B737 MAX Series Undifferentiated
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 250; Flight Crew Total 18000; Flight Crew Type 12000
Communication Breakdown; Situational Awareness; Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Undershoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Overcame Equipment Problem
Aircraft; Human Factors; Procedure
Inbound to ZZZ on ZZZZZ arrival and Level at FL330 the controller gave us descend via ZZZZZ Arrival.  FMC was Active in the Cruise page after a cruise descent earlier from FL 360 which is normal with a Top of Descent computed to start approximately 4 Miles prior to ZZZZZ to cross at or below FL320 as the VNAV is designed to do at IDLE Thrust.  We had 12;000 feet set in the MCP per normal FM 'descend via' procedures for the arrival.   Everything was set per FM procedures; light winds; Normal descent speeds.  Just prior to the Top of Descent the FMC transitioned from the Cruise page to the Descent Page and the throttles started retarding and the aircraft started descending.  Approximately 200 feet into the descent the Pitch Stagnated and shallowed and the Path indicator 'hung' and the Airspeed started bleeding off as the throttles retarded  instead of maintaining speed and descending on the path.  I saw the FMC/Autopilot was not acting correctly so I told the FO he needed to started inputting Vertical Speed down to make the restriction.   He wheeled in vertical speed and at that time the Path 'Unstuck' and dumped out showing us 400 feet high.  He wheeled in enough vertical speed to get the descent going but since the VNAV did its normal computation of TOD so close to the FIX we crossed it approximately 240 feet high as it was impossible to react fast enough to the VNAV glitch to stay on path.   The FO did everything right per the FM procedures.  So what happened here is a very common thing that I see about 20 percent of the time that should NOT be happening and started after the Planes were reconfigured to default into VNAV Speed from Path instead of honoring PATH even if it had to dive for it.  In this case a MAX.  Most of the time when it happens you are not right on top of a restriction so you have time to correct it. There is a occasional glitch in the VNAV system I believe tied to FMC software not aircraft specific that happens and I'm not sure why when the FMC transitions from Cruise Page to Descent page.  What Happens is the FMC hits top of descent and begins to retard the throttles and pitch over but for some reason the Pitch stagnates because the path incorrectly and temporarily shallows out instead of continuing to pitch over to descend and the result is the airspeed starts to bleed off.  After 5-10 seconds the FMC comes out of this Foggy zone and the path indicator then drops out to reflect where the plane should be and it Drops out of VNAV Path into VNAV Speed because when this happens the planes usually ends up 500 feet high.  When you have room to correct it you can get the plane back on PATH before hitting a restriction but in our case since it started down so close to the fix as its designed to do we did not have room to fix it.  The only thing you can do to possibly mitigate this from happening is to force the plane into the Descent page early by hitting the 'descent now' button; which is NOT Standard but not prohibited; to get it started down.  I have never seen this anomaly happen once the FMC is established in the Descent Page even when temporarily leveling off while it waits for the next TOD if an arrival has a level segment.  This is purely an issue during transition from FMC Cruise Page to Descent page and is very random as to when it happens and should be considered a known anomaly because I have seen it so many times.  Had we not been at FL330; 1000 feet above the FL320 restriction; and had we descended from a higher Altitude with more distance from the restriction we would have had time to correct the deviation in path but in this close in situation we were not.
Captain reported a crossing restriction was not met due to an occasional software glitch which reportedly causes the descent to shallow out temporarily.