A Pegasus equipped B767 transitioned to heading hold and dropped the active waypoints on an arrival after only the arrival Runway was changed. An apparent database or programming flaw is believed to be the cause.
Synopsis
A Pegasus equipped B767 transitioned to heading hold and dropped the active waypoints on an arrival after only the arrival Runway was changed. An apparent database or programming flaw is believed to be the cause.
Narrative
We experienced several apparent anomalies while operating the FMC. While in cruise we selected our best guess for an approach into DFW and then inputted the appropriate crossing altitude/speed restrictions. We then selected another approach; only to find that all the previous crossing restriction information had cleared and computer generated default data was back in place. We then inputted the correct crossing restrictions as before (all this on the CQY arrival). Approaching DIETZ intersection we were given expect Runway 17L. We had anticipated Runway 17C so we selected the RNAV to Runway 17C (for practice) and to our amazement found several things. First we were somehow in heading mode we had no active waypoint ahead of us. We had no active waypoint ahead of us. The active waypoint was now CQY some three or four waypoints behind us. We quickly re-established direct DIETZ and continued per normal. There was no departure from the arrival track. I many years of experience with FMC aircraft; I have never seen a problem like this. It is inconceivable that selecting a different runway; separate from the arrival would cause any of the above problems. Be most certain; there was a route discontinuity between HORNZ at the end of the arrival and any of the approached selected. We were fortunate that all of this happened on a nice relatively VFR day; not under stress or IMC which might well have delayed the recognition of the problem. We believe there is a flaw in the database or programming of the FMC on this aircraft and further; there may be a problem with like equipped aircraft.
Second reporter narrative
Flying a flat panel Pegasus aircraft approaching DIETZ; right autopilot on; LNAV engaged; on the CQY6 arrival into DFW. We thought it odd that changing the approach would disengage the LNAV arrival being flown when only the approach to landing runway was changed. Nothing was said by ATC; and less than a minute elapsed while briefing the new approach and correcting the lateral navigation. Check for this anomaly with navigation packages. Subsequent flight on non-Pegasus did not alter arrival when landing runway changed.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.