Citation V/Ultra/Encore (C560)
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical
Flight Crew Diverted; Flight Crew Took Evasive Action; General Declared Emergency
Aircraft
During our flight our passenger came up and said that is was getting pretty warm in the cabin and asked if we could cool it down a bit. We selected a cooler position on the temperature controller (about 7 o'clock). Approximately 1 to 2 minutes later the ACM O'HEAT and the EMER PRESS annunciators illuminated (the EMER PRESS valve did not open automatically). The cabin started climbing rapidly at 3400 fpm. We asked ATC for a lower altitude and the PF donned his mask (PM was already wearing mask). The steps in the ACM O'HEAT and EMER PRESS checklists (selecting a warmer temp and cycling the Pressurization Source Select knob) yielded no results. The cabin kept climbing; the CAB ALT HI annunciator illuminated and ATC was not very forthcoming with descent clearances; so we declared an emergency and initiated the emergency descent. When I looked back during the descent; the passenger O2 masks had deployed. Our pax was wearing her mask and gave me a 'thumbs up'. Once we were below 10;000ft; after accomplishing the Emergency Descent checklist and putting the masks away; we decided on a close and suitable diversionary airport with rapidly improving weather conditions. We sent a quick message off to the company about the diversion. I also went back and talked to our passenger for a few minutes and explained the situation to her. At 5000 ft; while being vectored for the ILS approach; the EMER PRESS valve finally did pop open. We reset it by selecting EMER PRESS; then back to NORM.
The MEL'd temperature contoller had been written up multiple times in the previous month.
CE-560E flight crew reported difficulty maintaining cabin pressure. An emergency was declared; descent executed; and the flight diverted to a nearby safe alternate.