Date: 202004
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pressurization Control System
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Critical
Detector: 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 Landed in Emergency Condition; Flight Crew Diverted
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: F/O (First Officer) and I noticed a series of 'thumps' while in cruise at FL410. We suspected pressurization and noticed cabin climbing slowly at 500 fpm. Cabin did not stabilize and was just over 9000 feet. We decided to start a normal but rapid descent to a lower cruise altitude. We got the Cabin Altitude aural alert and warning light during the descent. Accomplished QRC/QRH; which required an emergency descent and deployment of cabin O2 masks. Notified ATC and got clearance to 14;000 feet. Passing about FL230 we noticed cabin was below 9000 feet and stable. We stopped descent at FL210 and diverted to ZZZ. Checked with F/As (Flight Attendants) to make sure there were no injuries and to advise that they and passengers could remove O2 masks. The rest of flight was uneventful. Preventative Measures: F/O suggested that we start down as soon as we saw the cabin climbing after the thumps. I wanted to see if cabin would stabilize since the rate was slow. If we started down earlier; we may have avoided the emergency descent/masks since the cabin stabilized at FL230.


Second narrative: We noticed a thumping/ vibrating sound. It sounded like something that had to do with pressure; as in an air pressure change in the aircraft. Associated with that noise was a cabin climb of about 500 fpm on the cabin rate of climb indicator. We were at 41;000 feet and had been discussing starting a descent as soon as we could; since the cabin was definitely climbing. We called Center and started our descent when the cabin reached about 9000 feet we were already on our way down. As we were descending through about FL390 the cabin reached 10;000 feet we donned our oxygen masks; went through the QRH; complied with the required items and continued our descent to 14;000 feet. ATC was great; they asked questions; but not in a rushed 'need an answer right this second' kind of way and were very helpful in our divert plan. Our cabin stabilized passing through 23;000 feet and was trending down to a lower cabin altitude. When things settled down; we talked to the flight attendants to make sure they were ok and get a status check on the passengers. The flight attendants were fantastic; definitely handled this event professionally and took care of passengers above and beyond. The passengers that we interacted with hardly seemed rattled or concerned at all; I feel this was likely due to how the cabin crew handled the situation. We landed in [diversion airport]; taxied to the gate; shut the aircraft down and went on about our day.


Synopsis: B737-700 flight crew reported an uneventful emergency descent & diversion due to a loss of pressurization during cruise.