Date: 202101
Local time of day: 0001-0600
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Air Conditioning and Pressurization Pack
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Time Pressure; Communication Breakdown; Confusion; Situational Awareness; Human-Machine Interface
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Automation Aircraft Other Automation
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Slow Pressure Leak at FLXXX.This trip started as a 3 day ZZZ1 airport.  Flight X was the second segment on day 1 from ZZZ2 to ZZZ3.  My First Officer; was the flying pilot for this leg.  Loading and boarding were normal; and the flight was full.  At push time a passenger stood up and ran to the lav; so we waited and pushed off gate X at XA:XXz.  We taxied to Runway XL for takeoff; the weather was beautiful.  With [FO] flying we departed XL on the ZZZZZ2 RNAV Departure to ZZZZZ.  Our route would take us from ZZZZZ to ZZZ4 ZZZ5 ZZZZZ1 into ZZZ3.  It was a beautiful day; we could see the front range clearly with [Mountain Peak] in the distance and nearly unlimited visibility.  The departure was uneventful; passing 10;000 feet I turned off the Sterile Cockpit light and gave a double ding; checked the pressure which was operating normally.  (A habit ever since my [X type aircraft] days; as they tended to be leaky airplanes; and because I read a story about an airliner that took off with the mode selector in the wrong position and all lives were lost). As such; I try to check it every so often in the climb.  Again passing 18;000 we did our flows; I checked the fuel which was feeding evenly and normally and again we were pressurizing normally.  I checked fuel and cabin pressure one more time and all was normal. We had a continuous climb to FLXXX without a single level off; which is unusual for ZZZ2.In level flight at FLXXX I checked the fuel passing ZZZZZ Intersection and all was normal 2 minutes ahead.  We were both still on headset with our speakers off at this point.  Shortly after ZZZZZ when we were just starting to settle in at cruise from ZZZZZ to ZZZ4 intersection; the Master Warning Illuminated with a Siren and a Cabin Altitude EICAS.  I glanced up and saw the Cabin Altitude light and the Cabin altitude between the 10 and 15 mark.  We immediately donned our masks.  Upon donning my mask the expansion strap knocked the ear piece out of my left ear; so it was dangling somewhere off the O2 mask.  With the Boom/Oxy switch in Oxy; I could not hear myself until I turned up my speaker.  Donning the mask was interesting; with earpieces; eye glasses; and not to mention the removal of a COVID mask that [FO] was wearing.  It took a few seconds to get the mask on and check in.  In any event with both our speakers up we checked in on O2 and I requested priority handling while accessing the QRC.  FO immediately started the descent.  We initially started down with 14;000 feet in the window which I knew would clear the terrain until I could check the Grid MORA's more thoroughly.  Running the QRC; we confirmed O2 on 100% with communications established; Engine Bleed Air switches were on and both Packs were on.  The next step; Step 5 says; Choose one:  Cabin Altitude is Controllable: or Cabin Altitude is uncontrollable:  When I looked up I saw the cabin steady between the 10;000 and 15;000 marks on the gauge and was not climbing; but was obviously higher than a normal cabin.  The Auto Inop lights were not illuminated and we were currently in Auto 2; so I decided to select Auto 1.  At this point nothing changed; but the outflow valve appeared closed and the cabin seemed to be steady.  I chose to continue the checklist with step 6 PASS OXY (If cabin Altitude 14;000 feet) ON.  I knew the masks had deployed because I could hear the flight attendants on the PA telling the passengers to put on their masks.Structural integrity was not in doubt; so I confirmed with FO that all the steps in step 8 were accomplished.  FO had done a great job and had accomplished all the steps; MCP was set at 14000; FLCH; Thrust was IDLE; Speed Brakes were extended; Speed was set at VMO; so I turned on the seat belt sign; ATC had been notified; so I did not set 7700 on the transponder.  I looked back up and at this point the cabin was slowly decreasing.  At no point did the cabin ever go out of the 10;000 to 15;000 marks on the cabin pressure.  At the most the cabinis estimated to have reached approximately 14;000 or a bit less (Maybe 13;000 or so).  