B757 PIC EXERCISES EMER COMMAND AUTH WHEN ENCOUNTERING MODERATE TO SEVERE TURB AND DSNDS JUST DURING THE TRANSITION BTWN ZOA TO HCF (ZHN CTL FACILITY) AT BRADR INTXN ON R464.

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WE HAD JUST COMPLETED A POS RPT TO SAN FRANCISCO RADIO AND THE FLT CONDITIONS HAD BEEN SMOOTH. WE SUDDENLY AND RAPIDLY WERE BUFFETED BY MODERATE TURB -- JUST SHY OF SEVERE. WE HAD NO LOSS OF CTL; BUT SIGNIFICANT BUFFETING AND MAJOR WIND SHIFT AND VELOCITY WAS EVIDENT -- INDICATING WE LIKELY HAD FLOWN INTO THE JETSTREAM. IT DID NOT DISSIPATE AS RAPIDLY AS IT STARTED AND I DECIDED A DSCNT WAS NECESSARY. WE WERE STILL IN CONTACT WITH SAN FRANCISCO RADIO; HAVING JUST FINISHED A POS RPT FOR ZOA OVER BRADR INTXN ON TRACK 464. WE MADE A REQUEST FOR LOWER WITH THEM; KNOWING IT WOULD TAKE TOO LONG USING THAT METHOD. AN EMER DSCNT WAS IN ORDER. WE NEXT CALLED ON 123.45 AND ADVISED OTHER TFC ALONG OUR RTE THAT WE WOULD BE DSNDING DUE TO TURB. WE MADE ANOTHER CALL -- THIS TIME TO ZHN -- I BELIEVE ON 127.1; VHF. WE NORMALLY WOULD HAVE BEEN A LITTLE EARLY TO MAKE A VHF CALL FROM OUR POS. THEY ANSWERED US AND WE ASKED CTR FOR LOWER DUE TO 'MODERATE TURB.' THEY ACKNOWLEDGED IMMEDIATELY. AS SOON AS REQUESTED; THEY CLRED US TO FL310. THEY ASKED US IF WE HAD ANY INJURIES AND OUR REPLY WAS THAT WE HAD NO INJURIES RPTED. THEY ASKED US TO PHONE ZOA UPON LNDG. I PHONED AFTER LNG AND TALKED WITH A SUPVR AT ZOA. HE SAID THAT I MAY HAVE LEFT MY ALT A LITTLE EARLY. I READ FAR 91.3(C) AS GIVING ME (THE PIC) THE AUTH TO DEV FROM ANY RULE TO THE EXTENT NECESSARY; SUBJECT TO A REQUEST BY THE ADMINISTRATOR. NO SUCH REQUEST WAS MADE OF ME WHEN I PHONED ZOA. SUPPLEMENTAL INFO FROM ACN 516633: THE CAPT ASKED ME TO REQUEST FROM SFO RADIO; ON HF; A DSCNT IMMEDIATELY TO FL310. I ASKED FOR A DSCNT BECAUSE OF MODERATE TURB AND THEY SAID STAND BY. I TOLD THEM THAT WE WERE GOING DOWN NOW; AND I SWITCHED TO ZHN ON VHF-TURB AND THEY CLRED US IMMEDIATELY. WE DSNDED OUT OF FL340 TO FL310. THE TURB STOPPED PASSING FL320. WE CHKED WITH THE FLT ATTENDANTS IN THE BACK OF THE ACFT. THEY RPTED NO INJURIES BUT SAID IT WAS A VERY DISRUPTIVE ENCOUNTER AND THAT THEY HAD STRAPPED INTO THEIR JUMP SEATS. ZOA DID NOT KNOW WE HAD GOTTEN CLRNC FROM ZHN TO FL310.