B747 ACFT ENRTE ON OCEANIC RTE GOT OFF COURSE DUE TO ERRONEOUS NAV COMPUTERS.

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SO LOADED INITIAL POS. ON PREFLT ALL COMPUTERS WERE CHKED FOR INITIAL POS AT GATE IN HNL. RTE WAS LOADED BY COPLT AND VERIFIED BY CAPT. AT THE COAST OUT POS; THE CTLR TOLD US WE WERE 25 NM S OF A POINT WE WERE CLRED DIRECTLY TO. WE CHKED THE COMPUTERS FOR ACCURACY AND CHKED THE SYS ACCURACY IN TRIPLE MIX POS. THE #1 COMPUTER WAS FLASHING FAIL; WE THEN DISABLED TRIPLE MIX AND #2 AND #3 COMPUTERS SHOWED THE SAME POS. #1 COMPUTER SHOWED THE SAME LATITUDE BUT 2 DEGS DIFFERENCE EXACTLY IN LONGITUDE. SINCE #1 WAS FLASHING FAIL AND #2 AND #3 AGREED WITH LATITUDE/LONGITUDE RECEIVED FROM ATC ON COAST OUT; WE WENT WITH #2 AND #3 FOR THE REMAINDER OF THE TRIP. AS A XCHK THE MAGNETIC HEADINGS CHKED WITH THE FLT PLAN. AS THE WIND WAS SLIGHTLY DIFFERENT FROM THE FLT PLAN; THE HEADINGS APPEARED TO BE VERY CLOSE; GIVEN THE WIND DIFFERENCE. WHEN ARRIVING IN THE TOKYO AREA 8 HRS LATER; THEY TOLD US TO CHK OUR POS. THEY SHOWED US 30 NM N OF TRACK; GAVE US A HEADING OF SW TO JOIN A RADIAL OF OJC; A VOR S OF NARITA. THE DME WOULD NOT LOCK ON TO CVC VOR (WHICH HAD BEEN UNDER REPAIR FOR SEVERAL MONTHS AND WAS NOW BACK IN SVC). ONCE ON THE GND AND THE ACCURACY CHKS WERE DONE ON ALL COMPUTERS; #2 AND #3 WERE BOTH 96 MI OFF; #1 WAS CORRECT. WE TRIED TO INSTALL AN INCORRECT POS TO ACCOUNT FOR THE ERROR BUT COMPUTER REJECTED THE POS WITH FLASHING RED AND AMBER WARNING LIGHTS. WE WROTE UP COMPUTERS IN LOG. HAVE YET TO DETERMINE THE CAUSE OF LONGITUDINAL PROB.