B737 Next Generation Undifferentiated
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 125; Flight Crew Total 13781
Situational Awareness
Aircraft Equipment Problem Less Severe
Flight Crew FLC complied w / Automation / Advisory; Flight Crew Overcame Equipment Problem
Aircraft
Aircraft was MEL right air conditioning pack. Inbound crew informed us that cabin temperature control was difficult; and cabin was very warm almost entire trip. MEL procedure was bleeds off takeoff and bleeds off approach and landing. Inbound crew said that after APU was started on approach; cabin began to cool down just before landing. Our leg was 30 minutes long; and dispatched at 14;000 feet. We elected to use APU air for pressurization at cruise; since it is rated to 17;000 feet; and it had already demonstrated it cooled the cabin better than engine bleed air. Takeoff was uneventful; and right pack switch was kept in HIGH as directed. Just after we were cleared to descend via the arrival; the cabin pressure warning horn went off. I checked the cabin altimeter; and it had just crept to 10;000 feet. We implemented procedures; and started the descent as previously cleared. The cabin altimeter went below 10;000 feet almost immediately; and the horn went off about the time we reached 8;000 feet. The flight continued to an uneventful bleeds off landing.I wrote up the APU for failure to keep pressurization at 14;000 feet. I followed up with Maintenance Control a few hours later and asked what the fix was that allowed the airframe to return to [the origin airport]. I was told by Maintenance Control that they thought the maintenance configuration of the MEL on that right pack might be faulty; which may be the cause of the poor temperature control. They also surmised that the airframe was 'leaky' enough that the APU was unable to keep it completely pressurized with just the left pack. They were still discussing the possibilities with no resolution at that time.
B737NG Captain reported that the aircraft was unable to maintain pressurization and temperature control.