B737-800
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 217; Flight Crew Type 9000
Situational Awareness; Workload
Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation - Altitude Crossing Restriction Not Met; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance
General None Reported / Taken
Human Factors; Aircraft
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!
Cleared to descend via Arrival for Runway XXL. Captain selected ALT INT to start descent early due to light chop at current altitude. Nearing [the] waypoint; I reviewed STAR/approach plates and FMC LEGS page approach points to confirm points since I was expecting a further approach clearance by now or very soon. I briefly glanced at the altimeter in time for the 1000 ft. prior call then went back to look at the plates. I did not scan aircraft position in relation to the arrival. Leveling at bottom of descent at 9000 ft.; Approach Control queried if crew was given a different clearance other than a via clearance. We then realized the crossing restriction was not met and at some point the speed window became open. At no time did l see the Captain select another MCP (Mode Control Panel) vertical mode.Ensuring continuous monitoring of the aircraft on the arrival. Don't become distracted too long on reviewing [Approach] plates and FMC LEGS page. After a long uneventful leg; don't be a victim of automation expectation bias and complacency.
B737-800 flight crew reported missing a crossing restriction on descent when an FMC mode change out of LNAV/VNAV PATH was not noticed.