A321 flight crew reported diverting to an alternate airport after experiencing loss of cabin pressure. They had been dispatched with a pack inoperative.
Synopsis
A321 flight crew reported diverting to an alternate airport after experiencing loss of cabin pressure. They had been dispatched with a pack inoperative.
Narrative
We were operating with a single pack due to MEL of pack #1. Approximately 4 hours into the flight; while cruising at FL340; we noticed the cabin altitude beginning to climb. Concurrently; I heard a slight rumbling noise coming from behind my seat and to the left; possibly from the lavatory. At this point the flight attendants called the flight deck because we had previously requested a restroom break and they were ready for us. I told them to stand by because we may have a pressurization problem developing and we would call them back. This ended up being a good thing as they were not taken by surprise when the masks dropped. We immediately requested and received descent clearance to I believe FL300. I transferred control to my First Officer. I then attempted switching Cabin Pressure Controllers per QRH advisory page guidance. The cabin altitude continued to climb; turned red; and was followed by the ECAM warning CAB PR EXCESS CAB ALT. We donned oxygen masks; [advised] ATC; requested continued descent and a 180 degree turn from the high terrain ahead. We performed the ECAM and QRH procedures and began a divert to ZZZ. I advised the passengers and Flight Attendants. I advised Dispatch through ACARS. My First Officer (FO) notified Operations in ZZZ. We regained control of the cabin altitude during the descent. Because this all happened so quickly once the warning was triggered; and my primary concern was a safe descent in high terrain; I am not sure of the maximum cabin altitude reached. I do remember clearly that the ECAM was displaying CAB ALT PAX OXY MASKS....MAN ON. There was no conditional statement in front of it. So I don't know if they had already dropped and this was just a confirmation step. I confirmed with my FO that I had the correct guarded switch and pressed the button. After we parked at the gate the seatbelt sign remained illuminated even with the switch off. According to our manual; the seatbelt sign illuminates above 11;300 regardless of switch position; so that may indicate the cabin was at least that high at one point. Also; right before we went on oxygen; I definitely started to feel symptoms I recognized from my altitude chamber training over 30 years ago. My First Officer and entire cabin crew performed exceptionally. I was very grateful there were no injuries. The staff at ZZZ were exceptional as well. They accommodated us on very little notice.Pack 1 was on MEL. We also had a MEL Temp Ctl Class 2 Msg. It was a challenge keeping the cockpit and cabin at a steady temperature throughout the flight. This may have been due to MEL or possibly pack 2 was getting weak.I don't think we should dispatch over high terrain on a single pack.
Second reporter narrative
I was operating Flight ABCD with Captain A. This was my second flight of the day and first leg with [this] Captain. Everything started normally in the flight deck preparation. We both acknowledged and reviewed MEL. Everything before the time of the event appeared normal. Captain and I were both making periodical system checks and tracking fuel burn and time of crossings along our flight plan. Approximately 4 hours into the flight; I realized I was needing to clear my ears over and over and felt very mild symptoms of hypoxia. That clued me into the pressurization gauge. I noticed it appeared to be higher than normal. Approximately 7500 ft. and rising at a rate around 500FPM. We were at FL340 and from memory cabin altitude at that flight level would normally be in the 5500 ft. to 6500 ft. range. At that point I said to Captain 'Hey I think we might have a pressurization problem'. About that time the cabin altitude was flashing green and above 8000 ft. Captain transferred controls to me and we asked for a lower altitude. Captain tried to regain control of the pressurization system utilizing the QRH advisory pages. The procedure did not work. Shortly after the cabin altitude started flashing red and exceeded 10;000 ft. We received the CAB PR EXCESS CAB ALT ECAM message. We completed the ECAM; donned oxygen and [advised] ATC. Due to task saturation I am unsure of the max alt the cabin got to. Initially ATC gave us a descent down to FL260. Then down to 12;000 ft. During this time Captain did a great job of communicating with the cabin crew to clue them into what was going on. We had previously set up a restroom break with the cabin crew when all of this started. As I was making the descent I immediately realized terrain was going to be an issue so I talked with ATC to get vectors away from the mountainous terrain. During this time Captain and I were discussing diversion airports. We both had a few suggestions but ultimately landed at ZZZ after he consulted the diversion manual. Captain did a great job of speaking to Dispatch; the cabin crew and communicating with me. Once we were cleared direct to ZZZ and descending through roughly 15;000 ft. we noticed the cabin altitude appeared to be back in the normal range and I transferred control back to Captain. We began the task of setting up for and approach and arrival into ZZZ. [After arriving at the gate] we received a call from our #1 and he asked us to turn the seatbelt sign off. We both confirmed the switch was off and assured him it was ok to have passengers get up. Later upon review of the manual Captain noted that the seatbelt signs would automatically illuminate above a cabin altitude of 11;300. We coordinated with Operations and deplaned without further issues. I have roughly 400 hours in the aircraft and have been at Company less than a year. I was thankful to have a great Captain and cabin crew that handled this problem professionally.We had a few MEL's that I believe led to the problem. The remaining pack was having issues controlling temperature along the flight. It would get really cold and then warm. I believe the remaining pack was either freezing up or getting weak. We never received any failure message during our event. Just the cabin pressure starting to rise.Having a single pack and going over high terrain should be avoided. This is the first time since working at Company that I've had a long cross country flight; over terrain; with a single pack.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.