B767-200 CREW WAS REQUIRED TO DECLARE AN EMER IN ORDER TO AVOID RPTED SEVERE TURB IN CZQX OCEANIC.

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DURING CRUISE AT FL390 ON NAT TRACK 'E'; WHILE FLYING FROM MANCHESTER; UK TO PHILADELPHIA; I RECEIVED A SIGMET FORECASTING MODERATE TO SEVERE TURBULENCE. THE AREA WAS ON MY RTE OF FLT AND RANGED FROM FL310 TO FL390. A PHONE PATCH WITH MY COMPANY DISPATCHER CONFIRMED THAT A COMPANY FLT PRECEDING US HAD ACTUALLY EXPERIENCED SEVERE TURBULENCE AT OUR FLT LEVEL IN THE SIGMET AREA ON THE SAME TRACK. WE CALLED GANDER RADIO AND REQUESTED A LOWER ALTITUDE. AFTER 29 MINS AND NUMEROUS RADIO CALLS; GANDER ADVISED THAT THEY WERE UNABLE TO GIVE A LOWER ALTITUDE BECAUSE OF A PRECEDING ACR (Y) 777 ESTIMATED TO BE 10 MIN AHEAD. WE CONFIRMED WITH THE ACR (Y) FLT THAT THEY WERE ESTIMATING THE NEXT RPTING POINT EXACTLY 10 MIN PRIOR TO US. WE HAD VISUAL CONTACT WITH THE ACR (Y) AND THEY WERE AT FL380. WE INFORMED GANDER RADIO OF THESE FACTS AND THAT WE HAD ADEQUATE SEPARATION. GANDER RADIO STILL REFUSED TO GRANT A LOWER ALTITUDE. NUMEROUS CONTACTS WITH ACR (Y) ACFT ESTABLISHED THAT THEY WERE IN CONTINUOUS MODERATE AND 'APCHING SEVERE' TURBULENCE. GANDER RADIO WAS INFORMED OF THIS FACT BY US AND WE AGAIN REQUESTED A DSCNT. THE RESPONSE WAS THE SAME. I TOLD GANDER THAT IT WAS UNACCEPTABLE. THE RESPONSE WAS THAT 'WE HAVE OUR RULES.' I REPLIED THAT I WANTED GANDER TO CONFIRM THAT WE WERE BEING REQUIRED TO FLY INTO AN AREA OF SEVERE TURBULENCE AS CONFIRMED BY PLT RPTS. GANDER REPLIED THAT THE ONLY WAY WE COULD DSND WAS TO DECLARE AN EMER. IN MY JUDGMENT; REMAINING AT FL390 WOULD HAVE PLACED MY ACFT AND PAXS IN UNAVOIDABLE JEOPARDY. THEREFORE; IN COMPLIANCE WITH MY COMPANY FLT OPS MANUAL AND FAR'S. I EXERCISED MY EMER AUTHORITY; DECLARED AN EMER; AND DSNDED TO FL310 IN ACCORDANCE WITH APPROPRIATE PROCS. ALL THE WHILE ACR (Y) WAS IN SIGHT. UPON REACHING FL310 WE WERE CLRED BACK ON COURSE AND CONTINUED UNEVENTFULLY TO PHL. SUPPLEMENTAL INFO FROM ACN 500364: AN EMER WAS DECLARED AND A TURN OF 90 DEG TO PARALLEL TRACK WAS MADE. DSCNT WAS MADE TO FL 310 AFTER CLRING TRACK STRUCTURE. BROADCAST WAS MADE ON 121.5 AND 123.45 AS PER FAR'S FOR NO ATLANTIC RVSM AREA. GANDER WAS ADVISED. DSCNT WAS UNEVENTFUL AND AFTER LEVELING AT FL310; CLRNC WAS RECEIVED TO RETURN TO NORMAL TRACK STRUCTURE AND TO MAINTAIN FL310. REMAINDER OF FLT WAS NORMAL AND UNEVENTFUL.