Date: 202012
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Autopilot
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Situational Awareness; Communication Breakdown; Distraction; Training / Qualification
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Aircraft; Environment - Non Weather Related; Human Factors
Primary problem: Human Factors

Narrative: Captain (CA) was PF and First Officer (FO) was PM. We had a crew alert action of AUTOFLT FCU1 failure not far from our initial descent for the STAR. CA was working the crew alert issue and Center called saying what the CA heard was 'Flight ABC'; Direct ZZZZZ descend via ZZZZZ1' (CA was in the QRH; reading about or looking for the crew alert issue).  After talking with the FO on the ground in ZZZ; after the event; the FO thought the read back something like 'Flight ABC direct ZZZZZ for the ZZZZZ1.'  Then; PM made sure PF was inserting the bottom altitude of STAR in the FCU (Meaning PM understood the clearance to descend via the ZZZZZ1).  PF did insert the bottom altitude. Then; shortly afterwards; we hit some moderate turbulence and PF decided to start descend early because he could not read the QRH; hoping to get into some clear air.  Then we were told we descended and there may have been a flight deviation.  I did not think so at the time because of the clearance I understood.I believe there were several causal factors on this event. 1) Pilot Inexperience.  The FO was on the Type X aircraft and in March was awarded the Type Y aircraft.  But from March through December; all the FO did was go to Type Y training; and IOE.  This was the FO's first flight since IOE. The FO was going very slow making sure no mistakes were made. The PF was also going very slow to ensure all FO tasks were completed.  2) Task saturation (crew alert event; pilot inexperience; turbulence and clearance understanding). Combined; this put both pilots in a 'caution situation.'  The PF; was trying for something like this would not happen from the moment the flight started by making sure he was slow and deliberate on all his thoughts and actions. 3) Turbulence event. We descended early because PF had a hard time reading the QRH. 4) PF flying and completing crew alert actions.  In this case; it may have made a difference; but; I do not believe it would have made a difference; because the PM understood the clearance from Center was a clearance to descend Via;   and when the turbulence started; the person working the crew alert action would have said descend to get out of the turbulence the QRH could be read. The same outcome would have happened.Suggestions - Verify the clearance; no matter the circumstances...In this particular event; the PF should have verified the clearance knowing the proficiency of the other pilot may not have been 100%.  A pilot may be 'Current' with regards to the official written documents approved by the FAA; but they may have not been proficient...(I do not know how this can be fixed...)


Second narrative: In preparation for descent; Status Page showed an inoperative system the CA/PF noticed. CA was in the process of investigating the crew alert message that caused it. We had a seasoned CA in the Jump Seat who was assisting in the lookup for the crew alert actions; if any; and the code was send to Maintenance. In that time; we received a call from Center to proceed 'direct to ZZZZZ for the ZZZZZ1.' I read the clearance back 'direct ZZZZZ for the ZZZZZ1 arrival'. I then conveyed to the CA/Pilot Flying 'direct to ZZZZZ; we can descend via the ZZZZZ1' and I backed CA up to set direct ZZZZZ in the FMS and dialed in the lowest altitude in the arrival for south landings. We verified the altitude. While inbound to ZZZZZ; we were at FL390 and encountered some occasions of moderate turbulence and continuous light chop. The CA/PF commenced a descent to avoid remaining in the turbulent conditions. I contacted the Center to let them know we were descending out of FL390. They called back to ask what was the reason for the descent. I stated we had encountered moderate to light turbulence and needed to descend to avoid it. They called back asking me to confirm the reason for descent was turbulence. I confirmed. Then; before we were handed off to the next frequency; the controller asked us to contact a number for possible altitude deviation. There was a misunderstanding; on my part; of the vertical clearance in the call with Center with the initial clearance for ZZZZZ1 and I passed that misinterpretation on to the CA/PF.It was my first flight in over a month; preceded only by sim events way back in September and just prior to OE. This was a threat up front. Hadn't flown in this environment in over 18 months. We had multiple distractions due to the crew alert and system loss being addressed; icing conditions and turbulence. The ATC frequency was covering high traffic volume so there was a significant amount of talking on the frequency. We had both been up very early in order to be ready at report time. My expectation bias led me to believe we were cleared to 'descend via' when we were cleared only laterally for the approach.We could have briefed the arrival prior to addressing the crew alert and done a transfer of control of the aircraft and radios. I could have had ATC repeat the clearance; once the PF was back with me; so that they could communicate directly what the intended clearance was. I could have stated the clearance exactly as it was read back to eliminate any 'single point' of interpretation; or misinterpretation.


Synopsis: Air carrier flight crew reported an altitude deviation and cited the First Officer's lack of flying as a contributing factor.