B737-700
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Undershoot; Deviation / Discrepancy - Procedural Clearance
General None Reported / Taken
Human Factors; Aircraft
We were in a -700 model 737. In the descent to FL280; when ATC cleared us to cross ZZZZZ Intersection (on STAR) at FL240. We entered the new restriction in the FMC; noticed that the aircraft VNAV PATH symbol was erratic from high to low back to high etc. Eventually it settled down; we leveled at FL280 as programmed in the Altitude window of the FMC. We had 240 in the Altitude window and both LNAV and VNAV switched engaged and confirmed in the window. Aircraft started the descent as planned from FL280 to FL240. However; we crossed ZZZZZ Intersection at FL250 instead of FL240. Still showing on VNAV PATH and right on path. That was very puzzling. Later in the flight; the aircraft while on autopilot and level at 4000 feet; decided to pitch up after an increased speed was set into the Speed window; power came up and so was the nose. I took over manually and flew the approach by hand. On the ground in Airport ZZZ1 we had maintenance come out; do bite checks etc.; could not find anything wrong. I decided to shut down the aircraft completely and restart it and do some more bite checks. Nothing found both times. I was told Maintenance in Airport ZZZ just did an update (which one I don't know) to aircraft while it was in ZZZ. I have seen multiple times now in -700 series aircraft doing the same thing; where it shows on path but the path calculated by the aircraft will not make the next altitude restriction. During the XXX XXX and now Ground School I hear people getting close to overspeeds in the descents; that make me believe there is something big going on with the descent calculations and why we may be missing altitudes. The speed indexes do increase our speeds; but that of projected 284 in the descent is not even close to be considered in the neighborhood for overspeeds. My idea is that somewhere in the software is a glitch when it comes to the descent calculations; and I have a feeling it started around the time when we were getting the descent winds from outside source and entered them into the box.
B737-700 Captain reported the FMS seems to have difficulty making crossing restrictions.