Narrative:

This report identifies minor navigational error caused by a 777-200 autopilot and/or FMS autoflight system. I believe it was caused by an FMS anomaly. We observed the autopilot making an incorrect left turn crossing over resno; and we made necessary inputs to correct what could have become a gross nav error. En-route; after requesting our oceanic clearance; we confirmed and verified the ATC clearance per SOP. It was as filed; no changes. The clearance was; resno n55w020 n56w030 n56w040 n56w050 janjo mach .84 FL380. The first officer was flying; and we had recently climbed to FL380 for the crossing. Several hundred miles prior we were given direct to resno. As we were crossing resno; ATC advised us to contact shanwick oceanic for enroute HF's. We were in LNAV VNAV path and the first officer put in a R1 right slop offset while remaining in LNAV. After doing so; the aircraft (incorrectly) began a left turn. We then noticed that on the nd it showed an approximately 46R off course indication. In addition; the nd correctly showed we were still basically on the magenta line; with n55w020 as the next waypoint. What was confusing was that on the FMC CDU; the active way point was now n56w030. Somehow 20W had disappeared; or sequenced just as we passed over resno and did the offset; and 30W became the active waypoint. About this time shanwick called us to alert us they detected a navigation error for our route. (We were cpdlc and they were looking at our FMC routing and the position report). They made us read back our cleared route. I tried to explain what happened. We then corrected the situation by using heading select to maintain course. We re-entered direct to n55w020; and cleaned up the legs page. ATC later agreed all was well now; and we proceeded on without any other issues. I believe we were no more than several miles south (left) of course at resno before we corrected the problem. We are not sure what could have caused this anomaly; and for 20W just to drop off the legs page. It was interesting though that even though 20W was not on the legs page; it was on the nd. I took a snapshot with my ipad camera of the FMC legs page and also the nd. Also of the route 2 page which confirmed our cleared routing.

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Original NASA ASRS Text

Title: B777-200 flight crew experienced a track deviation crossing RESNO over the North Atlantic when the First Officer entered a 1R offset. The aircraft began a left turn and the N55W020 waypoint had disappeared from the FMS. The waypoint is reentered and the aircraft is returned to track.

Narrative: This report identifies minor navigational error caused by a 777-200 autopilot and/or FMS Autoflight system. I believe it was caused by an FMS anomaly. We observed the autopilot making an incorrect left turn crossing over RESNO; and we made necessary inputs to correct what could have become a gross nav error. En-route; after requesting our Oceanic clearance; we confirmed and verified the ATC clearance per SOP. It was as filed; no changes. The clearance was; RESNO N55W020 N56W030 N56W040 N56W050 JANJO Mach .84 FL380. The First Officer was flying; and we had recently climbed to FL380 for the crossing. Several hundred miles prior we were given direct to RESNO. As we were crossing RESNO; ATC advised us to contact Shanwick Oceanic for enroute HF's. We were in LNAV VNAV PATH and the F/O put in a R1 right SLOP offset while remaining in LNAV. After doing so; the Aircraft (incorrectly) began a LEFT turn. We then noticed that on the ND it showed an approximately 46R off course indication. In addition; the ND correctly showed we were still basically on the Magenta line; with N55W020 as the next waypoint. What was confusing was that on the FMC CDU; the active way point was now N56W030. Somehow 20W had disappeared; or sequenced just as we passed over RESNO and did the offset; and 30W became the active waypoint. About this time Shanwick called us to alert us they detected a NAV error for our route. (We were CPDLC and they were looking at our FMC routing and the position report). They made us read back our cleared route. I tried to explain what happened. We then corrected the situation by using Heading Select to maintain course. We re-entered direct to N55W020; and cleaned up the legs page. ATC later agreed all was well now; and we proceeded on without any other issues. I believe we were no more than several miles south (left) of course at RESNO before we corrected the problem. We are not sure what could have caused this anomaly; and for 20W just to drop off the legs page. It was interesting though that even though 20W was not on the legs page; it was on the ND. I took a snapshot with my IPAD Camera of the FMC legs page and also the ND. Also of the RTE 2 page which confirmed our cleared routing.

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.