Narrative:

First officer (first officer) and I left the hotel to have breakfast together at the airport as the food available at the hotel is limited. After breakfast we proceeded to the departure gate.the aircraft was extremely hot when we first boarded and there was no airflow. While I briefed purser and the cabin crew; first officer went to the cockpit to try to cool down the airplane.he noticed the compartment temperatures were 94 degrees and the APU (auxiliary power unit) was not running. He started the APU and turned the packs on. When he returned from the walk around; he mentioned that the pc air was hooked up but not turned on.I relayed the situation to station operations; and they said that they had contacted maintenance some time ago to start the APU. When boarding began the aircraft was still very warm; the compartment temperatures read 84 degrees. Later in the boarding process the customer service agent mentioned that they had called 3 times about the air conditioning issue as well.the departure; climb; and early cruise were normal. At FL390 and after approximately an hour into the flight we setup for a restroom break. When I left the flight deck; I noticed how warm it still was in the cabin. After returning to the cockpit I checked the cabin temperature indication and saw that they remained high at 82 to 84 degrees and that the temperature selectors were near full cold. I also noticed the cabin altitude seemed unusually high. While first officer was in restroom; I moved the temperature selectors to full warm to clear any ice that might have built up in the packs. After a minute or two there was no change; so I began the first steps of the cabin temperature hot procedure (2.20.24).step 8 in the procedure calls for turning off the left pack. I was reluctant to do that as the air flow seemed low already and I was concerned about pressurization. First officer had returned to the cockpit by this time; and I began to suspect the possibility that the recirculation fans may not be operatingcorrectly. I then asked first officer to try cycling the recirculation fans. While I meant in my mind the right recirculation fan only; I did say fans (plural). First officer promptly complied with my request and turned both recirculation fans off and then on. With a sinking feeling in my chest; I immediately heard the overboard exhaust valve open in the belly of the plane. I then checked the cabin pressurization panel and saw it starting to climb.I immediately called it to first officer's attention and asked him to contact ATC (air traffic control) for a lower altitude. I wanted to make sure that everyone in the cabin was near an oxygen mask; so I turned on the seat belt sign and called purser and told him to get the crew to their jumpseats. I then made a PA 'flight attendants; take your jumpseat.' while we were in the descent; the cabin altitude warning sounded. We completed the immediate action items; started the cabin altitude qrc; and advised ATC. Very quickly in the descent the cabin altitude began to descend. The cabin altitude warning went out; and we completed the qrc (quick reference checklist) as control of the cabin had been regained. We leveled off at FL230 as the cabin altitude was well below 10;000 ft. During the descent; the cabin temperature returned to normal at approximately 74 degrees.as there is no satcom; we contacted dispatch via ACARS 'call me'. We spoke to them on airinc VHF;but the frequency had gaps and we had difficulty discussing the issue with the dispatcher and [maintenance control]. I'm not certain for the cause of this. I was having no difficulty with ATC communication; but there werenumerous breaks where we could not talk to dispatch. We asked dispatch to calculate our fuel burn for a flight to ZZZ.I briefed the flight attendants on the situation. I made a PA to the customers explaining that we were having difficulties with the air conditioning system; and I apologized for the abrupt descent. I assuredthem that there was no need for concern and that we were considering an alternate route to ZZZ. I told them that I would keep them informed.with normal pressurization and temperature restored we climbed up to FL290; stopping at FL250 and FL270 to confirm that normal pressurization and air conditioning would continue. Both the FMC (flight management computer) and dispatch showed us landing at ZZZ with acceptable reserves at FL290. Dispatch advised us that therewere no delays into ZZZ. The only weather concern was a storm cell over ZZZ1 that we had in sight and was easily detoured around.after consulting with dispatch and first officer I cancelled the priority request. ATC then recleared us to ZZZ VOR and then ZZZ1 VOR for the STAR arrival. I updated the passengers and gave them our new ETA. Subsequent arrival and landing normal.after arrival at ZZZ; I completed elb (electronic flight bag) entries. Dispatch advised us earlier that a mechanic would meet the flight to discuss the write up. We called maintenance as no one met the flight. They told us they would call someone for us. We waited for another approximately :15 but no one arrived. We had to leave as our next flight was departing from the other concourse.before the cabin crew left; I asked them if there was any concern by the passengers and how things went from their eyes. They all complimented first officer and I and said that not a single passenger had expressed any concern at all. They appreciated the communication and thanked us for our consideration and professionalism.on our way to the next flight I contacted the fodm and debriefed her on the event. As we were on the concourse; I asked her to email me some sort of confirmation and her name; but as of this writing thathas not arrived.on our flight to ZZZ1 first officer and I reflected on what went well and what could have gone better.obviously starting the APU or turning on the pc air to cool the airplane prior to our arrival would have helped