Date: 201512
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Light: Night
Aircraft: B737 Undifferentiated or Other Model
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 System
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: On climb out as PF we got a cabin altitude warning passing 11;000. I immediately initiated a level off and then a controlled descent back down to 10;000 feet to analyze the problem. I don't think the aircraft was above 11;500 feet. Checked the bleeds and packs on. Saw the cabin altitude just above 10;000 feet. Then saw the pressurization system was in manual. Switched the system to auto and once the cabin began to descend continued our climb. I did not run the cabin altitude warning checklist as the problem was immediately identified and corrected. We debriefed the errors after reaching cruise altitude.Several errors led to this event occurring which started with the PM being late after missing a wakeup call. We arrived at the aircraft late; but had 30 minutes before scheduled push. I told the PM to take his time and that we had plenty of time to get things done. The aircraft was a Remain Over Night (RON) and with the cold temperature the outflow valve should have been mostly closed with the system in manual (the first missed item in FO preflight). We completed the preflight; briefings and started checklists about 5 or 6 minutes prior to push. In no hurry we ran the before start checklist during which I looked and responded with the pressurization set up correctly. It was quite dark; but I thought I saw the pressurization mode switch in auto vs manual (our second missed item). We then finished the rest of the checklist and pushed on time to deice. Taxi; takeoff and initial climb were normal. PM ran the after takeoff checklist. At 10;000 feet I ran my procedure and the PM stated cabin altitude was good. I glanced at the gage; but don't remember what it indicated; but I thought it was ok (our third missed item leading to the warning). A short time later the cabin altitude warning went off.Looking back I attribute this to my own fatigue. I had been sick for a few days prior and used two days of sick leave in addition to my 2 days off before this trip. I felt much better and ready to fly so I stayed on the schedule. Day one was a 3 legs. I went to bed at the layover location at a normal time expecting 8 hours of sleep. I felt ok; but tired and ready for sleep. The room was very hot; but managed to find a manual shutoff for the baseboard heater. After about 3 hours of sleep I woke up coughing. I slept off and on getting up several times before the early morning alarm. After getting ready I was concerned with the 12.5 hour day ahead; but felt okay to continue. As the day progressed I started feeling poor; coughing more and getting tired. After missed several radio calls (one of my first personal signs of fatigue) on my PM leg I started to think about getting off the flight. At the end of the second leg I discussed how I was feeling with the FO and decided to call in sick. I know now my own performance was degraded due to being tired and the missed items leading to a cabin altitude warning were directly related to inattention to detail brought on by fatigue.




Synopsis: During preflight checks; the auto pressurization switch was in manual. During climbout the cabin altitude warning system activated. Switch was placed in auto and system returned to normal operation.