Date: 201211
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Dusk
Aircraft: A321
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: FMS/FMC
Component problem: Design; Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Human-Machine Interface; Situational Awareness
Anomaly: ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented
Contributing factors: Chart Or Publication; Aircraft; Human Factors; Procedure
Primary problem: Procedure

Narrative: The ZZZZZ RNAV Arrival continues to be a challenge to both pilots and controllers. Knowing in advance of previous altitude deviations on the ZZZZZ; we both reviewed and discussed the arrival and speed constraints. The FMS was loaded/verified correctly for the south transition to RWY 23. After crossing ZZZZZ1 waypoint at the correct altitude towards ZZZZZ2; ATC assigned us a 190 heading; speed 270 knots; maintain 13;000 feet. We complied. After about 30-45 seconds; ATC re-cleared us direct ZZZZZ1; resume the ZZZZZ Two. With the ZZZZZ1 constraint of 260 knots and 13;000 feet loaded and verified in the FMS; we set the 6;000 foot bottom altitude at ZZZZZ3; entered a managed descent in the FCU; direct to ZZZZZ1. EVEN WITH THE 13;000 foot CONSTRAINT LOCKED IN AT ZZZZZ1; THE AIRPLANE BEGAN AN EARLY UNEXPECTED DESCENT! By the time I realized the early descent and selected vertical speed 'zero'; the airplane had descended 500 feet. The Controller 'Supervisor' took over and told us to maintain present altitude; re- cleared us to ZZZZZ4 waypoint; resume the arrival. We busted the ZZZZZ1 constraint altitude by about 500 feet. After ATC re-cleared us direct to ZZZZZ1 maintain 13;000 feet; resume the arrival; both pilots verified all altitude and speed constraints properly loaded. (A magenta constraint of 260 knots and 13;000 feet showing on Nav Display). PF put 6;000 feet bottom altitude in FCU and commanded a managed descent. The airplane began to descend early; approximately 2-3 miles east of ZZZZZ1. The PM noticed the descent first and brought it to my attention. I then selected vertical speed zero on FCU to stop descent. The obvious reason this occurred was that we did not stop the early descent soon enough. However; when we were vectored off the arrival just past ZZZZZ1 waypoint; this was the second or third time we were taken off the ZZZZZ RNAV Arrival and re-cleared to a new waypoint to rejoin the arrival. This constant vectoring on and off the arrival only leads to confusion and sets up the pilot for an altitude and/or speed deviation. Pilots need to maintain constant vigilance of what the airplane is 'actually' doing; not what you 'expect' the airplane to do. Also; controllers should assign the arrival and then leave us alone. If the situation does not allow a normal descent on any RNAV arrival; take us off the arrival and assign us headings; altitudes and speeds to the approach fix or airport.




Synopsis: An A320 (Series) flight crew failed to note when their autoflight system began a descent to comply with a subsequent crossing restriction on the RNAV STAR to airport prior to crossing their active waypoint; at the prescribed altitude. A contributing factor may have been ATC's vector off the arrival and then re-clearing them to the problem fix and to resume the STAR while they were already level at the charted crossing altitude.