Between the 3 of us in the cockpit; that is about the highest any of us saw.  You must interpolate because the gauge only has a marks for 10 and 15.  At some point during all this the flight attendants called the cockpit and I told the jump seater to take the call which he did.  I quickly checked the Grid MORA's and confirmed the 10;000 was safe; so FO requested 10;000 and I set it in the window.  At this point and altitude the cabin was stable.  We were well into the descent and the cabin had started descending.  I continued the checklist with the expanded items in the checklist; altimeters were set.  Initially we requested a divert to ZZZ6; as we were both familiar with the field and it was the nearest suitable airport.  I took the airplane from FO so he could set up which again he did very quickly.  I gave FO back the airplane and called the flight attendants.  They informed me that everything in the back was fine and I told them they no longer needed the masks.  Now we started a conversation about our divert options.With the passengers; all safe and no issues; we checked our fuel and determined that we had plenty of fuel to safely return to ZZZ2.  The cabin altitude was siting somewhere around 6 to 7 thousand feet.  I sent a quick note to dispatch to call me; so they sent me a frequency.  We were unable to reach them; so they sent me a second frequency which also did not work (Probably because we were at 10;000 feet).  I then established communication through the SAT Comm to the Dispatch; because there was no direct connection for sector X.  The dispatcher that answered handed me off to the sector X dispatcher; and he agreed that ZZZ2 was a good choice and said he would let everyone know.  After clearing with Dispatch; we discussed our plan and then informed ATC we no longer needed priority handling and that we were not an emergency and would like to proceed to ZZZ2.  They routed us direct ZZZZZ2...so I called the FAs and informed them of the new details and time.  They again informed me that everything was fine in the back and the cabin was secure.  I made a brief PA announcement to inform the passengers of our situation and our return to ZZZ2.  In the cockpit; we all started a discussion about the plan and confirmed that we had notified all the parties concerned and accomplished all the appropriate actions.  I asked Captain Y if there was anything else he could think of and he said; no you did everything you can do.  I requested landing data for [Runway] YL and FO briefed the Visual approach.  We accomplished the Descent Checklist and FO flew a beautiful uneventful visual approach to Runway YL.  Once on the ground we taxied back to gate X and handed the plane over to maintenance.Two things that are interesting about this situation:  First; when donning O2 masks and establishing communications.  Because we both turned our speakers all the way up; it made initial communication both inside and outside the aircraft difficult; due to feedback between the 2 speakers.  Also; I transmitted once outside the aircraft instead of inside with the rocker switch on the yoke.  The second and more interesting point is that whenever you practice a depressurization in the simulator; it is always an explosive rapid decompression.  This was not like that at all; this was an airplane that had a high cabin with a gauge reading between the 10;000 and 15;000 marks.   A higher than normal altitude but not climbing.  At the point on the checklist when it says 'Cabin altitude is controllable:'  It says 'Attempt to reduce cabin altitude by switching to the other auto controller (or manual mode if AUTO INOP is illuminated).'  I think it would be better worded to say Cabin altitude is controllable and can be returned to a normal cabin altitude.  In our case it was a high cabin; but not one that was continuing to climb. So is that controllable?  We decided to continue with the uncontrollable portion of the checklist and drop the masks etc.So; what went well?  Our planning and decision making; and workload management went well.  We constantly evaluated our situation and adjusted our plan as we gained more info and had the ability to discuss our options.  What could have gone better?  The communication while on O2 masks.  Next time I would make sure my ear piece stays in my ear and or I would turn down my cockpit speaker so that we do not get so much feedback over the speakers.  I feel that all in all; this was a well-executed event by all concerned ending with a safe return for passengers and crew.


