Narrative:

We were class ii on a random route in ETOPS airspace within reykjavik oca in cruise; having received our oceanic clearance and all oceanic airways procedures were accomplished. I had built an offset track waypoint of N61 degrees to identify crossing from shanwick oca into reykjavik oca. Our active waypoint was n62w20 then n63w30 next. Around the point of N61; I was identifying some alternate airports ICAO identifiers from the flight manual; the first officer was reading something from the flight manual; we both felt the aircraft in an uncommanded bank; and looked up and saw the aircraft turning right on its own on. I selected heading select and brought it back to the magenta line; and pulled up progress page 2 on the FMC CDU to note the cross track error distance. We were showing about 2.5 to 3 miles from the magenta line; and although we had an intercept angle; the distance was not decreasing. I looked at the legs page and nav display; the active waypoint was now n63w30; not the previous active waypoint of n62w20. Somehow the active waypoint; n62w20 dropped out on its own; and the aircraft began the uncommanded turn to intercept the course to the next waypoint n63w30. There was no activity on the CDU keys at the time of the event; it was all uncommanded. I reloaded the correct and cleared waypoint n62w20 back into the FMS; it was in small white font on the nd; not magenta; and not on the legs page of the CDU; and re-established the course. No navigation deviation occurred; we were performing offset procedures at the time of 1 NM right; and through the event was never more than 2-3 NM right of course until the correction made and re-activation of the correct active waypoint. I am aware of the anomaly of FMC blanking; page 1 of amsdis; and of previous reports related similar to this issue. In this case however; unlike the published anomaly; we never have a standby message or blank pfd or nd. The nd remained through the event; however; only the active waypoint dropped on its own; and the FMC cycled to the next waypoint on the legs page. The aircraft was ads B equipped; and within a minute or so; received a SELCAL from reykjavik; and they asked for a position report; as I would assume they noticed the active waypoint was no longer the same as the cleared route when n62w20 dropped out. Again there was no CDU activity at the time of the event. The event occurred near the previously built along track waypoint; but not specifically at the N61 position; and was recognized quickly and corrected quickly.

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Original NASA ASRS Text

Title: A B777-200 crew reported that the active oceanic random track waypoint dropped out of the FMC and the aircraft transitioned to the next waypoint. A slight track deviation occurred before the crew reprogrammed the FMC back to the correct active waypoint.

Narrative: We were Class II on a random route in ETOPS airspace within Reykjavik OCA in cruise; having received our oceanic clearance and all Oceanic Airways procedures were accomplished. I had built an offset track waypoint of N61 degrees to identify crossing from Shanwick OCA into Reykjavik OCA. Our active waypoint was N62W20 then N63W30 next. Around the point of N61; I was identifying some alternate airports ICAO identifiers from the Flight Manual; the First Officer was reading something from the Flight Manual; we both felt the aircraft in an uncommanded bank; and looked up and saw the aircraft turning right on its own on. I selected heading select and brought it back to the magenta line; and pulled up Progress page 2 on the FMC CDU to note the cross track error distance. We were showing about 2.5 to 3 miles from the magenta line; and although we had an intercept angle; the distance was not decreasing. I looked at the legs page and nav display; the active waypoint was now N63W30; not the previous active waypoint of N62W20. Somehow the active waypoint; N62W20 dropped out on its own; and the aircraft began the uncommanded turn to intercept the course to the next waypoint N63W30. There was no activity on the CDU keys at the time of the event; it was all uncommanded. I reloaded the correct and cleared waypoint N62W20 back into the FMS; it was in small white font on the ND; not magenta; and not on the legs page of the CDU; and re-established the course. No navigation deviation occurred; we were performing offset procedures at the time of 1 NM right; and through the event was never more than 2-3 NM right of course until the correction made and re-activation of the correct active waypoint. I am aware of the anomaly of FMC blanking; page 1 of AMSDIS; and of previous reports related similar to this issue. In this case however; unlike the published anomaly; we never have a standby message or blank PFD or ND. The ND remained through the event; however; only the active waypoint dropped on its own; and the FMC cycled to the next waypoint on the legs page. The aircraft was ADS B equipped; and within a minute or so; received a SELCAL from Reykjavik; and they asked for a position report; as I would assume they noticed the active waypoint was no longer the same as the cleared route when N62W20 dropped out. Again there was no CDU activity at the time of the event. The event occurred near the previously built along track waypoint; but not specifically at the N61 position; and was recognized quickly and corrected quickly.

Data retrieved from NASA's ASRS site as of April 2012 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.