Narrative:

On a flight from nrt to lax using inertial navigation we coasted in over trevr intersection and contacted ZOA. Once radar idented by ZOA, we were informed that we were about 20-25 mi north 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 intersection and had us fly directly over it to fix our position. We advised ZOA when our INS units showed us to be directly overhead the intersection. Again ZOA advised us that we were about 22 mi north. This confirmed that the INS system had developed large errors and were the cause of our course deviation. The primary check of the INS units was performed over vacky intersection as we coasted out of japan. The INS inertial position on all 3 units checked quite closely with our actual position over vacky. From this point on, we had no way of fixing our position and updating the INS units due to our distance from any landmasses or VOR/DME facilities. This deviation 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/hour. Previously this rate was limited to 2.0 radial mi/hour 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 deviations. By the time we reached trevr intersection we had been flying for about 8 hours. 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 reporter revealed the following information: the reporting first officer states the he has flown this same aircraft and has had similar navigation errors approaching 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 procedure. The reporter understands that his air carrier will be replacing within a yr all of the INS sets of this particular manufacturer with new ones of another manufacturer. The reporter has heard nothing from the FAA about this. This incident was discussed at length in the reporter's last recurrent training. Supplemental information from acn 273671: after 8 hours 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 flight with no warning lights and no apparent problems. In 12 yrs working with INS equipment I have never seen 3 INS's all err in the same direction at the same time.

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Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.