Date: 201208
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Daylight
Aircraft: Citation V/Ultra/Encore (C560)
Operator: Fractional
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pressurization Control System
Component problem: Malfunctioning
Reporter organization: Fractional
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Physiological - Other; Training / Qualification; Workload; Time Pressure
Second reporter function: Captain; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Critical; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Took Evasive Action; General Declared Emergency
Contributing factors: Aircraft; Weather
Primary problem: Aircraft

Narrative: En route we were at FL450 in order to top weather and provide a smooth ride for passengers. The cabin pressure suddenly began to climb at approximately 1;000 FT per minute. We donned the O2 mask; dropped the mask for our passengers and began a rapid descent to the requested and assigned altitude of FL340. We performed the checklist in accordance with Company SOP's and continued the descent to 10;000 FT and then with the Company's input; diverted and made a normal landing.


Second narrative: We were filed at and flying at FL400 as we approached a line of thunderstorms. I (pilot not flying) discussed with pilot flying and we decided to climb. I requested and was cleared to FL430. As we continued several minutes later the ride began to deteriorate again; so we discussed and requested FL450. About 15 minutes after this; while flying over some thunderstorms; my inner ear felt discomfort and my partner noticed it as well; he stated 'I think we've got a pressurization issue.' We both scanned the pressurization system and noticed that the cabin had begun to climb at about 600-1;000 FPM (the rate was varying quite a bit as we commenced the descent.) I was wearing my mask; and ordered my co-pilot to put his on and commence a descent. We both realized that we would be descending into severe weather; so we hesitated while he scanned the radar for the best route down; and I called ATC. I told them we had a pressurization issue and needed an emergency descent. She said 'Standby.' I replied that no; we were starting down now. She cleared us immediately to FL340. Pilot flying set that; I acknowledged; and he accomplished the memory items for the emergency descent. I dropped the O2 masks in the cabin; seat belt sign was already on; and turned to brief the passengers to don them immediately; and explained what was happening. We activated the anti-ice systems as we descended through the weather; but could not keep the wing anti-ice lights out without adding power; which pilot flying handled expertly. ATC then cleared us to 10;000 FT; and asked us where we wanted to go. I was in the process of finishing up the checklist for the pressurization event and assessing that question. I asked about nearby airports with 5;000 FT or better runways. We conferred with him and decided between Controller; myself; and the pilot flying on a nearby International Airport. I set about pulling the approach charts and weather. Once we descended to 10;000 FT; then; I believe; to 7;000 FT as cleared we removed our masks. I handed off my duties to the pilot flying then I briefed passengers on they could remove theirs; and of our plan to go to a divert airport; reassured them that everything was safe. I then called the Company Assistant Chief Pilot on the aircraft phone to bring company into the loop; effect a recovery flight; and to assess with him our landing choice. At this point everything was stabilized; safe; plenty of fuel. Assistant Chief Pilot assessed weather radar and suggested that we were at an equal distant between two suitable airports; and that the weather at another one was considerably better. I had pulled up weather after this suggestion; and agreed with him. I then conferred with pilot flying; and we decided the other airport was the better choice. Notified ATC; and were re-cleared. We landed without further incident.


Synopsis: A CE-560 pressurization failed at FL450 so an immediate descent was begun; an emergency declared; and while navigating around thunderstorms the crew diverted to a nearby airport.