Date: 202202
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: A319
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb; Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Air Conditioning and Pressurization Pack
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Human factors: Troubleshooting; Situational Awareness; Human-Machine Interface; Confusion; Communication Breakdown
Second reporter function: Captain; Pilot Flying
Anomaly: Aircraft Equipment Problem Critical; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Overcame Equipment Problem; Flight Crew Landed As Precaution; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Departure Airport; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft; Human Factors; Procedure
Primary problem: Human Factors

Narrative: Airplane had multiple write ups - one of which was an inop pack #2. Maintenance worked on and cleared a recurring smoke detector fault which delayed flight. The isolation valve was off and there was some discussion with Maintenance as to why it was turned off. After looking at MEL; we turned it back on. At some point during that discussion; the mechanic turned off the #1 pack. The #2 pack was left on. On taxi out (I think); we had ECAM that required us to turn off the isolation valve which we did. After departure we had several other ECAMs as well as the autopilot disconnecting several times. If I remember correctly they were: some flight control ECAM that flashed on with MW and then went away; another ECAM with smoke and air that also went away; smoke detector fault; and auto flight/autopilot with MW (Master Warning). Around FL270; CA (Captain) notices cabin altitude was 9;500 ft. We asked for 21;000 ft. initially and started descent. During the descent we got the CAB PR EXCESS/CAB ALT MW. We [requested priority handling]; donned oxygen masks and initiated an emergency descent to 10;000 ft. During the descent the airplane was in an over speed state twice. The PF (Pilot Flying) choose not to use autopilot as it was not staying connected. We were not over max landing weight and had a normal landing in ZZZ. The oxygen masks never deployed. The cabin altitude got to around 10;300 ft. at the highest. Classic hurry up and wait and then hurry up and go. Maintenance said plane probably will be out of service. I was already going to next plane in my head. Then; plane was fixed; 30ish people boarded and everyone was ready but us. We even went through MELs twice. Obviously we didn't pay attention to important details of which pack/valve we were talking about. I wish I had an answer for that as to why we didn't catch it. Slow down; repeat -Just like we do with engine masters; throttles; etc. I need to say out loud pack #2 and confirm pack #2 not just inoperative pack because it was too easy to look up and see that 'a' pack was off (even though it was the incorrect pack).


Second narrative: We arrived at the aircraft and were notified it was on a maintenance delay due to a recurring MEL 21-XXZ. After a slight delay; the mechanic indicated the aircraft was returned to service. During our Flight deck preflight; the Mechanic; myself; and the First Officer read through the MEL and configured the aircraft for our flight. The Mechanic reached up and turned off pack 1 while leaving Pack #2 on. I believe all three of us confirmed this twice; neither one of us realizing. Uneventful push back; but on taxi; we received an ECAM indication an Isolation valve fault. We worked that issue and continued on to ZZZ1. Uneventful takeoff but shortly after; while climbing through 10;000 ft. the autopilot disconnected (1 of 5 times during flight). As the Pilot Flying; I re-engaged and continued climbing. after approximately 23;000 ft.; once again the autopilot disconnected. After re-engaging; we momentarily received a Master Warning F/CTL ECAM; which disappeared in a matter of seconds. While passing through 27;000 ft.; I noticed the Cabin climbing through 9;500 ft. We asked for an immediate descent. Shortly thereafter we received a CAB PR EXCESS CAB ALT. We initiated an Emergency Descent; requested a turn back towards ZZZ and followed the ECAM procedure. Cabin pressure maxed out at 10;100 ft. Uneventful landing and deplaning at gate. No injuries/issues reported as the seat Belt sign and FAs (Flight Attendant) were told to remain seated for flights duration due to weather. Maintenance delays and confusion over the MEL procedure. The fact that this aircraft had a history of ECAMS and the recurring issue returned. Weather certainly played a factor as our route of flight took us in to the heart of the storm; with blowing snow; gusty winds and potential for icing. From Taxi to Cruise; several out of the norm warnings/events occurred; certainly a lot for a XX minute flight.A slow methodical approach to the MEL review. We took the initiative to review the procedures with the mechanics; just need to be more assertive if questions arise. The oxygen mask was scratched to the point of not being able to see much on the old 319 screens. It was a hand flown descent as the autopilot disconnected 5 times during the flight; however this led to a slight overspeed. Again; the screens are incredibly difficult to see when wearing the oxygen mask and the balancing act between getting lower and keeping the speed under control caught me on this one; over sped by 5 kts I believe.


Synopsis: Flight Crew reported confusion over MEL procedures led to a Cabin Altitude Master Warning. The Flight Crew performed an expedited descent and air turn back to make a precautionary landing.