Narrative:

Location: approximately 100 NM northeast of 51n50w. Flight ab marked 51n50w at XA00. Answered SELCAL on 3476 at XA05. Gander advised flight ab was 15-20 NM north of 51n50w. Told to contact moncton center on 122.37. At XA15 flight crew conducted 10 min position fix. FMC position was exactly on track. Moncton center confirmed flight ab was 15-20 NM north of course and instructed flight ab to proceed direct 52n40w. Flight crew crosschecked FMC and all 3 IRS and discovered discrepancy. Proceeded direct 52n40w and conducted flight using IRS. Exited track on course according to shannon center. Flight crew was advised by gander no reports were sent.

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Original NASA ASRS Text

Title: NAVING ON TRANSATLANTIC RTE FLC WAS USING THE NORMAL NAV SYS, FMC, WHEN THEY WERE CONTACTED BY GANDER RADIO ADVISING THEY WERE 15-20 MI N OF COURSE. FLC VERIFIED FMC ERROR AND USED IRS FOR NAV.

Narrative: LOCATION: APPROX 100 NM NE OF 51N50W. FLT AB MARKED 51N50W AT XA00. ANSWERED SELCAL ON 3476 AT XA05. GANDER ADVISED FLT AB WAS 15-20 NM N OF 51N50W. TOLD TO CONTACT MONCTON CTR ON 122.37. AT XA15 FLC CONDUCTED 10 MIN POS FIX. FMC POS WAS EXACTLY ON TRACK. MONCTON CTR CONFIRMED FLT AB WAS 15-20 NM N OF COURSE AND INSTRUCTED FLT AB TO PROCEED DIRECT 52N40W. FLC XCHKED FMC AND ALL 3 IRS AND DISCOVERED DISCREPANCY. PROCEEDED DIRECT 52N40W AND CONDUCTED FLT USING IRS. EXITED TRACK ON COURSE ACCORDING TO SHANNON CTR. FLC WAS ADVISED BY GANDER NO RPTS WERE SENT.

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.