Narrative:

During cruise at FL250 due to a placarded right air conditioning pack; we were sent 40 mi south of desired course to hold due to slow down into ZZZ1. PF turned the aircraft toward the holding fix and reduced airspeed to 250 KIAS in preparation of holding. While holding procedures were being worked out along with endurance calculations; the #1 air conditioning pack tripped due to an over-temperature. The system had been running normally in the manual mode with no problems other than a higher than normal sensitivity to small changes in cabin temperature. When I looked up the air supply temperature gauge was pegged at 160 degrees C and the cabin altitude was climbing at 500 FPM. We immediately manually closed the manual pressure control valve in order to minimize the release of pressure from the aircraft. 1 pack reinstatement was attempted without success. The decision was made to immediately descend before troubleshooting the problem in hopes of getting the aircraft to 10000 ft before the passenger oxygen system automatically deployed the masks. Center initially only would clear us down to 24000 ft; so we declared the emergency and started an emergency descent down to 11000 ft. Both pilots used supplemental oxygen even though the maximum cabin altitude reached was 10500 ft. Once level; we opened the ram air valve; which brought down the temperature on the air supply gauge and after 4-5 mins the pack came back on line. While maneuvering for landing in ZZZ2 the pack unexplainably tripped 2 more times with the cabin temperature selected full cold. We flew an uneventful ILS into ZZZ2 and turned the aircraft over to maintenance. As a side note; the support at ZZZ2 was fantastic.

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Original NASA ASRS Text

Title: MD 80 CABIN ALT ACTIVATED WITH ONLY ONE AC PACK OPERABLE. FLT CREW DECLARED EMER TO DESCEND. FLT CREW WAS UNABLE TO CONTROL CABIN ALT OR TEMP; SO A DIVERSION WAS NECESSARY.

Narrative: DURING CRUISE AT FL250 DUE TO A PLACARDED R AIR CONDITIONING PACK; WE WERE SENT 40 MI S OF DESIRED COURSE TO HOLD DUE TO SLOW DOWN INTO ZZZ1. PF TURNED THE ACFT TOWARD THE HOLDING FIX AND REDUCED AIRSPD TO 250 KIAS IN PREPARATION OF HOLDING. WHILE HOLDING PROCS WERE BEING WORKED OUT ALONG WITH ENDURANCE CALCULATIONS; THE #1 AIR CONDITIONING PACK TRIPPED DUE TO AN OVER-TEMP. THE SYS HAD BEEN RUNNING NORMALLY IN THE MANUAL MODE WITH NO PROBS OTHER THAN A HIGHER THAN NORMAL SENSITIVITY TO SMALL CHANGES IN CABIN TEMP. WHEN I LOOKED UP THE AIR SUPPLY TEMP GAUGE WAS PEGGED AT 160 DEGS C AND THE CABIN ALT WAS CLBING AT 500 FPM. WE IMMEDIATELY MANUALLY CLOSED THE MANUAL PRESSURE CTL VALVE IN ORDER TO MINIMIZE THE RELEASE OF PRESSURE FROM THE ACFT. 1 PACK REINSTATEMENT WAS ATTEMPTED WITHOUT SUCCESS. THE DECISION WAS MADE TO IMMEDIATELY DSND BEFORE TROUBLESHOOTING THE PROB IN HOPES OF GETTING THE ACFT TO 10000 FT BEFORE THE PAX OXYGEN SYS AUTOMATICALLY DEPLOYED THE MASKS. CTR INITIALLY ONLY WOULD CLR US DOWN TO 24000 FT; SO WE DECLARED THE EMER AND STARTED AN EMER DSCNT DOWN TO 11000 FT. BOTH PLTS USED SUPPLEMENTAL OXYGEN EVEN THOUGH THE MAX CABIN ALT REACHED WAS 10500 FT. ONCE LEVEL; WE OPENED THE RAM AIR VALVE; WHICH BROUGHT DOWN THE TEMP ON THE AIR SUPPLY GAUGE AND AFTER 4-5 MINS THE PACK CAME BACK ON LINE. WHILE MANEUVERING FOR LNDG IN ZZZ2 THE PACK UNEXPLAINABLY TRIPPED 2 MORE TIMES WITH THE CABIN TEMP SELECTED FULL COLD. WE FLEW AN UNEVENTFUL ILS INTO ZZZ2 AND TURNED THE ACFT OVER TO MAINT. AS A SIDE NOTE; THE SUPPORT AT ZZZ2 WAS FANTASTIC.

Data retrieved from NASA's ASRS site as of January 2009 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.