Date: 201808
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Light: Daylight
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: Failed
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Returned To Departure Airport; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Diverted; Flight Crew Landed As Precaution; Flight Crew Landed in Emergency Condition
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: The aircraft experienced a slow loss of cabin pressure due to the pre-conditioned air door and check valve becoming loose and ultimately departing the aircraft leaving a hole for pack air to escape. The initial takeoff and climb were normal. The event started above 10;000 feet in the climb when I noticed a pressure change in my ears. I looked at the pressurization panel and noticed the cabin was climbing at a rate higher than normal - but not excessive - at about 700 fpm. This happens sometimes with the system momentarily and then returns to normal. The rate quickly returned to about 500-600 fpm which was in the normal range. We continued the climb and decided to watch it carefully. At about 17;000 feet; I noticed a spike in the cabin climb rate again. This time it was more severe at about 1500 fpm. We checked that all air panel switches were in the correct position and that the duct pressure was good at 45 PSI. All was correct. We also checked for any abnormal indicator lights and door lights. All was normal. We then checked the ALT/ DIF schedule and realized that the cabin was 2000 feet higher than it should have been for our current aircraft altitude. At that point; we realized we had a problem and leveled off. I notified ATC that we had a pressurization problem; [advised ATC]; and we began an expeditious descent at 320 knots with speed brakes deployed to 10;000 feet. The cabin was still climbing at about 1500 fpm so we selected MAN on the controller and positioned the outflow to full closed in an attempt to slow the cabin climb. Passing through about 12;000 feet; the cabin began climbing more quickly - probably when the valve / door departed the aircraft. The cabin reached about 9800 feet when we encountered the Cabin Altitude warning horn. This occurred almost simultaneously as we reached 10;000 feet aircraft altitude. We completed the Cabin Altitude Warning Checklist and were not able to further control the cabin pressure and differential continued to drop. We notified the cabin crew; passengers; Dispatch; and [Operations] of the situation and that we would be returning to land. We completed the deferred items and diversion checklist items and landed without further incident. The passenger oxygen system was not utilized or deployed as the cabin altitude did not exceed 10;000 feet.


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


Synopsis: B737 flight crew reported being unable to pressurize the aircraft in flight.