things. The poor airflow in the aircraft was likely a factor as well. In my experience it takes an awfully long time to cool off an airplane that has been allowed to get that hot on the ground. But; normally once airborne with cooler exterior temperatures and the packs running on the engine bleeds the aircraft will cool before the top of climb.however; the most important error was when I asked first officer to cycle both recirculation fans. Equally important was not leaving the left recirculation fan off when I knew the overboard exhaust valve hadopened. Doing so would have put the left pack in high flow and it is possible that the cabin would not have climbed like it did. This lacking in systems knowledge on my part has been corrected with avengeance over the last 24 hours. Repair of this error took the next hour of the flight.my communication and monitor/cross check was lacking regarding the issue of 'fan' v 'fans.' this is a lesson I have learned extremely well from this and will use it to teach with in the future. The precision of the spoken word is often casual; but at times and situations like this it is critical that time be taken to convey a precise message. The B756 flight manual had the information and resources I needed to address the situation. However; my concerns over pressurization caused me to question turning off a pack in this situation. In retrospect; it would have been better to try turning off a pack that can be turned back on rather than open a hole in the bottom of the plane that can't be closed (even though that was not my intention).I think the planning and decision making went well. First officer and I weighed the pros and cons of cancelling the priority handling and the option of diverting enroute. In each case he provided good input that helped memake the decision. As mentioned before; we did have some challenges in communicating with dispatch to get dispatcher's input. As I was never able to have a conversation with [maintenance control]I did not have the benefit of their input in the decisions. I was ableto speak to [name removed] long enough to get concurrence on cancelling the priority request but most all the communications we had were through ACARS.our workload management went well overall. First officer and I were able to separate our duties; but then come together and recreate a shared mental model of the situation.purser; and the rest of the cabin crew also did a great job in handling the situation. The fact that none of the passengers expressed any concerns after the flight is a testament to their composure and professionalism. Please forward my thanks to them all.first officer did an excellent job during this and all the flights we flew. He is very capable and experienced; and managed the flying and ATC coordination perfectly while I was tending to the other duties. He is a fantastic resource on the flight deck. I truly look forward to flying with him again.

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Original NASA ASRS Text

Title: B757 flight crew reported accidental depressurization in cruise when troubleshooting a pack malfunction.

Narrative: FO (First Officer) and I left the hotel to have breakfast together at the airport as the food available at the hotel is limited. After breakfast we proceeded to the departure gate.The aircraft was extremely hot when we first boarded and there was no airflow. While I briefed Purser and the cabin crew; FO went to the cockpit to try to cool down the airplane.He noticed the compartment temperatures were 94 degrees and the APU (Auxiliary Power Unit) was not running. He started the APU and turned the packs on. When he returned from the walk around; he mentioned that the PC air was hooked up but not turned on.I relayed the situation to station operations; and they said that they had contacted maintenance some time ago to start the APU. When boarding began the aircraft was still very warm; the compartment temperatures read 84 degrees. Later in the boarding process the Customer Service Agent mentioned that they had called 3 times about the air conditioning issue as well.The departure; climb; and early cruise were normal. At FL390 and after approximately an hour into the flight we setup for a restroom break. When I left the flight deck; I noticed how warm it still was in the cabin. After returning to the cockpit I checked the cabin temperature indication and saw that they remained high at 82 to 84 degrees and that the temperature selectors were near full cold. I also noticed the cabin altitude seemed unusually high. While FO was in restroom; I moved the temperature selectors to full warm to clear any ice that might have built up in the packs. After a minute or two there was no change; so I began the first steps of the Cabin Temperature Hot Procedure (2.20.24).Step 8 in the procedure calls for turning off the left pack. I was reluctant to do that as the air flow seemed low already and I was concerned about pressurization. FO had returned to the cockpit by this time; and I began to suspect the possibility that the recirculation fans may not be operatingcorrectly. I then asked FO to try cycling the recirculation fans. While I meant in my mind the right recirculation fan only; I did say fans (plural). FO promptly complied with my request and turned both recirculation fans off and then on. With a sinking feeling in my chest; I immediately heard the overboard exhaust valve open in the belly of the plane. I then checked the cabin pressurization panel and saw it starting to climb.I immediately called it to FO's attention and asked him to contact ATC (Air Traffic Control) for a lower altitude. I wanted to make sure that everyone in the cabin was near an oxygen mask; so I turned on the seat belt sign and called Purser and told him to get the crew to their jumpseats. I then made a PA 'Flight Attendants; take your jumpseat.' While we were in the descent; the cabin altitude warning sounded. We completed the