Narrative:

Received aircraft with no logbook discrepancies. Upon start of the #2 engine; it was noted that insufficient pressure or an air leak of some sort would not allow for a normal start. Advised ground crew to tow aircraft back to the gate where a logbook entry was made and maintenance notified. Later; air conditioning pack #1 was placed on MEL and an amended dispatch release was obtained with restrictions to remain at or below FL250. Engines started normally; taxi and after takeoff checklist were completed. First officer was pilot flying; captain was pilot monitoring. A check airman occupied the flight deck jumpseat. Approximately 5-10 minutes into cruise at FL250; the cabin altitude warning horn sounded. We confirmed the cabin altitude had reached 10;000 ft and immediately accomplished the immediate action items; donning the masks; communicating; with captain assuming pilot flying duties. We declared an emergency and requested a turn toward the coast line to prepare for our descent. A descent clearance was received and the checklists for excessive cabin altitude was completed. When we leveled at 10;000 ft it was determined the safest course of action would be to return to the departure airport. I sent the dispatcher a diversion message via ACARS. We communicated with the flight attendants for a cabin report and I made a PA announcement to reassure the passengers of our safety and intentions. Weighing 127;000 lbs; the QRH was again referenced for overweight landing procedures. ATC asked if we wanted to continue as an emergency aircraft or complete the flight under normal; non-priority handling. We opted for normal traffic handling. Landing was accomplished under QRH guidelines with touchdown less than 200 fpm. Taxi in to the gate was uneventful and the flight was later operated with a different aircraft. Appropriate discrepancies were entered into the maintenance log and the dispatcher was contacted in accordance with company procedures. A mechanical failure of the operating pack or cabin air leaks may have prohibited the aircraft from maintaining normally scheduled cabin altitude. Manual operation of the outflow valve had no affect on reducing cabin altitude.

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Original NASA ASRS Text

Title: B737 Captain is dispatched with one pack inoperative and shortly after leveling off at FL250 the cabin altitude warning horn sounds. Emergency procedures are complied with and Crew receives clearance to 10000 feet. Manual control of the outflow valve has no effect and Crew elects to return to departure airport.

Narrative: Received aircraft with no logbook discrepancies. Upon start of the #2 engine; it was noted that insufficient pressure or an air leak of some sort would not allow for a normal start. Advised Ground Crew to tow aircraft back to the gate where a logbook entry was made and Maintenance notified. Later; air conditioning pack #1 was placed on MEL and an amended dispatch release was obtained with restrictions to remain at or below FL250. Engines started normally; taxi and after takeoff checklist were completed. First Officer was pilot flying; Captain was pilot monitoring. A Check Airman occupied the flight deck jumpseat. Approximately 5-10 minutes into cruise at FL250; the cabin altitude warning horn sounded. We confirmed the cabin altitude had reached 10;000 ft and immediately accomplished the immediate action items; donning the masks; communicating; with Captain assuming pilot flying duties. We declared an emergency and requested a turn toward the coast line to prepare for our descent. A descent clearance was received and the checklists for excessive cabin altitude was completed. When we leveled at 10;000 ft it was determined the safest course of action would be to return to the departure airport. I sent the Dispatcher a diversion message via ACARS. We communicated with the Flight Attendants for a cabin report and I made a PA announcement to reassure the passengers of our safety and intentions. Weighing 127;000 lbs; the QRH was again referenced for overweight landing procedures. ATC asked if we wanted to continue as an emergency aircraft or complete the flight under normal; non-priority handling. We opted for normal traffic handling. Landing was accomplished under QRH guidelines with touchdown less than 200 fpm. Taxi in to the gate was uneventful and the flight was later operated with a different aircraft. Appropriate discrepancies were entered into the maintenance log and the Dispatcher was contacted in accordance with company procedures. A mechanical failure of the operating pack or cabin air leaks may have prohibited the aircraft from maintaining normally scheduled cabin altitude. Manual operation of the outflow valve had no affect on reducing cabin altitude.

Data retrieved from NASA's ASRS site as of April 2012 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.