A B-747-250 MAY HAVE BEEN OFF COURSE AT TREVR.

Narrative

A MIL CHARTER FROM YOKOTA AIRFIELD; JAPAN; TO LAX. WE CONTACTED ZOA AT TREVR INTXN; THEY SAID WE WERE 12 MI NE OF TREVR AND CLRED US DIRECT AVENAL. ENRTE TO AVENAL THEY CALLED AGAIN AND SAID WE WERE CLRED DIRECT AVENAL; WE SAID WE WERE PROCEEDING DIRECT AND THEY SAID AGAIN THAT WE HAD BEEN 12 NE OF TREVR. AT THAT TIME WE STARTED PICKING UP AVENAL ON OUR VOR'S AND MADE A 5 DEG CORRECTION AND NOTHING ELSE WAS SAID ABOUT IT. WHEN WE LANDED IN LAX AND FLUSHED OUR INS'S; THEIR ERRORS WERE 20; 13; 8. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTING FO SAYS THAT THE FAA STARTED TO 'LOOK INTO' THIS INCIDENT; THEN DROPPED IT. THE FLT PLAN IN THIS INCIDENT AGREED WITH THE CHART COORDINATES AND ALWAYS DOES. THE INS TOLERANCES ARE 3 NM PLUS 3 NM PER HR OF FLT TIME; ABOUT 33 NM FOR THIS FLT. THERE IS NO WAY TO UPDATE THE INS APCHING TREVR. THE RPTR DOES NOT THINK THAT THIS ACFT WAS THE SAME ONE THAT IS DISCUSSED IN ACN 269411; BUT IT MAY BE HIS ACR IS CHANGING THESE OLD INS FOR NEWER; MORE PRECISE ONES. INS TOLERANCE EQUALS 3 NM PLUS (3 NM/HR) (10 HR) EQUALS 33 NM. A CALL TO ZOA OCEANIC CTL FOUND THAT AN ACFT IS CONSIDERED TO BE ON COURSE IF IT IS NO FURTHER THAN 20 MI OFF OF THE CTRLINE OF THE COURSE. IT CAN BE ANYWHERE ALONG THE COURSE LINE AND STILL BE ON COURSE. MANY OF THE ACFT COMING IN OVER TREVR ARE IN THE FLEXIBLE TRACK SYS AND APPEAR TO BE OFF COURSE WHEN THEY ARE ACTUALLY ON COURSE. MANY 'ONERS' (OCEANIC NAV ERROR RPTS) ARE FILED IN THIS AREA AND ARE LATER DISMISSED. THE ZOA RESPONDENT DOES NOT THINK THAT THERE IS DISTORTION IN ITS RADAR PRESENTATION AT TREVR. HE AGREES THAT THERE MAY BE A 'CRUSADE' BY CTLRS IN THIS SECTOR TO IMPROVE NAV. SUPPLEMENTAL INFO FROM ACN 269337: UPON LNDG AT LAX THE INS UNITS INDICATED WITHIN TOLERANCE; SO WE DID NOT WRITE UP THE INS UNITS. THIS NAV ERROR WAS RPTED TO OUR CHIEF PLT.