B737 Next Generation Undifferentiated
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 196; Flight Crew Type 9000
Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem; Flight Crew Returned To Clearance
Aircraft; Human Factors
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.
It would be nice to have better software - the aircraft constantly goes out of VNAV PATH and into VNAV SPEED for no reason; and sometimes the VNAV disconnects for no reason like it did to us today.
B737NG flight crew reported deviating from the cleared RNAV arrival altitude constraints when the FMS dropped out of VNAV.