GROSS INS NAV ERROR NORTH ATLANTIC.

Narrative

AFTER XING NORTH ATLANTIC ON RTE YQX; 51.50; 53.40; 54.30; 54.20; 53.15; BURAK; EINN. RPTED 54.20 AT ATA XA26Z; FL410; ETA 53.15 AT XA49Z; BURAK. JUST PRIOR TO 53.15 SHANNON CALLED US ON VHF 135.6 AND INFORMED US WE WERE OFF COURSE BY 38 MI N. WE REVIEWED HIS COORDINATES AND COMPARED THEM WITH OUR OMEGAS. WE SHOWED WE WERE NO MORE THAN 5-6 MI N OF COURSE. ON CHKING OTHER NAV INPUTS (#1 IRS AND #2 IRS INPUTS) ON THE #1 SIDE SHOWED US 35 MI L. #1 IRS AND #2 IRS INPUTS ON THE R SIDE SHOWED US NO MORE THAN 2 MI L. WE TOOK A HDG FROM SHANNON AND LANDED WITH NO FURTHER COMMENTS. PLOT AT 54.20 WAS N54.06 - W20.07; #1 SIDE; N54.05; W20.03 #2 SIDE. THIS WAS NOT SIGNIFICANT OFF COURSE FOR CONCERN. 3 NAVAIDS WERE SHOWING PLUS OR MINUS 5 MI OFF COURSE AND INBOUND TO N53.W15. WE WERE SURPRISED THAT JUST PRIOR TO N53.W15 WE WERE INFORMED WE WERE SO FAR OFF COURSE. AT NO TIME PRIOR TO 53.15 INCLUDING OUR RECORDED POS AT N54.20W INDICATED WE HAD TO CORRECT A SIGNIFICANT COURSE DEV. MAINT IS CHKING #1 NAV SYS FOR PROBS. (SUSPECT #1 VLF.) NAV SYS WILL BE FLT CHKED PRIOR TO NEXT OVERWATER FLT.