A B747-400 FO RPTS A RETURN TO LAND WITH RUDDER VIBRATIONS. THE CAPT WAS RELUCTANT TO EXERCISE CAPT'S AUTH AND DUMP FUEL BEFORE LNDG.

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AT CRUISE WITH AUTOPLT ON; FL320; A HIGH SPD VIBRATION FELT THROUGH RUDDER PEDALS. I REDUCED AIRSPD .84 TO .81 AND IT HELPED REDUCE VIBRATION. MY CAPT ACTED IN A MISSION STATUS -- KEEP GOING. I ASK THAT HE RETURN. HE SAID THERE WAS NO VIBRATION; HE THEN TRIED TO DETERMINE WAS IT HYD OR YAW DAMPER RELATED. I WAS COMFORTABLE AND ENCOURAGED TO FIND THIS REOCCURRING PROB. MAINT FAILED TO FLT TEST THE ACFT AND THIS PROB DID NOT FIX ITSELF. ACR X MAINT CTL IS PUSHING THE MAINT PERSONNEL. NO PARTS ARE COMMON. CREWS TAKE ACFT WITH SIGN-OFFS 'COULD NOT DUPLICATE.' MAINT CTL CONTACTED MANUFACTURER AND 'THEY' MADE THE DECISION TO RETURN. WE ISOLATED THE PROB TO NOT BE AFFECTED BY HYDS/YAW DAMPER; RETURNED TO ZZZ2 AND THE CAPT INFORMED DISPATCH OF OUR NEW ETA AND FUEL DUMPING OF 24 TONS. WE GOT A SATELLITE CALL; THE CAPT WAS TOLD PER THE FOM THERE WAS A PROC IN THE FOM AND THE CHIEF PLT APPROVED US LNDG OVERWT. THE CAPT AGREED; END OF CALL. I INFORMED THE CAPT HE WAS BREAKING A FAR ACFT LIMITATION; WE DIDN'T DECLARE AN EMER AND NO ONE CAN AUTH THIS; IT'S A CAPT'S AUTH; BESIDES THE FELLOW REFERRED IS THE DIRECTOR OF TRAINING. SO I TOLD THE CAPT WE MUST DUMP; HE AGREED THAT WE COULD BLOW TIRES ON LNDG; A SINGLE RWY OP. SO ONCE AGAIN WE WERE FORCED INTO A FALSE SENSE OF SECURITY THAT IT WAS OK TO BREAK THE FAR'S. WE DUMPED FUEL AND 'DID NOT DECLARE AN EMER' AND LANDED NORMALLY.