Narrative:

Deice and anti-ice in blocks. Push back and engine start then called for 'after engine start check list'. First officer asked for engine anti ice 'on'; and I called for it to be 'on'. ATC happened to call us for a runway change at that very same time. First officer answered ATC and we proceeded to reconfigure the fcp and FMS flight plan. Taxi; take off; and climb were normal. Just as we leveled at FL310; we received an ECAM alert; 'pack #1 and pack #2 fault'; which requires us to turn both packs off. Followed ECAM and realized we would depressurize; asked ATC for lower altitude of 10;000 ft and to return to ZZZZ. Started our descent and a turn back to ZZZZ. Passing around FL220; I scanned the overhead panel; while the first officer worked the FMS; and noticed the APU and APU bleed air valve were still 'on'. I turned the APU bleed air valve off and the aircraft pressurized back to normal. Requested ATC to level off at FL200; checked fuel status; and we decided to continue to [destination]. ATC granted us clearance back on route and a climb back up to altitude. No emergency was declared and ATC was very helpful. As I described the event above; our normal flow pattern was interrupted and we missed some key items on the 'after start check list'.this was a situation with an easy fix. It could have turned out much different. Checklists are important; not only for normal operation of aircraft systems; but for safety itself. I am just as much to blame for not scanning the overhead panel before takeoff and assuming check lists [and] switches are complied with and not confirming that they are. I will have to make an effort make sure I scan the ECAM memo page and overhead panels before each take off. I wouldn't be surprised if this scenario showed up in one of our [training] events.

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Original NASA ASRS Text

Title: A300 flight crew reported experiencing a pressurization anomaly just after reaching cruise as a result of not fully configuring the system because of distractions during preflight.

Narrative: Deice and anti-ice in blocks. Push back and engine start then called for 'after engine start check list'. FO asked for engine anti ice 'on'; and I called for it to be 'on'. ATC happened to call us for a runway change at that very same time. FO answered ATC and we proceeded to reconfigure the FCP and FMS flight plan. Taxi; take off; and climb were normal. Just as we leveled at FL310; we received an ECAM alert; 'Pack #1 and Pack #2 fault'; which requires us to turn both packs off. Followed ECAM and realized we would depressurize; asked ATC for lower altitude of 10;000 ft and to return to ZZZZ. Started our descent and a turn back to ZZZZ. Passing around FL220; I scanned the overhead panel; while the FO worked the FMS; and noticed the APU and APU bleed air valve were still 'on'. I turned the APU bleed air valve off and the aircraft pressurized back to normal. Requested ATC to level off at FL200; checked fuel status; and we decided to continue to [destination]. ATC granted us clearance back on route and a climb back up to altitude. No emergency was declared and ATC was very helpful. As I described the event above; our normal flow pattern was interrupted and we missed some key items on the 'after start check list'.This was a situation with an easy fix. It could have turned out much different. Checklists are important; not only for normal operation of aircraft systems; but for safety itself. I am just as much to blame for not scanning the overhead panel before takeoff and assuming check lists [and] switches are complied with and not confirming that they are. I will have to make an effort make sure I scan the ECAM MEMO page and overhead panels before each take off. I wouldn't be surprised if this scenario showed up in one of our [training] events.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.