B737-800 flight crew reported pressurization system malfunction during cruise flight. Flight crew diverted and landed safely.
Synopsis
B737-800 flight crew reported pressurization system malfunction during cruise flight. Flight crew diverted and landed safely.
Narrative
Flight ABCD was scheduled for service from ZZZZ to ZZZ. The aircraft experienced pressurization issues and required diversion to ZZZ1. Details of the event from the FO perspective are below.When I arrived at the gate for Aircraft X; the gate agent informed me that she would come with me to open the aircraft. The aircraft was very cold (-9 degree C outside with 15 kt winds depicted by ATIS) so I elected to turn on the APU for additional heating. I conducted the exterior inspection and found everything to be satisfactory in accordance with the checklist. Additionally; there were no indications of ice formulation on any aircraft surfaces. This information coupled with no precipitation allowed us to forego any de-ice or anti-ice applications. Nothing noteworthy occurred during the remainder of preflight; taxi; takeoff; or climb.Just prior to reaching our final cruise altitude of FL380; the master caution light; AUTO FAIL; ALTN; and AIR COND annunciator light illuminated. I was PF and the captain was PM. I maintained control of the aircraft while the captain conducted the AUTO FAIL or Unscheduled Pressurization Change QRH checklist. The alternate pressurization system was adequately controlling cabin pressure. After a couple minutes; the AUTO FAIL; MANUAL; and AIR COND annunciator light illuminated. I maintained control of the aircraft while the captain resumed the AUTO FAIL or Unscheduled Pressurization Change QRH checklist from step X. Manual operation of the outflow valve was responding properly to movement of the toggle switch and pressurization was controllable. Once the checklist was completed; we discussed contingency plans if further degradation occurred. This included referencing the emergency descent checklist and ensuring quick availability on my tablet; informing flight attendants of the situation; conducting a descent to FL320 to decrease differential pressure requirements; and discussing best suitable fields for divert. ZZZ1 was determined as the immediate plan if pressurization became uncontrollable.After about 30 minutes of manual control of cabin pressurization; the outflow valve position stopped responding to toggle switch manipulation. The cabin began depressurizing at a rate of approximately 2000 fpm. Within approximately 1-2 mins; the CABIN ALTITUDE warning illuminated as the cabin pressurization exceeded 10;000 ft. I maintained control of the aircraft while the captain resumed the AUTO FAIL or Unscheduled Pressurization Change checklist from step 6. Oxygen masks were immediately donned by myself and the captain. Priority handling was requested with ATC and descent was conducted with an initial safe altitude given of 11;000 ft and an altimeter setting of approximately 29.67. At that point the cabin altitude was approximately 11;000ft and aircraft altitude was at FL320. While the captain communicated emergency descent to the flight attendants; he additionally advised that he would manually deploy masks. PA was then conducted in accordance with the checklist. To my knowledge; the cabin altitude never exceeded 14;000 ft. Once cabin altitude was below 10;000 ft; flight crew masks were removed and the emergency descent checklist was continued. Non-routine landing considerations were completed by the captain while I coordinated additional descents and an approach to runway XXC at ZZZ1. ATC advised our altitude was indicating 300 ft off from assigned altitude and altimeter setting was corrected to approximately 29.89. All emergency checklists were completed and an uneventful Flaps 30 landing was completed at approximately 134;000 lbs.Upon landing; emergency and medical vehicles were standing by and offered assistance. It was determined by our flight attendants that all passengers were unharmed and did not require any medical assistance. The emergency and medical vehicles followed us to our gate. After completing engine shutdown and the associated checklist; we were informed that 2 of the flight attendants wanted to be evaluated by the medical professionals. After a self assessment and thorough discussion of threats; the captain and I determined the safest course of action would be removal from the remaining legs for our duty day.
Second reporter narrative
We conducted an emergency descent from cruise altitude due to uncontrollable cabin pressure.We arrived to the aircraft around XA10 for a XB10 departure. The aircraft was free of ice; snow; and frost; to include the tops of control surfaces and wings. The potable water was serviced by ground crew; as it had been drained. Preflight; push; start; taxi out and takeoff were uneventful at ZZZZ.While climbing to FL380; at about FL370; the Master Caution; AUTO FAIL; and green ALTN lights illuminated. As Pilot Monitoring; I conducted the AUTO FAIL procedure from the QRH to Step X; as cabin pressurization was controllable with the Alternate Controller. About 5-10 minutes later; the Master Caution; AUTO FAIL; and MANUAL lights illuminated; indicating a failure of the Alternate Controller. I resumed the AUTO FAIL procedure from Step Y of the QRH; reverting to Manual Control. The cabin pressure remained controllable using the manual toggle switch; with pressure oscillating +-1000 feet at FL380.We asked for and descended to FL320 to decrease differential pressure (externally; ATC was not yet aware of our issue). We advised dispatch of the altitude change and that we were in manual pressure control. We also developed a plan for if the pressure became uncontrollable. This included briefing the Flight Attendants of our increased work load and possible pressure fluctuations; pulling up the Emergency Descent Checklist on the FO's tablet and reading through it; and using ZZZ1 as a primary divert if necessary.After approximately 30 minutes of manually keeping the cabin press around 6000ft (per the placard); the outflow valve was no longer responding to manual switch positions. The valve remained slightly open; resulting in a cabin climb rate of 2000fpm. At 10k' cabin altitude; the CABIN ALTITUDE warning lights and horn activated. We donned masks; established communication and requested priority handling with ZZZ center; asking for an emergency descent to 10k'. I resumed the AUTO FAIL QRH procedure; starting with Step C (cabin press uncontrollable decision tree). I then started the Emergency Descent checklist from the QRH. As we began our emergency descent; ATC initially cleared us to 11k'. We requested to divert to ZZZ1. We were cleared to and descended to 10k'; after which crew oxygen masks were removed. I advised the flight attendants to remove masks and prepare for landing at ZZZ1. I also completed the descent checklist and Non-Routine Landing Considerations checklist.ZZZ1 center reported us as 300ft off altitude and provided an updated local altimeter setting; which corrected the altitude discrepancy.The FO coordinated further descent and setup the systems and briefed for an ILS approach to RWYXXC. I was able to manually fully open the outflow valve just prior to the final approach fix. We landed uneventfully and under weight; flaps 30; brakes 2 on XXC. Emergency vehicles inquired about assistance and if there were any medical concerns; which there were none. They followed us to gate XX; where we shutdown the aircraft. Upon passenger deplaning; I was informed that two flight attendants requested medical evaluation due to the stress of the event.Upon post landing inspection; maintenance suspected a ruptured waterline near the main wheel well caused a trail of ice to migrate into the outflow valve; preventing it from closing properly.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.