AN ACR B-747 HAD A NAV ERROR ENRTE NRT-LAX.

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ON A FLT FROM NRT TO LAX USING INERTIAL NAV WE COASTED IN OVER TREVR INTXN AND CONTACTED ZOA. ONCE RADAR IDENTED BY ZOA; WE WERE INFORMED THAT WE WERE ABOUT 20-25 MI N OF TREVR. WE RECHKED THE COORDINATES WE HAD LOADED IN THE INS UNITS FOR TREVR AND THE FOLLOWING WAYPOINT WHICH WAS PESCA. ALL WERE LOADED CORRECTLY. ZOA THEN GAVE US COORDINATES FOR ANOTHER INTXN AND HAD US FLY DIRECTLY OVER IT TO FIX OUR POS. WE ADVISED ZOA WHEN OUR INS UNITS SHOWED US TO BE DIRECTLY OVERHEAD THE INTXN. AGAIN ZOA ADVISED US THAT WE WERE ABOUT 22 MI N. THIS CONFIRMED THAT THE INS SYS HAD DEVELOPED LARGE ERRORS AND WERE THE CAUSE OF OUR COURSE DEV. THE PRIMARY CHK OF THE INS UNITS WAS PERFORMED OVER VACKY INTXN AS WE COASTED OUT OF JAPAN. THE INS INERTIAL POS ON ALL 3 UNITS CHKED QUITE CLOSELY WITH OUR ACTUAL POS OVER VACKY. FROM THIS POINT ON; WE HAD NO WAY OF FIXING OUR POS AND UPDATING THE INS UNITS DUE TO OUR DISTANCE FROM ANY LANDMASSES OR VOR/DME FACILITIES. THIS DEV OCCURRED BECAUSE ALL 3 INS UNITS DEVELOPED LARGE ERRORS. THIS; I BELIEVE; IS THE DIRECT RESULT OF THE AIRLINE'S CHANGE IN POLICY ALLOWING INS UNITS TO HAVE A DRIFT RATE OF UP TO 2.5 RADIAL MI/HR. PREVIOUSLY THIS RATE WAS LIMITED TO 2.0 RADIAL MI/HR AND AT THAT POINT THE UNIT WAS WRITTEN UP IN THE LOGBOOK. THIS POLICY CHANGE MAY SAVE THE AIRLINE MONEY BUT IT WILL ALLOW OR CREATE THE POTENTIAL FOR LARGER COURSE DEVS. BY THE TIME WE REACHED TREVR INTXN WE HAD BEEN FLYING FOR ABOUT 8 HRS. THIS WOULD MEAN WE COULD HAVE ABOUT A 20 MI DRIFT ERROR WITHOUT A WRITEUP. I BELIEVE THAT THE INS UNITS WE HAVE ARE BECOMING TOO OLD AND COSTLY TO MAINTAIN AND WILL HAVE TO BE REPLACED SOON. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTING FO STATES THE HE HAS FLOWN THIS SAME ACFT AND HAS HAD SIMILAR NAV ERRORS APCHING COASTAL FIXES. TREVR IS ABOUT 175 MI FROM BOTH FOT AND ENI ON THE CALIFORNIA COAST; JUST BARELY WITHIN DME RANGE AND OUTSIDE THE NORMAL RANGE FOR A RELIABLE UPDATE PROC. THE RPTR UNDERSTANDS THAT HIS ACR WILL BE REPLACING WITHIN A YR ALL OF THE INS SETS OF THIS PARTICULAR MANUFACTURER WITH NEW ONES OF ANOTHER MANUFACTURER. THE RPTR HAS HEARD NOTHING FROM THE FAA ABOUT THIS. THIS INCIDENT WAS DISCUSSED AT LENGTH IN THE RPTR'S LAST RECURRENT TRAINING. SUPPLEMENTAL INFO FROM ACN 273671: AFTER 8 HRS AND 56 MINS AIRBORNE; THE RADIAL ERRORS AT THE GATE AT LAX READ: INS #1 18 NM; INS #2 32 NM; INS #3 27 NM. THE INS'S HAD BEEN RUNNING TOGETHER FOR THE ENTIRE FLT WITH NO WARNING LIGHTS AND NO APPARENT PROBS. IN 12 YRS WORKING WITH INS EQUIP I HAVE NEVER SEEN 3 INS'S ALL ERR IN THE SAME DIRECTION AT THE SAME TIME.