Date: 202007
Location: ZZZ.ARTCC, US
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: VFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pressurization System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Detector: Automation Aircraft Other Automation
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: At cruise; the altitude warning horn came on. We donned masks and began a descent. Ran the QRC for rapid depressurization and then went to the QRH for bleed trip off light. That resolved the pressurization issue. We attempted to contact Dispatch for fuel info at our lower altitude but ACARS would not send and ARINC had no success. Later I learned Dispatch had received our send; but we did not receive their response. We had 8000 lbs of fuel so diversion was not necessary.We had just come out of an overhang at 380 with anti-ice on. The master caution flickered on with no SAP (System Annunciator Panel) to give us an indication WHY it came on. We both were surprised at the lack of SAP but noted the center fuel pumps depleted of fuel was the likely cause. Shortly thereafter the cabin altitude horn came on. We donned the masks and established communication; which is easier said than done since the masks are terribly distracting with their loud air noises. I requested 10;000 from ATC and the FO (First Officer) descended us. As I ran the QRC the FO scanned for possible causes and discovered both bleed trip off lights were illuminated and we turned to the QRH bleed trip off procedure which resolved the issue and we began to pressurize. I have X hours in this airplane as Captain. I have never touched an AI switch in my entire career. The Captains took care of the [type aircraft] AI switches and [another type of aircraft] had automated anti-ice systems not requiring pilot action at all... I was actually proud of myself for remembering to turn it on when we entered icing conditions! I had not yet selected it off as we emerged from the weather. I am sure glad this did not happen WHILE we were still in the weather. If the SAP would have illuminated we may have had a good chance at resolving the issue before [we requested priority handling from ATC]. We did hit the recall light to see if any SAP would illuminate. None did; leaving us no reason to suspect anything except fuel pumps.The final comment in the QRH procedure for bleed trip off light gave insight to why the bleeds tripped off and pressurization was lost: Flights above approximately 350 with Wing AI on may cause bleeds to trip and pressurization loss. Wow. I am shocked I learn about this limitation of the system as the final comment in a dramatic emergency procedure. Perhaps if the aircraft is not capable of maintaining cabin altitude with Wing AI on at 'approximately 350;' then it should be limited to non-icing conditions above 'approximately 350.' Perhaps a memory limitation could be added to prohibit this.




Synopsis: B737-800 Captain reported encountering an aircraft pressurization problem and having to start a descent.