FLT CREW OF AN F900 ENCOUNTERS SEVERE WAKE TURB IN TRAIL OF AN A320 AT SAME ALT 15 MI AHEAD.

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WHILE CRUISING AT FL200 ON FLT PLAN HTO TO SYR; FOLLOWING AN AIRBUS 320 AVERAGING 12 TO 15 MI DIRECTLY IN FRONT OF US ON SAME RTE TO SYR WE ENCOUNTERED THREE SEVERE TURB EVENTS. THE PREVAILING WIND AT FL200 WAS 36 TO 40 KTS EXACTLY ON OUR NOSE. FIRST 2 EVENTS; FOR 10 TO 15 SEC DURATION; WERE SEVERE BUT NOT STEADY. THE THIRD EVENT CAUSED AN UNCONTROLLABLE ROLL TO THE R FOR A FEW SECONDS; AFTER WHICH WE RECOVERED AND ASKED FOR IMMEDIATE CLRNC TO FL180 -- OUT OF THE PATH OF THE AIRBUS -- WHICH WE RECEIVED. WE NOTIFIED CTLR OF THIS AND I EXPLAINED TO ATC THAT THE WIND AT ALT (DIRECTION/VELOCITY) SHOULD HAVE BEEN NOTICE FOR MORE SEPARATION THAN 15 MI! CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED HE ENCOUNTERED TWO FAIRLY MILD ENCOUNTERS WITH THE WAKE AND THEN A THIRD HE DESCRIBED AS SEVERE. AT THAT POINT HE REQUESTED A LOWER ALT TO AVOID FURTHER ENCOUNTERS. HE STATED THE MOST NOTABLE DISPLACEMENT WAS IN THE ROLL AXIS OF APPROX 50 DEGS. ALTHOUGH THE AUTOPLT WAS ENGAGED AT THE TIME OF THE ENCOUNTER IT WAS NECESSARY TO DISENGAGE IT AND ARREST THE ADVERSE ROLL MANUALLY.