RPTR ENCOUNTERS BOEING 757 WAKE TURB.

Narrative

AT APPROX 30 NM S OF DCA WE ENCOUNTERED WHAT I BELIEVE WAS AN UPSET CAUSED BY WAKE TURB BEHIND A B-757 THAT WAS; ACCORDING TO ATC AND TCASII ABOUT 8-9 MI AHEAD OF US AND 2100 FT BELOW US. THE ACFT WAS ALSO FLYING THE 'IRONS' ARR INTO DCA. WHEN WE HIT THEIR WAKE OUR AUTOPLT KICKED OFF AND THE ACFT EXPERIENCED SEVERAL VIOLENT ROLLS BOTH L AND R 40 DEGS. THE FO WHO WAS FLYING; SAID HE DID NOT HAVE TO GO TO THE STOPS ON THE AILERONS BUT DID HAVE HIS HANDS FULL. IT REQUIRED AN EXPLANATION TO THE PAX ON THE PA. I ASKED THE CAPT OF THE B-757 LATER ON IF HE FLEW A NORMAL; IDLE; CLEAN; DSCNT. HE REPLIED 'YES' HIS WT WAS APPROX 180000 LBS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED HE WAS FLYING A 737-300. HE WAS 9 MI IN TRAIL OF A BOEING 757 AS DETERMINED BOTH BY TCASII AND CONFIRMED BY ATC. THE RPTR IN TALKING WITH THE 757 CAPT LEARNED THE 757 WAS ALSO DSNDING CLEAN AT IDLE THRUST. THE RPTR WANTED TO EMPHASIZE THE AIR WAS SMOOTH. THE RPTR SUGGESTED ATC ADOPT A MINIMUM OF 5 MI BEHIND A 757 FOR A NON-HVY IN TRAIL. THE MAJOR MOTION ASSOCIATED WITH THE WAKE ENCOUNTER WAS RAPID ROLL L AND R OF ABOUT 40 DEGS IN EACH DIRECTION. THE FO ORIENTED THE AIRPLANE AWAY FROM THE IN TRAIL PATH TO ESCAPE AND AVOID FUTURE ENCOUNTERS. THE RPTR ESTIMATED THE ENCOUNTER LASTED ABOUT 7-9 SECONDS. THE RPTR DID NOT GET ON TO THE CTLS WITH THE FO.