A B747 IS MANDATED TO RETURN LAND AFTER THE CREW DETERMINES THAT THE SOURCE OF INFLT VIBRATION WAS THE #2 ENG; NOT THE WHEEL VIBRATION AS PREVIOUSLY WRITTEN UP IN THE LOGBOOK. RJAA; NRT; FO.

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AFTER TKOF OUT OF NRT; ABNORMAL VIBRATION AFTER GEAR RETRACTED. FELT THROUGHOUT ACFT BUT MOSTLY LOCATION OF DOORS 3L&R. ACFT PREVIOUSLY WRITTEN UP AND INSPECTED FOR WHEEL VIBRATION SUSPECTED; OR TIRES OUT OF BAL; OR BRAKES (INFLT) NOT SLOWING GYROSCOPIC TIRE MOMENTUM. TIRES; BRAKES CHANGED; ALL WRITE-UPS CLRED IN LOGBOOK. DURING CLB WITH VIBRATION; ALL ENG PARAMETERS IN NORMAL RANGES BUT NOTED #2 ENG EGT; N1 N2 SLIGHTLY HIGHER THAN OTHER ENGS. REDUCED EPR ON #2 ENG AS PWR EPR REDUCED TO 1.12 VIBRATION WENT AWAY WHEN #2 ENG ADVANCED FORWARD TO NORMAL RANGE WITH OTHER ENGS; ABNORMAL VIBRATION RECURRED. REDUCED #2 ENG THRUST; VIBRATION CEASED AGAIN. CONTINUED TO OPERATE ENG #2 AT REDUCED PWR. COORDINATED WITH COMPANY DISPATCH AND MAINT OVER RADIOS. ADVISED OF PROB; AND TOLD THEM PREVIOUS WRITE-UPS HAD SUSPECTED WHEELS PROBS BUT ALL OUR INDICATIONS SHOWED A POSSIBLE INDICATED IMPENDING ENG FAILURE FROM SOMETHING INTERNAL TO ENG. OPERATED ENG AT REDUCED THRUST AND COORDINATED RETURN TO NRT ARPT. FLT ORIGINALLY ROUTED NRT-SFO.; RETURNED TO ORIGIN ARPT. KEPT ENG OPERATING; ALL OTHER PARAMETERS NORMAL. DID NOT DECLARE EMER. REDUCED WT THROUGH NORMAL FUEL USAGE -- WAS NOT NECESSARY TO DUMP FUEL. LANDED UNDER MAX LNDG WT. MADE PRECAUTIONARY LNDG NRT. NORMAL LNDG AND ROLLOUT. RETURNED TO GATE AND DEPLANED PAX. WROTE UP ENG PROB IN LOGBOOK AND ADVISED MAINT. SUPPLEMENTAL INFO FROM ACN 451593: UPON COMPANY'S CONCURRENCE; WE RETURNED TO NRT. NO FUEL WAS DUMPED DUE TO NORMAL BURN -- APPROX 4 HRS. PAX WERE DEPLANED AND PUT ON ANOTHER FLT TO SFO.