN 80000 LB; DC-9-30; BEHIND A B757 (WT UNKNOWN). ATC WAS PROVIDING SEPARATION; WE COULD SEE DTW; BUT WE COULD NOT SEE B757. ASSIGNED SPD 170 KIAS AND SLOWING TO NEWLY ASSIGNED SPD 150 KTS. APCH CTL GAVE US VECTORS TO INTERCEPT ILS RWY 21L (250 DEGS HDG?) THEN SAID SPACING WAS TOO CLOSE. THEY ASSIGNED US 290 DEG HDG TO SQUARE CORNER AND INCREASE SPACING. SUBSEQUENTLY WE WERE ASSIGNED HDG 250 DEGS TO INTERCEPT ILS RWY 21L; 4 MI BEHIND B757 (WE COULD NOT SEE THIS ACFT; ATC WAS PROVIDING SEPARATION). I BELIEVE WE WERE STILL IN A SLIGHT L TURN (15 DEGS BANK OR LESS) AT 3700 FT MSL; WHEN WE ROLLED INTO AN UNCTLED L BANK OF APPROX 45 DEGS. BOTH PLTS ATTEMPTED TO ROLL THE ACFT BACK TO WINGS LEVEL; ACFT WAS MOSTLY UNRESPONSIVE; BUT ROLLED BACK TO APPROX 25 DEGS L BANK BEFORE IT ROLLED L AGAIN TO APPROX 35 DEGS L BANK. WE THEN RECOVERED TO WINGS LEVEL; STOPPED DSCNT; AND TURNED L AWAY FROM ILS RWY 21L. UPON INFORMING ATC WE COULD NOT FOLLOW B757; WE WERE ASSIGNED 180 DEG HDG FOR 45 SECONDS TO 1 MIN AND THEN TURNED BACK TO ILS RWY 21L. ATC SAID WE HAD 4 MI SEPARATION BEHIND B757. ROLL RATES WERE NOT VIOLENT; OUR RESPONSE WAS PRETTY FAST. CONCLUSION: 1) THIS IS NOT A COMFORTABLE FEELING. 2) 4 MI SEPARATION BEHIND B757 IS NOT ENOUGH FOR DC-9. GREATER SEPARATION IS DEFINITELY NEEDED (5 MI). 3) HAND FLYING ACFT IS THE BEST WAY TO IDENT WAKE TURB. I BELIEVE WE RESPONDED FASTER DUE TO FACT COPLT WAS HAND FLYING ACFT. 4) TREAT B757 SAME AS B747 (I WILL). 5) RIGHT COMBINATION OF EVENTS COULD HURT SOMEONE: AUTOPLT ON (SLOW RECOVERY RESPONSE); ADVANCING THROTTLES FROM IDLE (WE WERE AT IDLE) AND HAVING WRONG ENG SPOOL UP FIRST; OVER YOU GO -- OOPS. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: THE RPTR EMPHASIZED THAT THE MINIMUM DISTANCE BEHIND A 757 FOR A NON HVY ACFT SHOULD BE 5 MI. HE ALSO FELT THAT THE FLC HAD AN EARLIER DETECTION TIME SINCE THEY WERE 'HAND FLYING' AT THE TIME OF THE WAKE TURB ONSET. HE FELT THAT IF THEY HAD BEEN 'ON AUTOPLT' THEY WOULD HAVE DELAYED THEIR RESPONSE WHILE WAITING FOR THE AUTOPLT TO REACT.