Date: 201810
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Air Conditioning and Pressurization Pack
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Workload; Troubleshooting; Time Pressure
Second reporter function: Pilot Flying; Captain
Anomaly: Aircraft Equipment Problem Critical
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Diverted; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Landed in Emergency Condition; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Departure Airport; Flight Crew Took Evasive Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: With a L-Pack deferred; we had a R-Pack Trip Off during climbout. We requested to level off (FL 180); and started the QRH procedure for a -700 Pack Trip Off. The procedure calls for increasing pack temperature to reduce load; which was unresponsive in both auto and manual modes; and a reset of the pack was also unsuccessful. During the reset attempts; cabin pressure began climbing; and a further descent was requested (10;000 feet). The Captain began an assertive descent; when around 12;000 feet MSL; the cabin altitude alarm sounded. We followed the memory items to don masks and establish communications; and leveled at 10;000 feet shortly thereafter. Since masks were no longer required; we took them off; elected to return to our departure airport; and landed without further incident. Being a very dynamic situation with limited time and procedures complicated due to an already deferred pack; some of the procedures and communications could have been better to facilitate CRM and lessen distraction and confusion during the event.


Second narrative: We departed ZZZ with MEL 21-1-XX; Left AC pack inoperative. Passing through approximately 16;500 feet; I noticed the cabin rate was climbing and the cabin pressure was approaching 8000 feet. We asked for a level off at FL 180. At level off; the Right Pack Trip Off light illuminated and I asked the FO (First Officer) for the Pack Trip Off Checklist. The cabin pressure continued to rise and we asked for (and were granted) a descent to 10;000 feet. As the Pilot Flying; I lowered the nose using Vertical Speed mode and the speedbrake to get the aircraft down to a safer cabin altitude. Passing through approximately 14;000 feet; the Cabin Altitude Warning horn sounded and the Cabin Altitude light illuminated. We immediately performed our QRC memory items; donning our oxygen masks with regulators set to 100% and commencing to establish Crew communications. After leveling off at 10;000 feet; I noticed the Cabin Altitude light extinguished and the cabin pressure altitude was at a safe level to remove our masks. We continued with all of our checklist procedures and the Right Pack would not reset. After requesting a return to ZZZ; we were given the ZZZZZ Arrival direct to ZZZ1; and subsequently ATC said they would be giving us vectors back to ZZZ. As we continued completing our checklists; I didn't come back and ask if the headings were given because of any navigational errors. ATC asked if we were [advising] and we told them 'No'. Landing and taxi to the gate were uneventful and the masks did not drop in the Passenger cabin. The flight attendants and First Officer confirmed they were doing fine; and I am not aware of any medical issues from the passengers after our return to the gate.Even though it was not the middle of summer in ZZZ; I am not sure it is good Customer Service to fly Revenue Passengers on a flight with only one operating AC Pack when considering passenger comfort. Taxiing out; I noticed the aircraft was not as cool as normal. As the Captain; I felt I gave a thorough Captains brief with the FO during our preflight briefings. Using both of our EFBs; we went over the entire MEL and Pressurized Flight with One Pack Inoperative. I noticed during climbout the cabin pressure was higher than normal; but attributed this to having the use of only one operating AC pack. In retrospect; it would have been a better idea to have asked immediately for a lower altitude the first time I noticed the cabin pressure increasing; instead of just asking to level off. Additionally; over the past few years we have had increased training using our oxygen masks in the simulator. Even with training; we initially had difficulty talking with the mask; so I recommend continuing to emphasize this training with our Aircrews.


Synopsis: 737 flight crew reported a return to departure airport due to a loss of pressurization.