Narrative:

Climbing through approximately 15;000 ft. We had a left pack trip. We leveled off at FL190 and accomplished the pack non-normal checklist. The left pack light extinguished; but re-illuminated after about a minute. We re-ran the pack checklist again and the left pack light extinguished again. I suggested to the first officer (first officer) to check the three supply duct temperatures after the second left pack trip. We observed the aft supply duct temperature to be excessively high; approximately 70 degrees C; and rising. I then suggested that we turn the aft cab temp selector to off. After we did that the aft supply duct temperature seemed to stabilize; but was still high. I noticed the left ram door full open light was illuminated as well. There was also a slight burning smell in the cockpit. I attempted to have the dispatcher call me in the hopes of getting a phone patch with [maintenance] via the call me prompt. We never heard the dispatcher over the frequency; and only briefly heard san francisco arinc. Arinc attempted several selacal's ; but we could hear no one on the designated frequency. I suggested a new frequency to dispatch via acars; but never heard back. At this point the left pack was stabilized; and we had burned some fuel by staying low. Both the first officer and myself felt that it was much safer to return to [departure airport] rather than climb to FL390 with an overheating; compromised pack. We diverted safely and contacted both [operations manager] and dispatcher. We also thoroughly briefed the mechanics at the return gate.

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Original NASA ASRS Text

Title: B737NG Captain reported returning to departure airport after twice experiencing left pack trip.

Narrative: Climbing through approximately 15;000 ft. we had a left pack trip. We leveled off at FL190 and accomplished the PACK non-normal checklist. The left pack light extinguished; but re-illuminated after about a minute. We re-ran the PACK checklist again and the left pack light extinguished again. I suggested to the FO (First Officer) to check the three supply duct temperatures after the second left pack trip. We observed the aft supply duct temperature to be excessively high; approximately 70 degrees C; and rising. I then suggested that we turn the Aft Cab Temp Selector to off. After we did that the aft supply duct temperature seemed to stabilize; but was still high. I noticed the left ram door full open light was illuminated as well. There was also a slight burning smell in the cockpit. I attempted to have the Dispatcher call me in the hopes of getting a phone patch with [Maintenance] via the Call Me prompt. We never heard the Dispatcher over the frequency; and only briefly heard San Francisco Arinc. Arinc attempted several SELACAL's ; but we could hear no one on the designated frequency. I suggested a new frequency to Dispatch via acars; but never heard back. At this point the left pack was stabilized; and we had burned some fuel by staying low. Both the FO and myself felt that it was much safer to return to [departure airport] rather than climb to FL390 with an overheating; compromised pack. We diverted safely and contacted both [Operations Manager] and Dispatcher. We also thoroughly briefed the mechanics at the return gate.

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.