Date: 201712
Local time of day: 0601-1200
Location: ZZZ.TRACON, US
Light: Dawn
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Pressurization Control System
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Human-Machine Interface; Confusion; Communication Breakdown; Troubleshooting
Second reporter function: Captain
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew Returned To Departure Airport; Flight Crew Overcame Equipment Problem
Contributing factors: Human Factors; Procedure
Primary problem: Human Factors

Narrative: After starting both engines the FO (First Officer) accidentally left one of the packs off without noticing. Neither Pilot noticed this during the Before Takeoff Checklist. It was a short taxi and the Crew was cleared to line up and wait upon reaching the runway. At some point around the lineup and wait clearance for Runway to 22L; the Captain noticed one of the packs off and erroneously turned off the other pack thinking he was turning the one that was off to on. The SIC was not aware any of this had happened so therefore unable to crosscheck. When climbing through 10;000 ft for 15;000 ft and executing the Climb Checklist; the FO pointed out that the differential did not look right. It was around 'zero' and should have been at 'four'. It may have been helpful if the FO articulated this better. The Captain looked at it and decided that it would catch up; neither Pilot thought to look at the packs and bleeds to verify proper position. Shortly after the Climb Checklist; both Pilots had started to notice discomfort in the ear at about the same time the Cabin Altitude light and warning horn came on. The FO notified ATC of a pressurization issue and the need to descend. Both Pilots donned their oxygen mask and the Captain took over the aircraft and the radios while the First Officer executed the QRC and the QRH. The aircraft was descended and vectored back towards the airport while the checklists were completed. The Captain [advised ATC] and the Crew executed the visual back into ZZZ. While executing the approach portion of the Deferred Items Checklist in the QRH; it was noticed that both packs were off around 4000 ft.  The Crew elected to continue to the airport since the Passengers and ATC had already been notified. During descent; the SIC attempted to modulate the pressurization manually per the checklist and probably could have done a better job. The APU and bleed was turned on during parking and made for an unneeded pressurization on the ground probably adding to everyone's discomfort as the door was attempted to be opened.


Second narrative: [Report narrative contained no additional information.]


Synopsis: B737 flight crew reported difficulty in managing the pressurization system.