B737NG flight crew reported deviating from the cleared RNAV arrival altitude constraints when the FM...

Attributes
ACN	1352331
Time
Date	201605
Local Time Of Day	0601-1200
Place
Locale Reference	DEN.Airport
State Reference	CO
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Make Model Name	B737 Next Generation Undifferentiated
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Route In Use	STAR DANDD8
Flight Plan	IFR
Component
Aircraft Component	FMS/FMC
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 196 Flight Crew Type 9000
Person 2
Function	Pilot Not Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 181 Flight Crew Type 3000
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Crossing Restriction Not Met Deviation - Altitude Overshoot Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy

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.

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.