Narrative:

Having just arrived on the flight deck after first break; the first officer had departed; the captain briefed to me the following: we are cleared to deviate up to 40 nm left of course. The last waypoint; now behind us; (S10 W160) did not cycle (still magenta) because we were deviating too far left (FM say more than 21 nm). He pointed out to me the map display course deviation readout which was 52 nm left of course but told me 'we're not really that far off course' it's just because the last waypoint did not cycle. He gave me the airplane in heading select; without LNAV armed with a 15 degree intercept angle back to course. The authorized deviation of 40L had not been input on the offset page. Consequently there was no dashed line depiction of our clearance limit on the map display for reference. The captain told me he thought the waypoint would cycle once we finally intercepted the course line. At that point I took control of the aircraft; the captain got up and departed for his break and the other [relief pilot] came up as well. Feeling a bit uncomfortable seeing the L52.1 on the map display; I used heading select to turn the aircraft further to the right increasing our intercept angle to 30 degrees and then 45 degrees. In addition I armed LNAV. The FMC was still programed with the original R1 offset. The course deviation began to click down 50L; 40L; 30L etc. When we approached the course line I shallowed the intercept angle. On top of our R1 offset the previous waypoint still did not cycle and LNAV did not capture. I had to use heading select to prevent going further right of course. The other international relief officer and I decided the best course of action at that point was to zero the offset; put the aircraft directly on the course line and select present position direct to the next waypoint in front of us: s05w157. This was accomplished; LNAV captured and then I subsequently offset R1 per SOP. This all occurred still within nzzo; aukland oceanic airspace. About 5 or 10 minutes later we got a cpdlc message to contact ZZZ1 on [numbers] as we were now crossing s05w157 and the ZZZ-ZZZ1 fir. We contacted ZZZ1; got a SELCAL check and frequencies for crossing 20N. It all seemed ops normal and ZZZ2 radio made no mention of any issues or any reports from ZZZ about navigational errors. 6 hours later all four of us discussed it some more. The captain asserted that the L52.1 was a reading left of an extended course line between the previous two waypoints and not an actual distance left of course from the point that did not cycle; as there was a slight left jog at that point. And this may very well be the case. The flight manual does state something akin to this and says waypoints must be manually cycled which we eventually did. My contention; having just arrived on the flight deck and not having the full situational awareness of the preceding half hour; was that at a minimum; we did not know precisely how far left we were. The configuration of the automation (uncycled waypoint; erroneous FMC deviation readout) probably was not ideal for the crew change. See two attachments.

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Original NASA ASRS Text

Title: First Officer reported a deviation in heading caused by the FMS not operating correctly.

Narrative: Having just arrived on the flight deck after first break; the First Officer had departed; the Captain briefed to me the following: We are cleared to deviate up to 40 nm left of course. The last waypoint; now behind us; (S10 W160) did not cycle (still magenta) because we were deviating too far left (FM say more than 21 nm). He pointed out to me the map display course deviation readout which was 52 nm left of course but told me 'we're not really that far off course' it's just because the last waypoint did not cycle. He gave me the airplane in heading select; without LNAV armed with a 15 degree intercept angle back to course. The authorized deviation of 40L had not been input on the offset page. Consequently there was no dashed line depiction of our clearance limit on the map display for reference. The Captain told me he thought the waypoint would cycle once we finally intercepted the course line. At that point I took control of the aircraft; the Captain got up and departed for his break and the other [Relief Pilot] came up as well. Feeling a bit uncomfortable seeing the L52.1 on the map display; I used heading select to turn the aircraft further to the right increasing our intercept angle to 30 degrees and then 45 degrees. In addition I armed LNAV. The FMC was still programed with the original R1 offset. The course deviation began to click down 50L; 40L; 30L etc. When we approached the course line I shallowed the intercept angle. On top of our R1 offset the previous waypoint still did not cycle and LNAV did not capture. I had to use heading select to prevent going further right of course. The other IRO and I decided the best course of action at that point was to zero the offset; put the aircraft directly on the course line and select Present Position direct to the next waypoint in front of us: S05W157. This was accomplished; LNAV captured and then I subsequently offset R1 per SOP. This all occurred still within NZZO; Aukland Oceanic Airspace. About 5 or 10 minutes later we got a CPDLC message to contact ZZZ1 on [numbers] as we were now crossing S05W157 and the ZZZ-ZZZ1 FIR. We contacted ZZZ1; got a SELCAL check and frequencies for crossing 20N. It all seemed ops normal and ZZZ2 Radio made no mention of any issues or any reports from ZZZ about navigational errors. 6 hours later all four of us discussed it some more. The Captain asserted that the L52.1 was a reading left of an extended course line between the previous two waypoints and not an actual distance left of course from the point that did not cycle; as there was a slight left jog at that point. And this may very well be the case. The flight manual does state something akin to this and says waypoints must be manually cycled which we eventually did. My contention; having just arrived on the flight deck and not having the full situational awareness of the preceding half hour; was that at a minimum; we did not know precisely how far left we were. The configuration of the automation (uncycled waypoint; erroneous FMC deviation readout) probably was not ideal for the crew change. See two attachments.

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.