B737-800 flight crew reported missing a crossing restriction on descent when an FMC mode change out ...

Attributes
ACN	1573867
Time
Date	201808
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Light	Daylight
Aircraft 1
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	Descent
Route In Use	STAR ZZZ
Flight Plan	IFR
Component
Aircraft Component	Autoflight System
Person 1
Function	Pilot Flying Captain
Qualification	Flight Crew Multiengine Flight Crew Instrument Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 217 Flight Crew Type 9000
Person 2
Function	First Officer Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 498
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Overshoot Deviation - Altitude Crossing Restriction Not Met Deviation - Procedural Clearance Deviation - Speed All Types

Narrative:

Cleared to descend via the RNAV arrival; we descended from cruise in LNAV/VNAV path. Just below 10;000 ft. MSL; we were informed that we had missed [a] restriction and noticed that we were fast; and the speed window was open. As hard to believe as this may sound; we did not do anything to change the mode out of LNAV/VNAV path. We were busy in the descent with radio calls; making sure we alerted the flight attendants at 10;000 ft.; etc; and failed to monitor the automation; assuming that we were protected. I don't know if there's a way that the coding of the FMC software or some other thing could allow this; but we were perplexed.look into whether this has happened with other aircraft; and obviously we need to monitor to make sure the aircraft does what it is supposed to do!

Original NASA ASRS Text

Title: B737-800 flight crew reported missing a crossing restriction on descent when an FMC mode change out of LNAV/VNAV PATH was not noticed.

Narrative: Cleared to descend via the RNAV Arrival; we descended from cruise in LNAV/VNAV PATH. Just below 10;000 ft. MSL; we were informed that we had missed [a] restriction and noticed that we were fast; and the speed window was open. As hard to believe as this may sound; we did not do anything to change the mode out of LNAV/VNAV PATH. We were busy in the descent with radio calls; making sure we alerted the flight attendants at 10;000 ft.; etc; and failed to monitor the automation; assuming that we were protected. I don't know if there's a way that the coding of the FMC software or some other thing could allow this; but we were perplexed.Look into whether this has happened with other aircraft; and obviously we need to monitor to make sure the aircraft does what it is supposed to do!

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.

B737-800 flight crew reported missing a crossing restriction on descent when an FMC mode change out of LNAV/VNAV path was not noticed.