B757 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 250; Flight Crew Total 26500; Flight Crew Type 15000
Confusion; Distraction; Human-Machine Interface; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Flight Crew FLC Overrode Automation; Flight Crew Returned To Clearance; Flight Crew Took Evasive Action
Aircraft
While cruising at FL370 the aircraft crossed the active waypoint; KURSE; and then made a hard right turn back to it. The subsequent waypoint; ONL; remained inactive on the HSI and disappeared from the FMC legs page; with the following waypoint; FOD; appearing at 1L. We used heading select and direct intercept to ONL to get back on course. ATC was notified. The POS REPORT page showed crossing KURSE nine minutes later than we actually did. Actual time XX:01; on POS REP page it was XX:10. Anomaly appears similar to that indicated in the Pegasus Bulletin.
The reporter stated that this is his second exposure to the PEGASUS uncommanded turn while navigating in LNAV. The first time; several years ago; he assumed that a flight crew error occurred. During this event; he was looking directly at the HSI and became very confused by the occurrence but knew very quickly what had happened. The HSI Map displayed the correct track KURSE the last waypoint; then ONL; FOD. However; a discrepancy existed between the HSI Map; and the PROG and LEGS pages. The LEGS Page deleted ONL and showed FOD as the next fix. The PROG Page kept KURSE as the active waypoint and showed an arrival time of XX:10 even though the aircraft had physically just crossed KURSE at XX:01. The HSI Map displayed the correct information with a magenta line but the aircraft commenced a right turn back to KURSE. The reporter was on either the 160NM or 360NM scale and could not see if the Map actually drew a circle track back to KURSE.
A B757 crossed the active waypoint; KURSE; and instead of the FMC sequencing to the next waypoint; it began a right turn back to KURSE which remained the active waypoint. The LEGS page skipped the next flight plan waypoint and a magenta track line was drawn to FOD. The HSI Map had the correct track fixes but retained KURSE; then ONL and FOD.