Narrative:

Inbound on the glen rose arrival. Clearance to cross fever intersection at 11;000 ft. During the descent; the first officer remarked that the egpws terrain warning system was no longer working. I also noticed that a warning on the FMS system that dead reckoning was displayed. The 2 systems are related in that the egpws uses position information from the FMS system. I suspected that the navigation information we had was faulty; but all fixes on the EFIS navigation display looked normal and a quick check with DME distance to dfw looked ok. The first officer tuned in jen VOR and determined that the fever fix displayed on the navigation display was not in agreement with the VOR/DME radial/distance for the fever intersection and that we were already past it; when the navigation display showed that it was still 10 miles in front of us. The first officer immediately reported the FMS failure to ATC and we were given radar vectors from that point on. Due to the failure of the FMS; the navigation display map display was no longer valid and we missed the turn to intercept the outbound radial and the crossing altitude restriction at fever. Before landing; the FMS system returned to normal. I wrote up the malfunction in the aircraft maintenance logbook for maintenance review of the FMS system. I don't know if the system actually malfunctioned or if the satellite signals to the system were unusable. There were no traffic conflicts to my knowledge and ATC did not seem concerned. What troubles me most is that all navigation fixes were still displayed and that we relied on our navigation display when it was clearly erroneous. Other than the warning on the FMS screen; which is not very instructive; we had no idea that we were flying off course. I think that if the navigation information is unreliable due to the system shifting to dead reckoning mode; then the navigation display should no longer show the fixes and route line at all; forcing the pilots to use other means of navigation. On this particular star we could have tuned in the jen VOR as a backup to the FMS system; but it is not standard procedure to do so. Also; there are many sids and stars which have no VOR basis at all which would give no backup other than the observation by an ATC radar controller.

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Original NASA ASRS Text

Title: MD80 flight crew reports track and altitude deviations due to FMS going into dead reckoning mode during arrival. System returns to normal prior to landing.

Narrative: Inbound on the GLEN ROSE arrival. Clearance to cross FEVER Intersection at 11;000 FT. During the descent; the First Officer remarked that the EGPWS terrain warning system was no longer working. I also noticed that a warning on the FMS system that Dead Reckoning was displayed. The 2 systems are related in that the EGPWS uses position information from the FMS system. I suspected that the navigation information we had was faulty; but all fixes on the EFIS navigation display looked normal and a quick check with DME distance to DFW looked OK. The First Officer tuned in JEN VOR and determined that the FEVER fix displayed on the Navigation Display was not in agreement with the VOR/DME radial/distance for the FEVER Intersection and that we were already past it; when the navigation display showed that it was still 10 miles in front of us. The First Officer immediately reported the FMS failure to ATC and we were given radar vectors from that point on. Due to the failure of the FMS; the navigation display map display was no longer valid and we missed the turn to intercept the outbound radial and the crossing altitude restriction at FEVER. Before landing; the FMS system returned to normal. I wrote up the malfunction in the aircraft maintenance logbook for maintenance review of the FMS system. I don't know if the system actually malfunctioned or if the satellite signals to the system were unusable. There were no traffic conflicts to my knowledge and ATC did not seem concerned. What troubles me most is that all navigation fixes were still displayed and that we relied on our navigation display when it was clearly erroneous. Other than the warning on the FMS screen; which is not very instructive; we had no idea that we were flying off course. I think that if the navigation information is unreliable due to the system shifting to Dead Reckoning mode; then the navigation display should no longer show the fixes and route line at all; forcing the pilots to use other means of navigation. On this particular star we could have tuned in the JEN VOR as a backup to the FMS system; but it is not standard procedure to do so. Also; there are many SIDs and STARs which have no VOR basis at all which would give no backup other than the observation by an ATC radar controller.

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.