A321
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Confusion; Human-Machine Interface; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem
Aircraft; Human Factors
During arrival preparation both the Captain and I checked all the crossing altitudes to be correct and verified that there was no discrepancy on predicted flight path. When issued descend via clearance; I put in 4;300 feet. At top of descent I started down. As we approached [the] waypoint I saw that the aircraft was going to fly below the required altitude and immediately hit ALT Hold to stop the descent. Even though the FMC (Flight Management Computer) had the correct altitudes in; AND the PATH showed we would cross in the required altitude window; the aircraft was diving low. ALL was done correctly and the FMA (Flight Mode Annunciator) windows indicated all was correct; the aircraft was doing its own thing. This arrival descends through high rocky terrain. Had this threat not been trapped; a GPWS event was a real possibility. Even though I hit ALT HOLD as soon as I saw what was going to happen; we still went approximately 300 feet low on the crossing window.Teach aircrews to never trust the Airbus programming and maintain constant vigilance. Even though I am very comfortable on the Airbus this was a new one on me.
A321 First Officer reported that even though the FMS was correctly programmed the aircraft descended below the entered altitude.