Narrative:

Took off after being delayed all day because of APU problems.  Climbing through 10;000 ft asl; I checked the cabin altitude and differrential pressure and the pressurization system appeared to be operating normally.  Climb continued normally until passing about 28;000 ft where the cabin altitude alerted.  The cabin altitude was just above 10;000 ft and climbing about 500 ft/min.  I immediately requested to stop the climb at 29;000 ft and was cleared to do so.  With the aircraft level and a/P on; the first officer and I quickly looked for the cause of the pressurization problem.  The cabin alttitude was still climbing about 500 ft/min; both air conditioning packs were on and indicating good flow; and the outflow valve indicator appeared to be fully closed.  The cabin auto controller was still in primary.  I confirmed the outflow valve was fully closed.  Consulted  the QRH quickly for pressurization problems.  Unable to stop the cabin from climbing in level flight and with the cabin alttitude passing 12;000 ft; I requested a descent  to 10;000 ft.  I informed ATC that we had a pressurization problem but was not declaring an emergency yet.  ATC granted the descent and the first officer began descending.  The cabin continued to climb during the initial descent.  Descending quickly and with the cabin climbing to about 13;000 ft; the primary controller automatically switched to standby and locked out.  The cabin altitude then began to descend.  I asked for clearance to level at 15;000 ft and ATC okayed.  The cabin continued to descend and the alert cleared at about 9;000 ft cabin altitude.  With the pressurization now operating normally in stby; I asked for a climb to 29;000 ft.  Where we continued to destination without further problems.

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Original NASA ASRS Text

Title: MD82 flight crew experienced a cabin altitude alert climbing through FL280. With the system apparently operating normally and the outflow valve apparently fully closed; the crew initiates a descent. When the cabin altitude reached 13000 feet the primary controller automatically switched to standby and locked out and the cabin altitude began to descend. Flight continues to destination.

Narrative: Took off after being delayed all day because of APU problems.  Climbing through 10;000 ft ASL; I checked the cabin altitude and differrential pressure and the pressurization system appeared to be operating normally.  Climb continued normally until passing about 28;000 ft where the cabin altitude alerted.  The cabin altitude was just above 10;000 ft and climbing about 500 ft/min.  I immediately requested to stop the climb at 29;000 ft and was cleared to do so.  With the aircraft level and A/P on; the FO and I quickly looked for the cause of the pressurization problem.  The cabin alttitude was still climbing about 500 ft/min; both air conditioning packs were on and indicating good flow; and the outflow valve indicator appeared to be fully closed.  The cabin auto controller was still in primary.  I confirmed the outflow valve was fully closed.  Consulted  the QRH quickly for pressurization problems.  Unable to stop the cabin from climbing in level flight and with the cabin alttitude passing 12;000 ft; I requested a descent  to 10;000 ft.  I informed ATC that we had a pressurization problem but was not declaring an emergency yet.  ATC granted the descent and the FO began descending.  The cabin continued to climb during the initial descent.  Descending quickly and with the cabin climbing to about 13;000 ft; the primary controller automatically switched to standby and locked out.  The cabin altitude then began to descend.  I asked for clearance to level at 15;000 ft and ATC okayed.  The cabin continued to descend and the alert cleared at about 9;000 ft cabin altitude.  With the pressurization now operating normally in stby; I asked for a climb to 29;000 ft.  where we continued to destination without further problems.

Data retrieved from NASA's ASRS site as of July 2013 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.