Widebody; Low Wing; 3 Turbojet Eng
Flight Crew Last 90 Days 118; Flight Crew Total 9000; Flight Crew Type 3032
Human-Machine Interface; Confusion
ATC Issue All Types; Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented; Flight Crew Returned To Clearance; Flight Crew FLC Overrode Automation
Aircraft
Our aircraft had just made the turn passing 58N020W towards 59N030W. The FMS had cycled through these points correctly. I elected to enter a SLOP of 1 nm right; by using the normal procedure of a lateral offset off lL; and inputting a 1 mile right offset into the offset prompt. I then selected the prompt to input the offset. The FMS flight plan then changed with  a T-P as the from point (instead of 58N020W) and showed 59N040W as next (instead of 59N030W). We immediately noticed this discrepancy. We called up the secondary flight plan; confirmed the correct waypoint sequence; and activated the secondary flight plan; and the aircraft remained on track to 59N030W. We received an automated ADS shortly thereafter stating 'ADS position reporting indicates that you are deviating from cleared route / indications are that you are routing direct to your 40W coordinate / please check and confirm your next waypoint'. We immediately resent the position report showing the correct waypoint sequence. At no time was the aircraft off course. I do not have an explanation for why the FMS behaved as it did. Vigilent adherence to waypoint passage procedures; kept this event from becoming a gross nav error.
I was in the crew rest module asleep when this event occurred. At the end of my rest period the Capt described to me what happened. After passing 58N020W the Capt attempted to put in a 1 mile right SLOP in the FMS. When he activated the SLOP the FMS dropped the 58N020W as the from point and just made the from point a T-P. The FMS also cycled to 59N040W when it should have been tracking to 59N030W. The Captain and First Officer (FO) immediately recognized the problem and activated the secondary flight plan which corrected the problem. The aircraft was never off course due to the rapid reaction of the Captain and; but we did get a message from ATC via ADS that there was a problem with our flight plan. We did not try to put in an offset for the rest of the flight and the rest of the mission was normal. The cleared FMS flight plan routing was checked and correct so I am not sure what caused the FMS to drop the from point and skip the 59N030W point. I use the lateral offset function on all crossings; but I have never seen this happen.
An aircraft flight crew attempted to enter a 1R in between fixes on a North Atlantic Track. The FMS dropped the proper 'from' waypoint and also dropped the proper 'to' waypoint. They activated the secondary flight plan and remained on course.