immediate action items; started the CABIN ALTITUDE QRC; and advised ATC. Very quickly in the descent the cabin altitude began to descend. The cabin altitude warning went out; and we completed the QRC (Quick Reference Checklist) as control of the cabin had been regained. We leveled off at FL230 as the cabin altitude was well below 10;000 ft. During the descent; the cabin temperature returned to normal at approximately 74 degrees.As there is no SATCOM; we contacted dispatch via ACARS 'CALL ME'. We spoke to them on AIRINC VHF;but the frequency had gaps and we had difficulty discussing the issue with the Dispatcher and [Maintenance Control]. I'm not certain for the cause of this. I was having no difficulty with ATC communication; but there werenumerous breaks where we could not talk to dispatch. We asked dispatch to calculate our fuel burn for a flight to ZZZ.I briefed the Flight Attendants on the situation. I made a PA to the customers explaining that we were having difficulties with the air conditioning system; and I apologized for the abrupt descent. I assuredthem that there was no need for concern and that we were considering an alternate route to ZZZ. I told them that I would keep them informed.With normal pressurization and temperature restored we climbed up to FL290; stopping at FL250 and FL270 to confirm that normal pressurization and air conditioning would continue. Both the FMC (Flight Management Computer) and Dispatch showed us landing at ZZZ with acceptable reserves at FL290. Dispatch advised us that therewere no delays into ZZZ. The only weather concern was a storm cell over ZZZ1 that we had in sight and was easily detoured around.After consulting with Dispatch and FO I cancelled the priority request. ATC then recleared us to ZZZ VOR and then ZZZ1 VOR for the STAR arrival. I updated the passengers and gave them our new ETA. Subsequent arrival and landing normal.After arrival at ZZZ; I completed ELB (Electronic Flight Bag) entries. Dispatch advised us earlier that a Mechanic would meet the flight to discuss the write up. We called Maintenance as no one met the flight. They told us they would call someone for us. We waited for another approximately :15 but no one arrived. We had to leave as our next flight was departing from the other concourse.Before the Cabin Crew left; I asked them if there was any concern by the passengers and how things went from their eyes. They all complimented FO and I and said that not a single passenger had expressed any concern at all. They appreciated the communication and thanked us for our consideration and professionalism.On our way to the next flight I contacted the FODM and debriefed her on the event. As we were on the concourse; I asked her to email me some sort of confirmation and her name; but as of this writing thathas not arrived.On our flight to ZZZ1 FO and I reflected on what went well and what could have gone better.Obviously starting the APU or turning on the PC air to cool the airplane prior to our arrival would have helped things. The poor airflow in the aircraft was likely a factor as well. In my experience it takes an awfully long time to cool off an airplane that has been allowed to get that hot on the ground. But; normally once airborne with cooler exterior temperatures and the packs running on the engine bleeds the aircraft will cool before the top of climb.However; the most important error was when I asked FO to cycle both recirculation fans. Equally important was not leaving the left recirculation fan off when I knew the overboard exhaust valve hadopened. Doing so would have put the left pack in high flow and it is possible that the cabin would not have climbed like it did. This lacking in systems knowledge on my part has been corrected with avengeance over the last 24 hours. Repair of this error took the next hour of the flight.My communication and monitor/cross check was lacking regarding the issue of 'fan' v 'fans.' This is a lesson I have learned extremely well from this and will use it to teach with in the future. The precision of the spoken word is often casual; but at times and situations like this it is critical that time be taken to convey a precise message. The B756 Flight Manual had the information and resources I needed to address the situation. However; my concerns over pressurization caused me to question turning off a pack in this situation. In retrospect; it would have been better to try turning off a pack that can be turned back on rather than open a hole in the bottom of the plane that can't be closed (even though that was not my intention).I think the planning and decision making went well. FO and I weighed the pros and cons of cancelling the priority handling and the option of diverting enroute. In each case he provided good input that helped memake the decision. As mentioned before; we did have some challenges in communicating with Dispatch to get Dispatcher's input. As I was never able to have a conversation with [Maintenance Control]I did not have the benefit of their input in the decisions. I was ableto speak to [name removed] long enough to get concurrence on cancelling the priority request but most all the communications we had were through ACARS.Our workload management went well overall. FO and I were able to separate our duties; but then come together and recreate a shared mental model of the situation.Purser; and the rest of the cabin crew also did a great job in handling the situation. The fact that none of the passengers expressed any concerns after the flight is a testament to their composure and professionalism. Please forward my thanks to them all.FO did an excellent job during this and all the flights we flew. He is very capable and experienced; and managed the flying and ATC coordination perfectly while I was tending to the other duties. He is a fantastic resource on the flight deck. I truly look forward to flying with him again.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.