Date: 201206
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Daylight
Aircraft: Citation II S2/Bravo (C550)
Operator: Air Taxi
Operating under: Part 135
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pressurization Control System
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Flight Instructor; Flight Crew Air Transport Pilot (ATP)
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Diverted; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Weather conditions at the time of departure were heavy rain and low overcast clouds due to a tropical storm. On departure the cabin was set in automatic temperature control mode and set for approximately 40;000 as our planned cruise altitude was initially FL390. After takeoff the cabin showed a normal climb and was holding pressure appropriately. Approximately 1 hour and 15 minutes into the flight we had been level at FL360 and initiated a climb to FL380 with a block from FL370 to FL380 due to the slow rate of climb. At 37;400 the First Officer noticed the cabin begin climbing at 1;500 FT per minute. I immediately stopped the climb and began back down towards FL370. I advised the First Officer to place his oxygen mask on and also did so myself. I then asked the First Officer to initially request FL300. While the First Officer was securing his mask I grabbed the emergency checklist and directed him to begin the checklist as appropriate. He began the Cabin Altitude Above Selected checklist and as he began doing so the cabin increased its rate of climb to nearly 4;000 FT per minute. As First Officer was completing the initial checklist items the 10;000 FT cabin altitude light came on and shortly thereafter the passenger oxygen masks automatically deployed. After the First Officer placed the pressurization source to emergency pressure; we noted a change in cabin rate however it was still climbing at 2;000 FT per minute and above 14;000 FT. Unable to secure the cabin from climbing we initiated an emergency descent as per the checklist. At this time we asked for 10;000 FT from ATC and after initially receiving FL240; then 11;000 FT we stopped our descent at 10;000 FT. During the descent we verified that the passengers were using their oxygen as appropriate. Upon reaching 10;000 FT we advised ATC that we would like to divert and received a clearance to do so. At this time I advised the passengers that we had an issue with the cabin pressurization and that it was under control at that time and we were diverting. In an effort to comfort the passengers we placed the cabin control back to normal to reduce the heat and noise and turned on the air conditioning. We landed without incident and taxied to the FBO.




Synopsis: A CE550 lost cabin pressure while climbing through FL370. Attempts to control the cabin pressure were unsuccessful and an emergency descent and diversion were made.