B-737 air carrier crew reported a pressurization malfunction at cruise before entering oceanic airspace. The crew returned to an alternate airport and landed safely.
Synopsis
B-737 air carrier crew reported a pressurization malfunction at cruise before entering oceanic airspace. The crew returned to an alternate airport and landed safely.
Narrative
The preparation and departure for ETOPS flight from ZZZ to ZZZ1 [Airports] was routine. All was still normal as we climbed through 10;000 feet and I checked/announced the pressurization panel (Cabin Altitude indicated near Sea Level"). We received a clearance to climb to a final cruising altitude from ZZZ [Approach] of FL340 (changed from original FL350); changed the FMC and pressure controller altitude to reflect 340 - we didn't notice anything out of the ordinary. We were cleared direct ZZZZZ. We reached flight level 340 and were there approximately 8 minutes. ZZZ [Approach] cleared us to switch to enroute frequencies. Soon after; FO (First Officer) scanned the overhead and acutely recognized / announced that the differential pressure was displaying approx 6.5 which is less than we typically see. With that awareness; I noticed and pointed out that our cabin altitude pressure was very close to 10;000 and moments later the cabin altitude warning horn began to sound. FO and I immediately proceeded with our immediate action items (O2 on 100%; crew comm. established; seat belt sign on). I pulled out the QRC for the Cabin Altitude Warning and began the checklist. Since we had just left ZZZ freq. and we had not yet left the FIR; I switched back to ZZZ [Approach] and asked for a turn back to ZZZ2 [Airport]. I [requested priority handling] and proceeded with the checklist and descent. The Outflow valve appeared fully closed before confirmed with checklist. PASS OXYGEN switch was selected ON and the masks were dropped in the back. In the course of the descent; I believe the cabin altitude got to 12k but didn't see anything beyond that. I spoke with the flight attendants and the passengers. When we reached 10k and were in hold to burn fuel to reach max landing weight; we put controller back to auto and it seemed to control cabin fine at the lower altitude. We proceeded with vectors to an RNAV approach and uneventful landing to runway XXR at ZZZ2. During the process; we communicated several times with the flight attendants (they were very calm and professional throughout) and that was reflected by the passengers cooperative and calm nature as well. FO was equally professional with his focus in continuing to prioritize flying the aircraft first in a very skilled manner. His recognition of the pressure differential abnormality certainly helped lessen the startle effect that could have made the situation more difficult. There were no other faults associated with the event (no failure of the pressure controller or pack faults). The cabin altitude warning horn and the cabin altitude/differential pressure gauge were our only indications of faults with the system. Initial speed in the hold at ZZZZZ1 was inadvertently 250 kts but corrected in the first turn. One concern and difficulty was communicating with audio panel that used a rubberband to hold switch in the intercom position. The rubber band appeared on but was not holding in the select position."
Second reporter narrative
Ground ops/preflight for this flight were normal. On climb out CA (Captain) and I both noted cabin pressure was normal climbing through 10000. It was near sea level; I do not remember the exact altitude. Upon reaching cruise altitude; approximately 5-10 minutes after leveling off I happened to look up at the pressurization and noticed differential pressure was at about 6.5 units. I remembered that typically I've seen it around 8 at cruise and I told the captain. We then both noticed cabin altitude was at 10000 ft and mentioned this was very abnormal. We noticed the outflow valve indicator was in the full closed position. About 5-10 seconds later the cabin altitude warning alarmed and I immediately said masks; took off my headset and donned the oxygen mask. I configured my panel and said FO (First Officer) ON O2; captain said CA ON O2. He then immediately called ZZZ [Approach] and asked for immediate vectors off course and a lower altitude. We got 10000 and while this was going on Captain started running the descent QRC followed by QRH; while I was flying the plane. Initially I bugged 10000 and pushed level change; and set speed to 280. We started descending and I focused on flying while he did checklists. After maybe one minute I noticed we had more space for flying faster on the speed tape so we accelerated to final speed of 330 knots eventually. At some point during the checklist CA dropped the passenger oxygen masks in the back; spoke with the flight attendants and advised ATC while I prepared us for landing in ZZZ2 [Airport]. At some point in the mid teen altitudes we noted cabin altitude was back to normal and we removed our oxygen masks and put on our headsets again. We leveled off at 10000 and slowed to 250. We found the manual valve controller to be a little difficult so we tried AUTO mode again and it seemed to work completely normal so we left it like that. I got our clearance to ZZZ2 and headed direct to ZZZZZ for the ZZZZZ arrival. We were below 18000 and I was aware of this; but I chose to work in the FMC due to CA task saturation; while being fully aware of aircraft state/flight path management at all times and making sure that was the #1 priority. I got the weather; landing data; messaged dispatch and configured the FMC for arrival and approach. I noted we would be landing about 5000 lbs overweight so the captain had us request a lower altitude and we received 8000. We then flew with flight spoilers deployed and did a few laps of holding as published at ZZZZZ1 to burn fuel for reach our max landing weight. We did the first turn in holding at 250. We then remembered max holding speeds and slowed to 220ish and extended to Flaps 5 to burn more fuel. Ca and I had a discussion about who would be flying; and I told him I would prefer him to fly due to me having less experience in the aircraft than him. He briefed while I flew; then we swapped controls; finished holding and received vectors for the RNAV (gps) RWY XXR. We then landed without any further notable events and taxied to the gate. The only warning we got during the flight was the cabin altitude warning.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.