Narrative:

We were on a cargo flight across the north atlantic to mildenhall, england. Our clearance after 51n30w read 51n20w 51n15w tivki. The flight was approximately scheduled for 8 hours. The flight was uneventful until we were approaching 5100.0n01500w. Approximately 5 mins before reaching 51n15w a major discrepancy was noticed between #1 and #2 comp (navigation). The #2 navigation indicated we were approximately 47.3 NM south of course. After checking by procedure it was determined that the #1 machine was in error. We then immediately made a turn to the north 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) south of course. We informed shanwick that we had discovered a navigation error and at that time we were in the process of correcting. At the time of the incident the aircraft was navigation on #1 machine. The #1 HSI was selected ONS. The navigation computer was navigation page 1. The #2 navigation computer was selected navigation page 4, the #2 HSI was on navigation. The #2 obs was selected on the inbound radial of crk VOR. The captain was flying the aircraft. He did get up to make a cup of coffee. I was completing the paperwork for the crossing and watching the operation of the aircraft. It was not until I went to do a xchk when I noticed the discrepancy. The navigation computer never did alert us as to the discrepancy. I feel that if I, the first officer, had left my HSI selected on ONS the deviation from course would have been noticed much sooner, and the deviation from course would not have occurred.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of August 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.