Date: 202110
Local time of day: 0001-0600
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Weather elements / visibility: Turbulence
Light: Night
Aircraft: B777 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pressurization Control System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Second reporter function: Captain; Pilot Flying
Anomaly: Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Departure Airport; Flight Crew Overcame Equipment Problem; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Around 1.5 hours into flight from ZZZ to ZZZZ at a cruise FL of 380 the Captain and I noticed an auto populated amber cabin pressure altitude of around 8;400 feet with a steady climb. We looked through our manuals to try to find a solution to get the pressure under control. We asked for a lower altitude to see if that would fix the problem but the cabin continued to climb. After manually trying to close the outflow valves which already indicated full closed; the pressure altitude exceeded 10;000 feet. We performed the emergency procedure for cabin altitude annunciation and ran the QRH which we had already reviewed.   We communicated the issue with ZZZ Center and told them we would need to descend to the lowest altitude possible with a return to ZZZ. We sent an ACARS message to Operations and discussed the issue with them over Satcom after. Around 17;000 feet is when the cabin altitude started to stabilize around 10;400 cabin altitude so a further descent was still required to be off oxygen. We descended to the MSA of 9;000 feet in the area and landed safely in ZZZ advising the Tower we would not need further assistance.


Second narrative: Around 1.5 hours into flight from ZZZ to ZZZZ at cruise FL 380 observed amber cabin pressure altitude of around 8400 feet with a steady climb of +100 ft/min. FO and I looked through our manuals to try to find a solution to get the pressure under control. I asked for a lower altitude to see if that would fix the problem but the cabin continued to climb. After manually trying to close the outflow valves which already indicated full closed; the pressure altitude exceeded 10;000 feet. We performed the emergency procedure for cabin altitude annunciation and ran the QRH which we had already reviewed.   We communicated the issue with ZZZ Center and told them we would need to descend to the minimum safe altitude to return to ZZZ. Sent an ACARS message to Dispatcher and discussed the issue with them and Maintenance Control.  Around 17;000 feet MSL is when the cabin altitude started to stabilize around 10;400 cabin altitude so a further descent was still required to be off oxygen. We descended to the MSA of 9;000 feet in the area. As cabin altitude stabilized we climbed up to 14000 ft for the return to ZZZ. Normal approach and landing advising the tower we would not need further assistance.


Synopsis: B777 flight crew reported loss of cabin pressure control and elected to make an air turn back and precautionary landing.