B737-800
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Type 10000
Workload
Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Illness / Injury
Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew Overcame Equipment Problem; Flight Crew Requested ATC Assistance / Clarification; General Maintenance Action; General Physical Injury / Incapacitation
Aircraft
Upon descent the aircraft pressurization system failed; leading to a loss of cabin pressure.  Until point of malfunction aircraft operated with no observed anomalies or deficiencies. Flight was smooth and routine until shortly after reaching Top of Descent and leaving FL380. Descending from FL380 to FL300 Auto Fail light illuminated on pressurization panel. Estimate the illumination of the Auto Fail light occurred around FL370. First Officer acting as Pilot Monitoring located appropriate checklist in QRH and we commenced working through the items contained therein. Completed QRH 2.4. The Auto Fail light extinguished and we continued normal operation with panel now set to ALTN mode. Shortly thereafter there was erratic behavior in the differential pressure and cabin altitude gauge. The changes were noticeable in ears as the pressure/cabin altitude changed quickly in either direction. Scanned overhead panel and noticed the ALTN light no longer illuminated. Verbalized to First Officer that we should put our masks on. At this point estimate the aircraft was between FL360 and FL300 which was the initial floor of our assigned descent from ATC.  We both donned masks and within moments the Cabin Altitude light illuminated with corresponding warning horn and a very noticeable drop in pressure. Called for and First Officer immediately secured and began with the Cabin Altitude Warning QRC. PASS OXY ON light illuminated as well indicating Cabin Oxygen masks had deployed. Accomplished Cabin Altitude Warning QRC. Cabin altitude spiked but can't recall to what altitude. At one point; cabin descended rapidly and diff pressure climbed to redline on gauge.Outflow valve seemed to be uncontrollable; which made the onboard pressurization issue uncontrollable.  Coordinated with ATC for lower altitude and maintained an expedited descent that was essentially unbroken until landing. At completion of QRC we were directed to accomplish QRH 2.1. Used 2.1 through entire descent and uneventful landing.  During descent while coordinating with cabin crew; flight attendants informed flight deck crew that one passenger briefly lost consciousness but was awake again and being attended to by cabin crew. Notified ATC of need for EMS to meet aircraft at gate upon arrival. ATC facilitated request and local EMS met aircraft and evaluated passenger.  Upon arrival coordinated with Dispatch; both local Company Maintenance and Maintenance Control and Chief Pilot; to ensure aircraft abnormalities were properly documented in logbook; crew was taken care of and all other pertinent issues had been addressed.  I think this was an anomaly and part of flying aircraft as a business. Things break. Just glad it turned out the way it did. As far as self evaluation goes; I did forget a few items that bug me. One; I should've let the First Officer handle communication with the cabin as I ended up unnecessarily task loading myself. Also; I forgot to send an ACARs to Dispatch or radio them letting them know about the situation. We were pretty busy but no excuse. I'm pretty sure ATC notified Company if only just the local ops folks. The checklist was handled correctly but as ALTN system failed and we were directed to go to manual. The cabin pressure spiked and dropped several times and then failed. That was certainly an enormous distraction and it took us a minute to get masks on and fully reengaged with the checklist. Feel like I could've done a better job leading in those moments. Will say that I was very happily surprised by how much our training kicked in; when the event got bad. Kudos to the training folks!
A routine flight changed as we began our initial descent. Aircraft pressurization began to fail which led to the loss of cabin pressurization. Between FL380 and FL360 the Auto Fail light illuminated; along with corresponding dashes in the FLT ALT window and LAND ALT window on the landing altitude selector panel. I was the PM for this leg; and as such I completed the QRH 2.4 Auto Fail Checklist. This drove the pressurization mode selector to be placed in the ALT mode. This did extinguish the Auto Fail light and temporarily allowed for control of the outflow valve. At this point the CPT and I felt pressure changes in our heads and noticed somewhat erratic oscillations in both the cabin differential and cabin altitude. The Captain made the decision to don the O2 masks at this point. I referred back to the QRH 2.4 Checklist as the cabin altitude was now not controllable. The checklist drove us to selecting the manual mode on the Pressurization Mode Selector. The Captain communicated with the cabin crew to; as to their status and the status of the passengers. We were approaching FL300 at this point and communicated with ATC that we needed lower; due to a cabin pressurization problem.Very shortly after donning our O2 masks the CAB ALT light illuminated along with the corresponding warning horn. Captain called for QRC Cabin Altitude Warning Checklist. We were granted lower by ATC as well as handoff to APP Control. Thanks to both ATC and Approach and Tower; we were able to basically keep our descent going all the way down from cruise to 6000'; and subsequent visual approach. The passenger O2 masks deployed automatically as we worked through the QRC Checklist. The Captain; again; checked in with the Cabin Crew to verify O2 masks had been deployed as well as their wellbeing. We learned here that a female passenger had fallen unconscious temporarily. She regained consciousness as we descended. As I continued to monitor the cabin altitude and manipulate the outflow valve the CPT let APP know we needed EMS standing by for her. At the completion of the QRC we continued on with 2.1 of the QRH and stayed with this checklist for the duration of the flight. The cabin altitude depressurized; as necessary; as we descended below 10;000 feet. The approach and landing were uneventful. I contacted Operations taxiing in to the gate and verified with them our request for EMS to meet us. They had not received word about the Passenger needing assistance. I fully thought I had contacted them airborne. This must have been missed on my part in all the commotion. Nonetheless; EMS showed up within a few minutes of setting the parking brake. The Captain proceeded to contact the Chief Pilot; Maintenance; and Dispatch to proceed with the necessary paperwork and communications. The things that I see could have been improved upon are communications and understanding task loading. Verifying each of our required tasks were accomplished could have been better. As these events unfolded rapidly; it definitely emphasized the importance of keeping a good pace; usually a slower pace; as tasks have the potential to be compounded.
B737-800 flight crew reported a normal landing following the loss of pressurization at top of descent.