Date: 201311
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Light: Night
Aircraft: B737-300
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Cruise
ATC / advisory: Center ZZZ
Aircraft component: Pressurization System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Before taking the flight; it was noted that there had been extensive work completed to one of the Pneumatic Air Conditioning systems.  There were no abnormalities on climbout or initial cruise.  [Enroute] the crew noted the Cabin Pressure light illuminated and the Cabin Altitude Warning Horn sounding.  Within seconds the crew had masks donned and within half a minute; communications were established.  After quickly determining that the cabin altitude was slowly and steadily increasing by 300 FT per minute; I called for the QRH and the Cabin Altitude Warning checklist.  We followed the checklist by placing the pressure controller in manual mode and attempting to close the outflow valve further.  The valve appeared to be fully closed and this action did not change the situation at all.  Since the rise in cabin altitude was very slow and steady and we were over congested airspace; we contacted ATC and requested diversion to [a nearby suitable airport] (selected to be less congested and with excellent services for 737s) with lower altitude and priority handling rather than the emergency descent which we felt would put us and other traffic in danger.  Since the situation was progressing slowly we opted to not declare an emergency at that time.  Descending through FL240 we noticed the cabin altitude change from climbing 300 FPM to descending at nearly 1;000 FPM and the cabin began to re-pressurize.  We continued to [diversion airport].  [An extra crewmember] was extremely helpful in locating the charts; however we were immediately frustrated to discover that some of the approach plates we needed for this airport were missing or misplaced from of one of the sets of charts.  We were able to locate one complete set of approach plates and continued to the field without further difficulty.  After landing; we assisted in a series of ground tests as directed by Maintenance using the Boeing procedures for testing pressurization on the ground.  We were unable to pressurize the aircraft on the ground; but in hindsight the concern was raised about the crew pressurizing to 4.0 PSI differential and potentially experiencing decompression sickness if later at altitude.  We're not sure if 4.0 PSI positive differential is enough to cause decompression sickness.  After the freight was picked up by another aircraft; we returned to [our departure airport] under a ferry permit at 10;000 FT unpressurized.Suggestions: As this was likely caused by an unexpected mechanical failure; I'm not certain it could be completely avoided in the future; however; having working crew issued iPads on the flight deck with complete sets of updated charts would be extremely helpful in the chaos of an unplanned diversion to an enroute airport. Thanks to checklist usage; aircraft systems knowledge and teamwork using all resources available in the flight deck; this potentially serious event became very routine.




Synopsis: B737 flight crew member reported loss of cabin pressure while enroute. The flight elected to deviate to a nearby suitable airport.