Date: 201606
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: Citation X (C750)
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Air Conditioning and Pressurization Pack
Component problem: Malfunctioning
Reporter organization: Air Taxi
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: Pilot Flying; Captain
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; Flight Crew Landed As Precaution
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Our aircraft started the day with the cockpit PACK deferred on MEL; so we were restricted to cruise at FL410 or below. [Captain] was flying in the left seat and I was monitoring in the right. During the climb out to FL400; I noticed the cabin pressurization was higher than normal for the given altitude but was stable. We were experiencing light to moderate turbulence at FL400; and climbed up to FL410 for a smoother ride for the passengers. During the climb; our cabin pressure showed about 500 fpm climb; passing through 8000ft of pressure. Less than 1 min after leveling at FL410; we then received an amber CABIN ALTITUDE CAS message as the cabin surged to 9000ft. [Captain] donned his oxygen mask at the same time I requested FL380 to see if the lower altitude can help our weak PACK with pressurization. We barely leveled off at FL380 when I noticed the cabin pressure not stabilizing. At that point; we made a quick decision as a crew to initiate a [rapid] descent and divert to [a suitable alternate]. I briefed our startled passengers from my pilot seat and donned my oxygen mask. We landed safely. [Captain] and I heard one of the children in the cabin crying during the descent; which was caused by the abrupt changes in cabin pressure. The passengers were startled but had no injuries. According to the Citation X MEL; the aircraft is supposed to have normal pressurization with one PACK up to FL410. This was clearly not the case on our oldest Citation X with 15;200 hours. Depending on the exact cause of the problem; maybe we need to reduce that upper altitude limit while flying with one operating PACK.


Second narrative: The crew acted in compliance with our training and a positive outcome was achieved. The event occurred fairly quickly and the crew was very busy throughout. Due to how quickly the event occurred; no ABNORMAL/EMERGENCY checklist was used. A rapid descent procedure was performed and the proximity of the diversion airport didn't allow any time for troubleshooting the problem. Once the descent was initiated it was obvious to the crew that the trip would not continue to its intended destination and the priority was to get the aircraft safely on the ground as quickly as practical. It is my opinion that the crew worked well as a team and used good CRM to ensure a positive outcome for all involved. The crew's decision to descend and divert was unanimous. It was also decided that since the descent was necessary due to the inadequate cabin pressurization; the expeditious landing at a suitable airport was also necessary because it was obvious that at the lower altitude the trip could not continue to the intended destination. In light of those decisions there was no need loiter or enter a holding pattern to troubleshoot the pressurization problem. It is the opinion of the crew that we acted in a safe and efficient manner.


Synopsis: CE-750 flight crew reported a loss of cabin pressure when operating with one PACK at FL410.