Boeing 747-800 flight crew reported a gradual loss of cabin pressurization; resulting in decision to descend and return to the departure airport.
Synopsis
Boeing 747-800 flight crew reported a gradual loss of cabin pressurization; resulting in decision to descend and return to the departure airport.
Narrative
We had been level at FL320 for at least 5 minutes. The First Officer had gotten up to start his break and the Captain had gotten up to use the lavatory. The Chief Pilot was sitting in the First Officer seat and he noticed that our cabin altitude was climbing at +200 feet per minute. I got back in the Captain seat and we looked at the QRH to see what we might do to stay ahead of the issue. Nothing was amiss. All packs and outflow valves were operating normally. The door synoptic page was also normal. We waited to see if the pressurization would stabilize but it continued to climb. At 10;000 feet the CABIN ALT master warning light and aural began. We ran the ECL (Electronic Checklist); donned our oxygen masks; advised ATC and descended to 14;000 feet. We also deployed the supernumerary oxygen per the ECL. One of the masks did not deploy.We spoke with dispatch via SATCOM. We all agreed that due to the nature of the issue that returning back to ZZZ was the best course of action.
Second reporter narrative
I was the Chief Pilot on this flight. Aircraft leveled off at FL320; I replaced the FO so the FO could start his break. The Captain then got out of the seat to use the lavatory. I noticed the cabin altitude climbing at a rate of approximately +200 feet per minute. The Captain got back in his seat and we began looking at the QRH to see what we might do to stay ahead of the issue. Nothing was amiss. All packs and outflow valves were operating normally. The door synoptic page was also normal. We waited to see if the pressurization would stabilize but it continued to climb. At a cabin altitude of 10;000 feet the CABIN ALT master warning light and aural began. We ran the ECL (Electronic Checklist); donned our oxygen masks; advised ATC; and descended to 14;000 feet. We also deployed the supernumerary oxygen per the ECL. One supernumerary mask did not deploy. We spoke with dispatch via SATCOM. We all agreed that due to the unknown nature of the issue returning back to ZZZ was the best course of action. In the meantime we also spoke with the engineer on duty and he agreed with our course of action. CRM was excellent and we successfully landed the aircraft back at ZZZ.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.