A B-747 FREIGHTER CREW WAS OFF TRACK OVER THE PACIFIC.

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WE WERE ADVISED BY ZAN THAT THEIR REMOTE RADAR SHOWED THAT WE WERE 18 MI N OF TRACK WHILE ON A-590 WHICH IS AN INS AIRWAY ACROSS THE NORTH PACIFIC. WE WERE 5 HRS INTO A FLT FROM TPE TO ANC AT FL330 ON AUTOPLT COUPLED TO THE #1 INS. ALL 3 INS UNITS WERE TRIPLE-MIXED. AN INS ACCURACY CHK WAS ACCOMPLISHED AT OSHIMA (XA); JAPAN; AND INDICATED THAT INS UNITS WERE RIGHT ON COURSE (2 HRS INTO FLT). 3 HRS LATER WE WERE 18 MI L OF TRACK. THE REMAINDER OF THE FLT WAS FLOWN IN THE HDG MODE TAKING ADF AND VOR BEARINGS. I PERSONALLY DON'T BELIEVE IN TRIPLE-MIXING INS UNITS JUST FOR THIS REASON; ALTHOUGH IT IS OUR COMPANY PROC. THIS ACFT WAS RECENTLY ACQUIRED FROM ACR X AND THE INS PROGRAM HAS AUTOMATIC TRIPLE-MIX FEATURE SO YOU HAVE TO MANUALLY SEPARATE THE UNITS; WHICH WE DID NOT DO UNTIL THE ERROR WAS DETECTED. WHEN THE INS UNITS RUN SEPARATELY; YOU CAN MONITOR THE 3 AND; BY XREFING; DETECT ONE THAT RUNS AMOK. IN TRIPLE-MIX; THEY ALL AVERAGE TO SELECT THE COURSE; SO THE BAD ONES DEGRADE THE GOOD ONE. THE RAMP POS AT ANC READINGS WERE #1 - 23 MI OFF; #2 - 8 MI OFF; AND #3 - 20 MI OFF. INS UNITS ARE DELCO CAROUSEL IV'S.