B767 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Total 20796; Flight Crew Type 9034
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Flight Crew Overcame Equipment Problem
Aircraft
Upon returning to the cockpit from first break; the Captain provided me with the standard relief pilot change briefing. After that; he told me about an anomaly that they experienced upon coasting out at RESNO. Evidently; after executing R1 the FMC dropped the active waypoint and the aircraft turned right to intercept the offset in LNAV. The aircraft did not intercept the outbound course and continued to turn to the north. Both pilots recognized the anomaly and quickly intervened with HDG SEL keeping the aircraft well within GNE limits. They noted that the active waypoint was N5430 and redirected the aircraft to N5520 which had dropped out of the FMC. After confirming that the aircraft was back on track and navigating properly; the clearance was reconfirmed with both EGGX and Shanwick. The three of us formally debriefed after the flight and later the Captain provided us with feedback on NAV anomalies.
Upon coast out at Resno; we entered an offset of R1; the FMC then dropped the next waypoint and never caught the offset (LNAV course) and continued to turn to the right. We put the aircraft to HDG SEL to stay on course and had to reinstall the next waypoint N5520. Needless to say all class II procedures were followed; and the routing prior to coast out was confirmed at least three times. The aircraft began heading to the North; and should have headed to the SW towards the now ACTIVE waypoint of N5430; it should have been navigating to N5520; (N5520 had dropped out). We were well within GNE limits; however we did need to confirm routing with EGGX as they called us not more than a minute later. We were very busy entering the proper course and navigating the aircraft in the correct direction. We advised Shanwick to standby while we BOTH re-entered the correct routing and course. Once we corrected course; we then called Shanwick back. We had just been switched over to the HF frequencies with Shanwick control and had just tuned in HF 8879. Shanwick simply wanted to confirm our routing after RESNO. We believe this to be an anomaly. The aircraft turned North off course and dropped out the next waypoint. Please research. Further inquiry revealed possible anomalies in the Pegasus FMC/honeywell data base.
B767 flight crew reported upon coast out at RESNO an offset of R1 is entered and the FMC dropped the next waypoint. The FMC never caught the offset (LNAV course) and continued to turn to the right. The next waypoint had dropped from the FMC.