FLC ENCOUNTERS BOEING B757 WAKE TURB ARRIVING JFK.

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WERE OPERATING FLT INTO JFK; WERE SEQUENCED ON JFK VOR RWY 13L BEHIND UNKNOWN ACFT WHICH WE COULD SEE BUT NOT WELL ENOUGH TO MAKE OUT TYPE -- TCASII SHOWED TFC ABOUT 4-5 MI AHEAD -- ATC TOLD US TO KEEP THE SPD UP; CLRED US FOR THE APCH AND TURNED US OVER TO TWR. AFTER PASSING CANARSY VOR; STARTED DSCNT TO MDA. AT APPROX 2 DME NE OFF CANARSY; NOTICED CAPT BECOMING UNEASY. HE THEN MADE COMMENT ABOUT THE PRECEDING ACFT'S WAKE AND (ON CUE) WE STARTED ENCOUNTERING WAKE TURB; SHORT; CHOPPY BUMPS; L AND R ROLLS WHICH REQUIRED SOME POSITIVE CTL INPUTS TO KEEP (RETURNED WINGS LEVEL; PUSHED PWR UP AND CLBED OUT OF IT). ASKED TWR WHAT WE WERE FOLLOWING; TURNED OUT TO BE A B757. COMPLETED APCH UNEVENTFULLY AND ABOVE VASI. WE MUST HAVE INADVERTENTLY DSNDED INTO THE B757'S WAKE; BUT FLYING OUT OF IT WAS NOT A PROB. COMPARED NOTES WITH CAPT A WK LATER AND WE BOTH AGREED THAT WE WERE AMAZED AT HOW MUCH WAKE WE ENCOUNTERED CONSIDERING THE B757 WAS A GOOD 4 MI AHEAD OF US. LESSON LEARNED: FROM NOW ON; I WILL TREAT B757'S WITH THE SAME PRECAUTIONS AS ANY 'HVY.' CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: THE RPTR STATED THE ACFT WAS A DEHAVILAND DASH 8. THE RPTR WAS NOT ON AUTOPLT. THERE WAS GPWS BUT IT DID NOT ACTIVATE. THE TCASII INDICATED THE B757 WAS 4-5 MI AHEAD. THE WINGS ROLLED ABOUT 20 DEGS L AND R. THERE WAS ABOUT 7 SECONDS DURATION BEFORE MANAGING TO ESCAPE THE WAKE. IF THERE WERE ADVISORIES OF FOLLOWING A B757 AHEAD AND OR CAUTION WAKE TURB THE FLT PATH WOULD HAVE BEEN FLOWN HIGHER AND PERHAPS MORE OFFSET. THE RPTR WISHES THE CTLRS WOULD RIDE MORE IN THE FLT STATION OF AIRPLANES TO GAIN FIRST HAND EDUCATION ON THE VIOLENCE ASSOCIATED WITH WAKE ENCOUNTERS.