Second narrative: Aircraft X departed ZZZ2 for ZZZ3 on [Date] with Captain X (pilot monitoring); First Officer Y (pilot flying); and Captain Y (flight deck jumpseater). Takeoff from Runway XL and the ZZZZZ2 departure were uneventful; with no clouds; unlimited visibility; and light winds. Shortly after reaching FLXXX; with the autopilot and autothrottles on; the CABIN ALTITUDE EICAS message was displayed and its associated siren sounded.All flight crew members donned oxygen masks and crew communications were established. I noticed the cabin altitude indicator reading approximately 10;000 feet. Captain X ordered an immediate descent; and advised ZZZ Center that we were descending to 14;000 feet. ZZZ Center cleared us to descend; and advised that there was no conflicting traffic. I initiated the descent by putting 14;000 in the altitude window and pushing FLCH; followed by deploying the speed brakes and selecting a higher indicated airspeed. I continued increasing the airspeed as VMO increased.As I was flying the descent; Captain X ran the CABIN ALTITUDE checklist. Once the checklist was complete; Captain X brought up the Alternates page in the FMC and we discussed possible diversion airports. ZZZ6 was chosen as the nearest suitable airport and ZZZ Center cleared us direct to ZZZ6. Captain X focused on diversion planning and communication while I flew the airplane and talked to ATC; who cleared us to continue our descent to 10;000 feet.Once we were level at 10;000 feet; we reconsidered the decision to land at ZZZ6. Captain X confirmed that the situation in the passenger cabin was under control; the situation was stabilized; and our fuel state was sufficient to continue to ZZZ2. After coordinating with Dispatch; the decision was made to change diversion airports to ZZZ2.The flight to ZZZ2 at 10;000 feet was uneventful. ZZZ6 Approach provided a route to ZZZ2; which was then simplified to radar vectors by ZZZ Center and ZZZ2 Approach to a visual approach to Runway XL. A normal approach and landing were followed by a normal taxi to gate X.Other notes and observations:When the alarm sounded; I was wearing a surgical mask; glasses; and a headset; and holding my company iPad. I was startled by the warning; and before donning my oxygen mask; I reflexively checked the cabin altitude; and saw that it was approximately 10;000 feet. As I donned my oxygen mask; I knocked my glasses askew and my headset off; and left my surgical mask on. I had to release my oxygen mask to remove the surgical mask; and then get my glasses situated on my face. I abandoned the headset in favor of the speaker.Captain X had some difficulty with his earpiece; as well as with the comm panel (transmitting on the radio rather than the intercom at one point). Additionally; we both experienced feedback when attempting to communicate with each other. This resulted in poor crew communication during the initial stage of the descent; while Captain X was running the checklist. Once the checklist was complete; we were able to troubleshoot our communications and they improved. However; I was lacking situational awareness of the situation in the early stages due to the communication issues.During the descent; Captains X and Y reported a maximum cabin altitude between 14;000 and 15;000 feet. I did not observe the cabin pressurization system during the descent; as my focus was on flying the airplane.Captain X had difficulty coordinating with Dispatch. He sent a 'Call Me' request via ACARS and received several VHF frequencies; but all failed; possibly due to our low altitude (10;000 MSL; 5;000 AGL). He reverted to satellite communications; and was again hampered; this time because our dispatch sector did not appear in the SATCOM directory. He was eventually transferred to the correct sector and was able to coordinate our diversion.Captain X did an outstanding job of crew resource management; using me to fly the plane and coordinate with ATC; and using Captain Y as an additional resource for communication with the cabin. Additionally; he asked for and received input from both Captain Y and me; and used that input to guide his decisions about our diversion.All ATC agencies (ZZZ Center; ZZZ6 Approach; ZZZ2 Approach; and ZZZ2 Tower) were extremely helpful and professional; providing all requested assistance without delay.Captain X left the flight deck promptly once the parking checklist was complete to reassure the passengers and flight attendants. Several people commented that they were happy to see him in the cabin during the deplaning process.Debrief Items:What went well? Decision making. Looking back; the decision to divert to ZZZ6 initially was well considered and sound. We knew that Company had service to ZZZ6 and without knowing if there were any sick or injured passengers or flight attendants; a close airport was appropriate. Once the situation was stabilized; other factors could be considered (passenger accommodation; maintenance resources) and the decision to divert to ZZZ2 made the most sense.Workload and automation management also went well. By utilizing me to fly the plane and coordinate with ATC; and having Captain X assist with cabin coordination; Captain X was able to run the checklist; troubleshoot the problem; and stabilize the situation. And by keeping the autopilot and autothrottles engaged; I was able to initiate and manage the descent effectively.Finally; as I stated earlier; Captain X's leadership effectiveness was excellent. He remained calm when things got busy; and he actively sought out advice from all available resources; which led to a successful outcome.What could have gone better? My reaction to the alarm should have been to immediately put on my mask; rather than check the cabin pressure gauge. And I could have donned my oxygen mask more smoothly and quickly; rather than forgetting to remove my surgical mask and knocking my headset off. We could have done a better job establishing effective crew communications; which could have led to better situational awareness and crew coordination. Thankfully; all of these items were dealt with promptly; and none of them stood in the way of a successful outcome.


Synopsis: Air carrier flight crew reported a pressurization problem during climbout resulting in a return to the departure airport.