Date: 201808
Local time of day: 0601-1200
Location: ZBW.ARTCC, NH
Flight conditions: IMC
Light: Daylight
Aircraft: A321
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZBW
ATC / advisory: Center ZBW
Aircraft component: FMS/FMC
Component problem: Design; Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Fatigue; Human-Machine Interface
Second reporter function: Pilot Not Flying; Captain
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Contributing factors: Human Factors; Aircraft
Primary problem: Ambiguous

Narrative: I was Pilot Flying. We were assigned to descend via the JFUND2/Runway 04R transition. We had separately confirmed all constraints in the FMC; and I had briefed the arrival. We had left FL270 prior to the first altitude restriction using managed descent. The first constraint was at or above FL240 at MNSTA. We were in both managed airspeed and managed vertical path. I was reviewing the taxi plan and gate information when we noticed the aircraft had descended below FL240 about 800 feet prior to the fix. I immediately selected VS (Vertical Speed) and then initiated a short climb to FL240. We continued to used managed descent and the aircraft met all [other] constraints. I have no idea why the aircraft did not perform as expected and desired. This was the way we were trained to fly the Airbus. We were tired and that could have contributed to the problem. Pilots should be trained not to trust the managed descent mode or not use it.


Second narrative: I'm aware of some of the weaknesses of the A320 [series] aircraft in managed descent; but I had never seen this happen. However; pilots I've flown with who have much more time on it than I have warned me of this potential. When we left FL270 'ALT CNST' was armed; and we both fully expected the aircraft to honor the FL240 restriction. I do not understand why it did not; but I know why I missed it. Divided attention; combined with fatigue at the end of a trip; and the timing of a frequency change took us both a step further out of the loop of careful monitoring. We both caught it quickly; and the rest of the arrival the managed descent mode behaved as expected; so it did not appear to be a maintenance problem. A more careful selection of the time for me to check back for write-ups; while we were still level; would have most likely avoided this problem. Divided attention; especially when tired and already a little slower; created the situation where my monitoring was not up to the requirements. As for the aircraft/PF (pilot flying) interface; I've seen the technique of using VS (Vertical Speed) and setting each individual altitude for at or above restrictions; but that is not the primary method taught on this fleet. I'm still unsure of the technical part of why the aircraft continued below the restriction.


Synopsis: A321 flight crew reported the aircraft failed to honor a properly programmed altitude constraint on descent in managed speed and managed vertical path mode.