Date: 201310
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Weather elements / visibility: scattered clouds; 10
Light: Daylight
Aircraft: Citation Excel (C560XL)
Operator: Personal
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Air Conditioning Distribution Ducting; Clamps; Connectors
Component problem: Failed
Reporter organization: Personal
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Workload
Anomaly: Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Diverted; General Declared Emergency; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: On the afternoon of October 2013 at approximately XA:15 local time; our aircraft was cruising at 40;000 FT. The Captain and I had completed our Cruise Checklist and approximately 30 minutes later; along with our passengers; we noticed that air had ceased to flow from the overhead vents. As with many jets; this air serves not only to control the air temperature in the aircraft; but also to pressurize the aircraft vessel. The Captain and I immediately turned our attention to the Cabin Altitude indicator as well as the Automatic Pressure Controller (APC) to see if there were any abnormalities. We initially noticed a climb in Cabin Rate of Climb of between 200 and 300 FT per minute while the cabin altitude itself began a climb from its initial value of 6;000 FT. We scanned the face of the Automatic Pressure Controller for any error indications thinking we may have had a failure of the APC. It was indicating normally. Within a matter of 10 or 15 seconds; we determined that the best course of action would be to begin a descent while we continue to troubleshoot the unexplained climb in cabin altitude. At this time; we both pulled our emergency oxygen masks from storage and put them in our laps in case a faster loss of pressure occurred. I also pulled the Emergency Checklist and opened it to the emergency descent tab and laid it open and ready. It is important to note that we had no other indications or annunciators during the duration of the flight. I immediately asked for a lower altitude from ATC and we were given 38;000 FT. ATC asked the nature of our situation and I told them it was a sudden drop in pressurization. The Captain initiated an aggressive descent and I asked for lower altitudes repeatedly as the urgency of our descent increased. The cabin altitude continued to climb until the cabin Outflow valve fully closed and momentarily slowed the loss of cabin pressure. Because we were descending aggressively; the cabin altitude never exceeded its upper limits; no annunciators illuminated; and the cabin oxygen masks never fell from their overhead containers. It is important to note that during the descent; we heard loud hums; and abnormal squeals coming from the aft bulkhead area. This confirmed our suspicion that it was not a computer issue; but rather a failure of an Outflow valve or duct. In addition; during the descent; we decided to make a precautionary landing at ZZZ2. With no emergency indications in the cockpit; and because we were able to descend so quickly; we elected not to declare an emergency. The safe completion of the flight was never in question. However; as we were exchanged from the Center Controller to the ZZZ2 Approach Controller; we were asked of souls on board and fuel remaining and were notified that emergency personnel and fire trucks had been dispatched to the runway. We were asked if we had a hydraulic problem and we confirmed that it was a loss of cabin pressure. I also informed them at that time that Emergency services were not required and that cabin pressure was stable and descending at this low altitude. In any event; the trucks and personnel followed us in to the local FBO. After inspection of the aircraft by mechanics; it was determined to be a cracked duct line bringing compressed air to the cabin.

Callback: Reporter stated they were flying a Cessna C-560XLS aircraft. The XLS has a similar fuselage as the XL series; but with improved avionics. The XLS does not have an EICAS System. Flight crews rely on the Annunciator lights for malfunctions.  The lack of any Annunciator indication was one of the concerns they had when he and the Captain; and the passengers in the cabin; noticed the airflow had stopped. They; (he and the Captain); did not go to the 'Manual' Mode because they were not in an emergency situation. Also; by placing the AUTO-Manual switch to MANUAL; cuts out the Auto Pressure Controller (APC) communication to the Climb and Dive solonoids on the Outflow valves. Since there was so many unusual noises; sounds from the Aft bulkhead area; they did not want to cut-out any auto functions at that time and possibly make the situation worse. They did an abbreviated descent; no speedbrakes; no roll and dive procedures; because they were proactive and ahead of the pressurization loss.Reporter stated their aircraft was a 2007 model with only 700 hours on the airframe. Maintenance at one of the aircraft Manufacturer's Service Centers noted he had never seen the type of lengthwise failure of a pneumatic duct that he found after they had arrived. The pneumatic duct is a composite/Kevlar material that had a 16' inch long opening; that looked like someone had cut the duct with a razor blade. The composite duct could be squeezed; but the cut; or opening could only be felt; because the duct failure was on the opposite side from view; when looking in the Aft Equipment Bay in the Tailcone section. The duct was fairly large and routed with cold air from the Air Conditioning Packs to the cabin.



Synopsis: A First Officer reports that he and the Captain flying a C-560XLS aircraft noticed that pressurization air had ceased to flow from the overhead vents while in cruise flight. No Annunciator lights illuminated; indicating a loss of pressurization source. A non-emergency abbreviated descent was accomplished. Maintenance found a large composite cold air duct that had split open approximately 16' inches long; that looked like it had been cut with a razor blade.