WDB FLT ENRTE TO NARITA WAS OBSERVED BY NARITA CTL AS BEING OFF COURSE. CORRECTION WAS BEING MADE BACK ON COURSE BY FLC; NOTHING MORE SAID.

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FLT ORIGINATED SINGAPORE NONSTOP TO TOKYO 'NARITA.' THE FLT WAS OPERATED NORMALLY ENRTE USING INS AND XCHKS VARIOUS POINTS ALONG OUR RTE WITH ADF AND VOR FACILITIES. WE OPERATED ALONG FILED RTE #2J 2970 NM DISTANCE AT 5:54 FLT PLAN TIME. AS FLT PROGRESSED INTO TOKYO AREA; FLT WAS REQUESTED TO CONTACT TOKYO CTL ON 125.9 AT BOBOT INTXN. AT BOBOT I GAVE POS RPT TO TYO RADIO AND CHKED IN WITH TOKYO CTL 125.9. AT THIS TIME I NOTICED 'BY REF MY VOR SELECTED TO MJE VOR 117.8 'MIYAKEJIMA' AIRWAY G581 248 RADIAL. THAT WE WERE TRACKING R OF COURSE. I BROUGHT THIS TO THE ATTN OF FO WHO WAS FLYING THE ACFT. HE IMMEDIATELY STARTED CORRECTION TO L TO REGAIN CENTERLINE. WHILE WE HAD ALREADY INITIATED THIS CORRECTION TOKYO CTL CALLED US AND SAID 'I SHOW YOU R OF COURSE.' I ADVISED AS CORRECTING. WE REGAINED CENTERLINE. TOKYO CTL MADE NO FURTHER COMMENT. WE RECEIVED NORMAL DSCNT CLRNCS; AND DSNDED INTO TOKYO 'NARITA ARR AREA' AND LANDED IN NARITA ON RWY 34 ILS STRAIGHT IN. WX VFR. THIS SITUATION DID NOT CAUSE ANY TFC CONFLICTS. CAUSAL RELATIONSHIP: COCKPIT CREW VERY FATIGUED FROM SERIES OF EVENTS. 9 DAY PLANNED TRIP BECAME A MARATHON OF ENDURANCE BECAUSE OF CONSTANT RESCHEDULING OF THIS CREW BY THE COMPANY! CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: OUR COMPANY SCHEDULERS GO STRICTLY BY THE BOOK OF DUTY RULES -- NEVER CONSIDERING HOW OUR BODIES MAY BE SCREAMING ILLEGAL -- IF IT IS LEGAL BY THE BOOK WE HAVE TO GO. THIS RESULTS IN BONE WEARY FATIGUE FREQUENTLY; RESULTING IN MANY FLC HUMAN FACTOR ERRORS. IT IS OUR OWN DOING OF COURSE. WE NEGOTIATE THE DUTY RULES AND ASK FOR MORE MONEY SO THE COMPANY WANTS TO GET THEIR MONIES WORTH. TOO OFTEN; HOWEVER; WE SUFFER EXTREME FATIGUE.