Date: 201603
Local time of day: 1201-1800
Location: EGGX.ARTCC, FO
Light: Daylight
Aircraft: B777-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
ATC / advisory: CTAF EGGX
Aircraft component: FMS/FMC
Component problem: Design; Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem; Flight Crew Returned To Clearance
Contributing factors: Aircraft; Human Factors
Primary problem: Ambiguous

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.


Second narrative: [Report narrative contained no additional information.]


Synopsis: 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.