Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe
Flight Crew Landed As Precaution; Flight Crew Diverted; General Declared Emergency
Aircraft
While cruising at FL400 I heard and felt a 'pop' in my ears which seemed to emanate from somewhere in the back of the passenger cabin. I looked back and saw the 2 passengers looking around the rear seats and lavatory area (on later discussion with the passengers; they thought a shampoo or lotion bottle had burst in the cabin). At first; I thought that one of the outflow valves had become momentarily stuck and then released. Because I felt a change in pressure in my ears; I proceeded to monitor the cabin pressure altitude indicator. At first no change was discernible. However; after a few minutes it became obvious that the cabin altitude was slowly climbing. Due to Part 135 regulations I had already donned my oxygen mask before climbing above FL350. I instructed the PM to advise ATC of our situation and ask for an immediate descent. We decided to divert to [a nearby airport] since we were almost directly overhead the airport at that time. ATC provided vectors and an immediate descent to lower altitudes as we were directed away from possible conflicts with other aircraft. We were able to descend to 10;000 FT just as the cabin altitude gauge indicated 10;000 FT. I flew the airplane and the PM handled all communications; passenger briefings; and checklists. The airport was VFR and we were vectored for a visual approach and uneventful landing. Although we were met by the airport's fire department on taxiing in to the ramp.  At no time during the event did we get any annunciators or other system warnings of a problem. The passenger oxygen masks never deployed nor were they needed. We did not declare an emergency and because of ATC's quick reaction we did not have to do a full emergency descent; although our descent rate at times was around 4;000 fpm. The company sent a recovery aircraft and the passengers were back on their way after about an hour and a half on the ground.After speaking with a Tech at a Cessna Service Center; it seems that the pink (or orange) couplings around the air ducts that feed air from the ACM to the cabin (CE560XLS) may get loose and back pressure from the system at altitude may cause that coupling to slide back from the duct thus allowing pressurized air to escape and not go into the cabin. One of the things we noticed was that air conditioned air out of the overhead wemacs stopped after we heard and felt the 'pop'. At lower altitudes and on the ground the condition may seem to correct itself but then reoccur at altitude. I have now included checking these couplings more closely as part of my preflight inspection.  I am thankful that we monitored the cabin altitude after the anomaly occurred and recognized a situation that could have been tragic for our passengers; ourselves; and the company had we not caught it in time. Instead it turned out to be a mild inconvenience for our passengers and just another day of flying for us. I would highly advise anyone who experiences unexplained changes in airflow through the wemacs or senses pressure changes (especially in your ears) in flight to vigilantly monitor the cabin altitude indicator and be prepared for an immediate descent to a lower altitude. Many thanks to my SIC and ATC for the smooth and professional support during this incident.
CE-560XLS Captain reported loss of cabin pressure while cruising at FL400. The pressure loss was slow enough to allow the aircraft to descend to 10;000 FT before they met the cabin altitude. Reporter mentioned air duct couplings as a probable cause.