B767 Undifferentiated or Other Model
Flight Crew Last 90 Days 250; Flight Crew Total 16000; Flight Crew Type 60000
Confusion
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Flight Crew Became Reoriented; Flight Crew Returned To Clearance; Flight Crew Overcame Equipment Problem
Aircraft
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 Dispatch and Maintenance Control 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.
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.