Aircraft Equipment Problem Critical
Air Traffic Control Issued New Clearance; Flight Crew Overcame Equipment Problem; General Maintenance Action
Aircraft
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.
Climbing to FL370; we experienced cabin altitude warning through FL290. The cabin altitude was right at 10;000 FT and climbing at 1;000 FT per minute. The outflow valve was almost fully closed with good flow on both packs. We contacted ATC; and descended to 15;000 FT. At about 21;000 FT in the descent; the cabin altitude controller switched to standby and transfer lock out. The cabin began to descend and operate normally. Once the cabin went back below 9;000 FT; we requested FL290 and continued to [destination]. What I find odd about the event is that even though the number 1 controller was obviously not operating properly; I do not understand why the cabin was climbing. It does not matter which controller is operating; but rather the position of the outflow valve and the flow rate into the cabin. When the controller switched to standby; neither variable changed; but all of a sudden the cabin descended.
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.