Date: 202101
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Human factors: Training / Qualification; Communication Breakdown; Time Pressure
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: We were on the ZZZZZ1 arrival into ZZZ and we had a clearance cross the ZZZ1 VOR at Flight Level 260 and then to descent via the ZZZZZ1. We were in a managed descent with 4000 feet set for the bottom of the ZZZZZ1 arrival. Nearing ZZZ1 I thought that we were high on the profile. I selected open descent intending to level off at FL260 and then continue on profile but I did not select FL260 in the FCU window. The airplane was very light at 113;000 pounds and descended very quickly right through the assigned FL260. I intervened by leveling off and we continued in a managed descent on profile for the remainder of the arrival.My estimation that we were high on the profile was inaccurate due to the fact that the airplane was light and capable of descending very quickly; I had not taken that into consideration. The pilot monitoring was heads down and I did not tell him I was changing the mode of descent from managed to open and explain why I was doing so. Had I done so; he would have caught it. This was my first day back to work after two months of leave. I had prepared to return by studying flows; call; procedures; etc. and had briefed the other pilot hoping to prevent errors from happening.Suggestions - Keep the other guy in the loop. Tell him when making a change especially if he is heads down. Always check the altitude selected when changing modes. Always focus on the next restriction.


Second narrative: We were south of ZZZ1 arrival validation completed; arrival briefing completed clearance to cross ZZZ1 at FL260.  We were then cleared to descend via ZZZZZ1 Runway XXL.  The Captain set 4;000 feet in the FCU.  We both verified the altitude selected and the aircraft was in managed decent.  I verified the aircraft was on profile and would cross ZZZ1 at FL260.  I then updated the MCDU with the new ATIS; did an in range and verified arrival gate and set ramp frequency in the number 2 standby radio.  The Captain then said; 'I messed that up'; or something like that.  I didn't realize the Captain had changed modes to open descent and had descended through the FL260 restriction at ZZZ1.  He said he felt the aircraft was not descending quickly enough.  We crossed ZZZ1 approximately 1;000 feet low.  There was no call from ATC.  The Captain then went back to managed descent and we continued to ZZZ without and further incident.Cause - Lack of communication from the Captain letting me know he went to open descent.  Captain had been on extended time off the last 7 months.Suggestions - Communication of mode changes in decent to Pilot monitoring.  Resetting FCU to next altitude constraint when operating in open decent.


Synopsis: Air carrier flight crew reported an altitude deviation due to failed crew communication and the Captain's recent extended time off.