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.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of April 2012 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.