B767-300 ACFT ON ATLANTIC OCEANIC RTE AT FL340 WAS PASSED BY A B747 AT FL350. APPROX 30-40 MI IN TRAIL OF THE B747 RPTR ACFT EXPERIENCED WAKE TURB WHICH IMMEDIATELY PUT THE ACFT IN A 15 DEG R BANK. RPTR CAPT ALTERED COURSE TO THE R; THEN DSNDED 250 FT AND ALTERED BACK TO A L OFFSET DUE TO WIND DIRECTION.

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AT FL340 WE WERE PASSED BY A B747 AT FL350. WITH THE B747 SEVERAL MI AHEAD AND IN SMOOTH AIR; WE SUDDENLY ENCOUNTERED MODERATE TO SEVERE WINGTIP VORTICES/TURB. PART OF THE ENCOUNTER INVOLVED AN UNCTLED 15 DEG BANK TO THE R. THERE WERE NO INJURIES OR CABIN DAMAGE RPTED. THE REMAINDER OF THE FLT WAS SMOOTH AND UNEVENTFUL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR SAYS THAT THE WAKE TURB WAS ONLY MODERATE; BUT THE RPT WAS SUBMITTED BECAUSE OF THE LACK OF ACFT CTLABILITY. WHEN THE ACFT INITIALLY ROLLED INTO 15 DEG BANK TO THE R; RPTR CAPT LEFT IT ON AUTOPLT WHICH COULDN'T COUNTERACT THE FORCES FROM THE WAKE. AS THE ACFT TURNED TO THE R; THE AUTOPLT REGAINED WINGS LEVEL AND RPTR OFFSET ABOUT 2 MI TO THE R. AFTER RECONSIDERING THE WIND BEING FROM THE S HE DECIDED TO DSND 250 FT AND ALTERED COURSE TO OFFSET TO THE L. AS THE ACFT WENT THROUGH THE WAKE TRAIL THE ACFT STILL BUMPED BUT MUCH LESS SEVERE. RPTR SAYS THAT THE VERT SEPARATION OF 1000 FT IS INADEQUATE FOR WAKE SPACING AND THE FACT THAT EVEN AFTER DSNDING 250 FT WHICH NOW MADE A 1250 FT SEPARATION; THEY STILL FELT IT.