Narrative:

Approximately 35W; with the C autopilot on and VNAV/LNAV sel; the aircraft began an uncommanded right turn (south). We noticed a pos-shift of course line so began looking at fmcs/irus. The left IRU showed appreciably larger error than C or right; both gpss mirrored irus C and right (9.5 left; 2.0 all others). Assuming the GPS held priority over the IRS we assumed a GPS receiver failure as GPS no longer guiding aircraft (FMC showed left IRU as priority; the one with massive drift). We were still in contact/troubleshooting with chidd and samc via satfon when the anp exceeded the rnp. Should have told gander oceanic prior to this but were busy troubleshooting and; based on the proximity of other aircraft we had the aircraft back on course ('kentucky windaged' a correction to get us back in the line; offset L1). We woke the captain who took over my part of the issue. It took several minutes to maybe an hour before it was ascertained that the GPS was a secondary system to the IRU's and in a discrepancy would be shed. It turns out that the left IRU has a maintenance history and the insidious nature of its drift over a long period (7hrs or so) before it became an issue led to the confusion.

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Original NASA ASRS Text

Title: The crew of a B767 experienced an uncommanded right turn during cruise flight and were initially confused as to which aircraft system or systems between 3 IRU's and 2 GPS units was causing the course deviation. Commendably the crew manually turned the aircraft back to the approximate course and then began the troubleshooting process.

Narrative: Approximately 35W; with the C autopilot on and VNAV/LNAV sel; the aircraft began an uncommanded right turn (south). We noticed a pos-shift of course line so began looking at FMCs/IRUs. The L IRU showed appreciably larger error than C or R; both GPSs mirrored IRUs C and R (9.5 L; 2.0 all others). Assuming the GPS held priority over the IRS we assumed a GPS receiver failure as GPS no longer guiding aircraft (FMC showed L IRU as priority; the one with massive drift). We were still in contact/troubleshooting with CHIDD and SAMC via SATFON when the ANP exceeded the RNP. Should have told Gander Oceanic prior to this but were busy troubleshooting and; based on the proximity of other aircraft we had the aircraft back on course ('Kentucky windaged' a correction to get us back in the line; offset L1). We woke the Captain who took over my part of the issue. It took several minutes to maybe an hour before it was ascertained that the GPS was a secondary system to the IRU's and in a discrepancy would be shed. It turns out that the L IRU has a maintenance history and the insidious nature of its drift over a long period (7hrs or so) before it became an issue led to the confusion.

Data retrieved from NASA's ASRS site 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.