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.

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Original NASA ASRS Text

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

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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.