Air carrier flight crew reported a gradual loss of pressurization during cruise. Flight crew then diverted and completed a safe landing.
Synopsis
Air carrier flight crew reported a gradual loss of pressurization during cruise. Flight crew then diverted and completed a safe landing.
Narrative
Taxiing out of ZZZ1 we noticed that the 'LEFT COMM PANEL PTT' was inoperative since Tower was calling the airplane a couple of times to give us climb instructions. Eventually the Captain realized of the malfunction and used the PTT from the yoke. During climb and already past FL180 the Captain also noticed the standby attitude indicator showing 400 ft. above normal altitude and he was showing me where I can find the altimeter reading limitations on the Fault Reporting Code manual. During cruise he also noticed that the radar was showing a lot of 'noise' on the display and we tried to troubleshoot by resetting with no success. The Captain was filling out the [form] with the faults that we had encountered so far while I was still at the controls when he noticed that the 'return to seat' light in the galley came on. He expressed his concern out loud to which I responded that from training I remember that one of the reason the light comes on is because of a depressurization of the 'main cargo deck'. He then looked at the pressurization gauges and read out loud 'Cabin 10K ft.; differential 3.8'; a couple of seconds later it dawned on him what he had just said and immediately proceeds to pull the oxygen mask; don it and broadcast PAN PAN with ZZZ Center. It took me a couple of seconds later to do the same because I just couldn't believe what was happening but shortly after I don my mask and established comms the 'CABIN ALTITUDE' master warning and EICAS (Engine Indicating and Crew Alerting System) came on. ZZZ instructed to descend to 16;000 ft. all while I pulled the thrust levers to idle; speed breaks extended; FLCH; and set manually the speed to MMO and the Captain was reading the checklist out loud. After we concluded the checklist and still in a descent we were doing about 5;000 ft./min the Captain noticed that the cabin was staying at 10;000 ft. but the differential was slowly coming down. When we level off at 16;000 ft. ZZZ asked if we wanted to descend to 10;000 ft. The Captain consulted with me and I agreed since we had no high terrain in the area based on the depiction of the terrain feature. We continued the [descent] to 10;000 ft.; removed our masks when we leveled off and the Captain transferred left comm to me so he could talk to [dispatch].ZZZ was asking information about our intentions to what I responded to give us a few minutes since the Captain was conferring with [dispatch] on the next best course of action. I also decided to pull the speed back to 250 KTS to help with the fuel consumption in case we decided to continue to ZZZ2. We had about 30k lbs. of fuel and were burning about 9;700 pounds per hour. Distance to ZZZ2 was showing about 315 nm. ZZZ handed me off to ZZZ3 Approach.The Captain finished his conference with [maintenance] and informed me that they have decided to divert to ZZZ3 since the software used to determine fuel burn numbers wouldn't give information below FL200. I mentioned that we had enough gas to go to ZZZ2 and since the weather was perfect I didn't have a problem continuing but he reaffirmed that without 'official numbers' the safest course of action was to divert. He contacted ZZZ3 Approach; informed our decision. While he was still with [dispatch] I had already pulled out info of frequencies; altitudes; got the ATIS for ZZZ3 and were pretty much ready to brief and run checklists. I did request the Captain to ask for delayed vectors because I didn't feel comfortable with the rush to get us down since we were not in an urgency anymore. When on final approach we noticed the LAND 2; NO LAND 3 on the auto land status. We made an uneventful landing in ZZZ3 and Tower gave us an apron on [Taxiway] 1 to wait for further instructions. During interaction with Airport Authorities I provided some input on informing them of the nature of our cargo (flammable) and seal the airplane since Captain was security coordinator in a situation like this.
Second reporter narrative
I was the PM (Pilot Monitoring); and the FO (First Officer) was PF (Pilot Flying). We were somewhere south of ZZZ en route to ZZZ2 from ZZZ1. I was looking at my iPad when I saw the RTS (Return To Seat) light in the galley illuminate. There was a brief period of processing as I tried to confirm that this was not normal and then understand why it happened. All I could think was that something was not right. My mind first went to cargo fire; but there was no fire indication. I mentioned it to the FO; who said it could be pressurization. I looked up at the pressurization panel. The differential pressure was indicating something less than 8 psid. There was again some time required to process that this was not normal. I then looked at cabin altitude; which indicated 10;000 [ft.]; and the rate was near 0. There was additional time spent coming to the realization that this was not only not normal; but possibly bad. Training from years ago then kicked in - when dealing with a pressurization problem; the first thing you do before you do anything else is put the oxygen mask on. I did that; then started looking at the panel to try to figure out what was wrong. After a few breaths of 100% oxygen; clarity returned and I came to the conclusion that doing this at FL360 was a bad idea and we should get down now. Reasoning that we were not yet in distress but needing urgent action to prevent becoming a distress situation; I made a PAN-PAN call to ZZZ Center; stated the problem and requested descent. We were cleared to 16;000 ft. I started the CABIN ALTITUDE checklist even though we had not received a warning because it seemed applicable to the situation anyway. The CABIN ALTITUDE warning did activate as I was completing the checklist; but cabin altitude was still showing 10;000 feet steady. Center asked me to advise if we would need a lower altitude. Passing FL240; the cabin was still at 10;000 and steady so I requested a lower altitude. We were given 11;000; and then 10;000. Somewhere below FL200; the cabin did start descending slowly. After reaching 10;000 (and after some struggle to stow the oxygen mask because it was a CY and I wanted to be able to communicate using a headset without having to hold the oxygen mask to my face to talk) I transferred left comm to the FO and called Dispatch (HOORAY SATELLITE PHONE!). At this point we were about equidistant from ZZZ and ZZZ3. Since the situation seemed to be under control with the cabin still slowly descending; we discussed whether to continue to ZZZ2 or divert. We had not declared an emergency; so that option was still reasonably available. The FO; Dispatch and I were all in agreement that we could continue to ZZZ2. Our own calculations said that should be fine. The FMC indicated we would arrive with 24;000 lbs of fuel. The only snag came when Dispatch tried to get new fuel burn numbers and the computer threw up its hands; refusing to provide anything below FL200. At that point; with ZZZ3 directly in front of us and good VFR conditions in the area; I decided the most conservative course of action would be to divert to ZZZ3. Dispatch gave me a quick brief on the airport and amended the release for the diversion to ZZZ3 with ZZZ4 as the alternate. We communicated our intent to ZZZ3 approach; receiving vectors to a visual approach to Runway XX. We took the necessary time to brief the approach and then landed normally.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.