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

Attributes
ACN	1585947
Time
Date	201810
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Light	Daylight
Aircraft 1
Make Model Name	B737-700
Operating Under FAR Part	Part 121
Flight Phase	Climb
Flight Plan	IFR
Component
Aircraft Component	Air Conditioning and Pressurization Pack
Person 1
Function	Pilot Not Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 107 Flight Crew Type 366
Person 2
Function	Pilot Flying Captain
Qualification	Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument
Experience	Flight Crew Last 90 Days 118 Flight Crew Type 11055
Events
Anomaly	Aircraft Equipment Problem Critical

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.

Original NASA ASRS Text

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

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.

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.