Date: 202104
Local time of day: 1801-2400
Location: ZZZZ.Airport, FO
Flight conditions: IMC
Weather elements / visibility: Thunderstorm; Turbulence
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Aircraft component: Pressurization System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Workload
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Overcame Equipment Problem; Flight Crew Diverted; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Flight ZZZZ - ZZZ1 (all nighter). Runway XXL departure IMC conditions with TS (thunderstorms) in terminal area. Crew briefed to remain seated due to expected turbulence on departure. Passengers briefed in welcome aboard announcement that the Flight Attendants would remain seated due to rain showers and bumps on departure. Performance for A/C weight of 145k; Flaps [set]; Runway XXL bleeds on reduced thrust T/O. Normal T/O executed with a climb to cleared altitude FL230 commenced. Passing approximately 11;000 [ft.; high elevation airport]. Cabin Alt Red warning light illuminated and horn sounded. Horn silenced - pressurization panel checked - and confirmed correct positioning of switches as terrain noted and communication coordinated with ATC for 'safe' altitude and significant weather avoidance required. Aircraft climbing at normal rate and leveling at 11;000 [ft.] was not an option. O2 masks donned and immediate action items of QRC executed. Remainder of checklist executed as Flying Pilot coordinated deviation with ATC and Captain with regard to safe altitude and course for primarily terrain avoidance and significant weather avoidance as a secondary threat. 17;000 ft. deemed to be safest altitude based on position of aircraft. Requested and set. Executed checklist items concurrently with NO resolution to regaining aircraft pressurization. All indications were normal; but obvious that aircraft was not pressuring at all. Plan determined by flight crew to coordinate safe corridor passage through the 'valley' - again based on terrain primary and weather secondary. Coordination with FAs indicated that passenger O2 had deployed automatically (and of note was deployed manually too during procedure); and that crew and passengers were 'safe'. ACARS NO COMM was illuminated likely due to A/C altitude and mountains blocking signals. Crew coordinated for lower altitudes as safe MORA (Minimum Off-Route Altitude) conditions permitted to account for passenger O2 eventually being depleted. Initially 15;000 [ft.]; then 12;000 [ft.] coordinated to attempt to get cabin altitude below 10;000 [ft.]. At 12;000 ft. with the pack switches selected to high - crew was able to get the cabin altitude to below 10;000 ft. On initial communication with the cabin crew - communication with the forward cabin was nearly impossible due to loud noise by door 1L (our apparent source of no pressurization). Crew communication conducted via aft galley inter phone to determine crew and passenger status. Cockpit crew determined return to ZZZZ not best course of action due to terrain; weather; heavy weight of aircraft and passenger O2 depletion. ACARS comm with Dispatch established as determination for diversion and potential diversionary airports weighed. Both ZZZZ1 and ZZZZ2 considered. ZZZZ1 determined to have more threats due to lack of familiarity; time and rising terrain by the airport. ZZZZ2 considered and subsequently determined (in coordination with Dispatch) due to familiarity; the fact that we have been routinely using it as an alternate for ZZZZ flights; safe passage from our current location to airport at 12;000 ft. and relatively easy weather avoidance to get to airport. Aircraft integrity was addressed and deemed not to be an issue to choose the more distant diversionary airport. Coordination with ATC-1 and ATC-2 Control completed to receive priority handling (even though very few aircraft were in the sector). Coordination with Dispatch continued throughout with updated runway data; weather and ZZZZ2 [company] ground operations taken care of. Clearance given to descend for the ILS [Runway] XXR approach to ZZZZ2. Crew understood and briefed overweight landing procedures for ZZZZ2 having previously agreed that the threat of landing at 133;000 lbs outweighed the threat of remaining airborne for another 45 min to burn down below max landing weight. Of note; on descent through 7;000 ft. a loud sound [was] heard in [the] cockpit - aircraft integrity deemed safewith no abnormal engine indications. Aircraft executed a normal approach and landing to Runway XYR in ZZZZ2. Upon arrival maintenance items entered for O2 deployed; aircraft pressurization; and overweight landing. Upon post flight it was determined the aircraft had at least two bird strikes which were also entered in maintenance log book. Maintenance quickly determined that door 1L had failed to seat properly but had given a closed condition indication (never any door indication illumination in the cockpit). Challenges existed in ZZZZ2 for both crew and passengers due to aircraft flight originating in [foreign country] with current COVID restrictions in place and requiring adherence.




Synopsis: B-737 Captain reported executing a challenging mechanical diversion due to pressurization failure during climb. The diversion was concluded with an uneventful overweight landing.