Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Workload; Physiological - Other; Troubleshooting
Aircraft Equipment Problem Critical
Air Traffic Control Provided Assistance; Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Landed in Emergency Condition; Flight Crew Diverted; Flight Crew Overcame Equipment Problem
Aircraft
Unable to control cabin altitude; [expedited] descentIn cruise FL430; 'CABIN PRESS LOW' Red CASS message illuminated; cabin began to climb 400-600 FPM; Ask for and received lower altitude. Performed checklist; when unable to control cabin rate of climb; [advised ATC] and began [expedited descent]. Regained limited control of the cabin at FL180; continued descent to 12;000 ft. and diverted to suitable airport ZZZ. Approach and landing uneventful.
Gulfstream IV Captain reported a pressurization problem that resulted in an expedited descent and precautionary landing.