Narrative:

While descending on the dandd arrival into den; we were told to descend via. We re-cruised the current altitude while setting the bottom altitude in the altitude window. Somewhere close to dandd intersection; the aircraft dropped out of its vertical mode (cwa P) and; before we realized it; we descended below the 17;000 ft assigned at dandd intersection to an altitude of nearly 16;000 ft. At once I kicked off the autopilot and began to climb back to 17;000 ft; which we did before crossing the dandd intersection. Reviewing the incident; we still don't know what happened. We had it dialed in and the vertical mode reverted to cws P.since our software is not the best; and we have no aural warnings of VNAV speed or cwa P; alas; we must watch it ever more closely--like a hawk.

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Original NASA ASRS Text

Title: B737NG flight crew reported deviating from the cleared RNAV arrival altitude constraints when the FMS dropped out of VNAV.

Narrative: While descending on the DANDD Arrival into DEN; we were told to descend via. We re-cruised the current altitude while setting the bottom altitude in the altitude window. Somewhere close to DANDD intersection; the aircraft dropped out of its vertical mode (CWA P) and; before we realized it; we descended below the 17;000 ft assigned at DANDD intersection to an altitude of nearly 16;000 ft. At once I kicked off the autopilot and began to climb back to 17;000 ft; which we did before crossing the DANDD intersection. Reviewing the incident; we still don't know what happened. We had it dialed in and the vertical mode reverted to CWS P.Since our software is not the best; and we have no aural warnings of VNAV SPD or CWA P; alas; we must watch it ever more closely--like a hawk.

Data retrieved from NASA's ASRS site 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.