Date: 202006
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: Climb
Airspace: Class E ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pressurization System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument
Human factors: Confusion
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport
Contributing factors: Aircraft; Human Factors
Primary problem: Ambiguous

Narrative: We were climbing out of ZZZ on our way to ZZZ1. Passing through 12;500 feet we received a Master Warning-Cabin Press. I immediately looked up thinking either the bleeds or packs were off; but the bleeds were both on and the packs in auto. We donned our masks. Once our masks were on we accomplished the Cabin Press Warning checklist (IAC) and asked to level off. The First Officer was flying and I asked him to also take over the radios while I moved on to the Pressurization Failure checklist in the EAC. We started back towards ZZZ during this checklist and started down to a lower altitude; setting up for the RNAV approach to Runway X which was in use. I called back to the flight attendants and explained what was going on and also asked them to give the passengers a brief of the return as I didn't want to make them uncomfortable hearing me try to talk with the oxygen mask on. The First Officer called ops and let them know we were coming back. We flew the approach; landed and taxied to the gate with no further issues. When we pulled into the gate I made a PA explaining to the passengers why we returned and asking them to go back to the terminal while we figured out the plan. On the ground I called Maintenance; Dispatch; and a duty officer. Maintenance confirmed with us if any CBs were popped; they weren't. They asked about the position of the bleeds; and packs which I said were on. All in all everything went smoothly and safely and we agreed to continue on; got swapped into a new plane in ZZZ and continued to ZZZ1. The only thing I can think of is that looking back I wasn't 100 percent sure if on the EAC checklist when it says to go into manual mode if it was originally in auto or manual. I remember on my flows thinking it looked like it was in auto but when I did the checklist I can't remember physically switching it to manual. I mentioned that to Maintenance on the phone. They asked if it was in dump mode because it was acting like the aft outflow valve was wide open as the cabin was at the same altitude as outside. It definitely was never in dump mode; but I just can't remember switching it from auto to manual so there was a chance the cause was because it was in manual all a long but I can't say for sure either way.That plane had been sitting for a while and wasn't quite taken out of storage as on that same flight we discovered the pitot 1+2 heat CBs were popped and was told on the phone by Maintenance that this was part of their storage procedure and that they had forgotten to push them back in. I'm wondering if the pressurization issue was also something similar that got forgotten when the plane was taken out of storage. All in all I felt we worked good as a crew and my first officer who is very close to upgrading did a fantastic job and I can tell he will make a great Captain. This was my first time flying in two months due to the coronavirus slow down but he was on the ball and I'm definitely thankful I did some chair flying or else I would have been way rustier. If I were to do it over again I might have tried to contact Dispatch in the air instead of once we got on the ground. I could have done a better job at keeping them in the loop; it just all happened so fast. I could have asked for some delay vectors or something.




Synopsis: Air carrier Captain reported receiving a cabin pressure master warning during departure and executed a return to departure airport.