Date: 201209
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pressurization Control System
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Human factors: Situational Awareness; Workload; Training / Qualification
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Maintenance
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; Flight Crew Took Evasive Action; General Maintenance Action; General Declared Emergency
Contributing factors: Aircraft; Procedure
Primary problem: Aircraft

Narrative: Previous flight en route had equipment cooling fan lights with both fans inoperative. Contract Maintenance performed corrective action and completed logbook entry. On subsequent flight same problem occurred after passing approximately 10;000 FT MSL. [We were] cleared to FL340. Passing FL270 Cabin Altitude Warning horn sounded. The pilot flying (Captain) pointed to Cabin Altitude indicator at approximately 9;500 FT. Pilot flying immediately initiated descent. Pilot flying and pilot not flying donned O2 masks and established communications. Pilot not flying broadcasted to ATC that we had experienced a loss of pressurization; declared an emergency; and stated we were executing an emergency descent to 10;000 FT MSL. Pilot not flying opened checklist and commenced Cabin Altitude Warning Horn/Abnormal Pressurization Checklist and Emergency Descent Checklist while communicating with ATC. Packs were noted in high. (Pilot not flying had selected high after flap retraction for cooling.) Engine bleeds were ON and APU bleed OFF. There was no indication of a failed pressurization controller. 'Man DC' was selected and the outflow valve was closed manually. Cabin altitude began to descend. [We] leveled at 8;000 FT MSL (per ATC Instructions). Pilot flying spoke with cabin crew and made a PA to the passengers as the pilot not flying coordinated approach and landing preparation with ATC. Pilot not flying programmed the FMC and got performance data for landing at the divert airport. Pilot not flying contacted Operations and coordinated gate arrival. Pilot flying and pilot not flying discussed overweight landing considerations. [We] received vectors to visual final to Runway 1. Landing was uneventful. After landing we both heard a loud tone and were unable to determine the source. We later discovered it was the lavatory fire/smoke warning. Completed checklist and pulled circuit breaker. No fires noted. Upon arrival at the gate; pilot flying observed the cabin was still pressurized. [Pilot flying] directed pilot not flying to slowly open outflow valve to depressurize the cabin. Pilot not flying (me) had failed to open the outflow valve after the aircraft was below 1;000 FT MSL.




Synopsis: At FL270 a B737 Cabin Altitude Warning horn sounded so an emergency was declared and the flight descended to 8;000 FT. On this flight and the previous flight the Equip Cooling OFF light illuminated indicating a lack of air flow.