GROSS NAVIGATION ERROR ON NORTH ATLANTIC TRACK SYSTEM BY ACR HVT.

Narrative

WE WERE ON A CARGO FLT ACROSS THE N ATLANTIC TO MILDENHALL; ENGLAND. OUR CLRNC AFTER 51N30W READ 51N20W 51N15W TIVKI. THE FLT WAS APPROX SCHEDULED FOR 8 HRS. THE FLT WAS UNEVENTFUL UNTIL WE WERE APCHING 5100.0N01500W. APPROX 5 MINS BEFORE REACHING 51N15W A MAJOR DISCREPANCY WAS NOTICED BTWN #1 AND #2 COMP (NAV). THE #2 NAV INDICATED WE WERE APPROX 47.3 NM S OF COURSE. AFTER CHKING BY PROCEDURE IT WAS DETERMINED THAT THE #1 MACHINE WAS IN ERROR. WE THEN IMMEDIATELY MADE A TURN TO THE N TO GET BACK ON TRK. WE HAD JUST COMPLETED THE TURN WHEN SHANWICK OCEANIC CALLED US ON THE UHF RADIO. SHANWICK INFORMED US THAT WE WERE OBSERVED 5020.0N01500W (40 NM) S OF COURSE. WE INFORMED SHANWICK THAT WE HAD DISCOVERED A NAV ERROR AND AT THAT TIME WE WERE IN THE PROCESS OF CORRECTING. AT THE TIME OF THE INCIDENT THE ACFT WAS NAV ON #1 MACHINE. THE #1 HSI WAS SELECTED ONS. THE NAV COMPUTER WAS NAV PAGE 1. THE #2 NAV COMPUTER WAS SELECTED NAV PAGE 4; THE #2 HSI WAS ON NAV. THE #2 OBS WAS SELECTED ON THE INBND RADIAL OF CRK VOR. THE CAPT WAS FLYING THE ACFT. HE DID GET UP TO MAKE A CUP OF COFFEE. I WAS COMPLETING THE PAPERWORK FOR THE XING AND WATCHING THE OPERATION OF THE ACFT. IT WAS NOT UNTIL I WENT TO DO A XCHK WHEN I NOTICED THE DISCREPANCY. THE NAV COMPUTER NEVER DID ALERT US AS TO THE DISCREPANCY. I FEEL THAT IF I; THE F/O; HAD LEFT MY HSI SELECTED ON ONS THE DEV FROM COURSE WOULD HAVE BEEN NOTICED MUCH SOONER; AND THE DEV FROM COURSE WOULD NOT HAVE OCCURRED.