Gulfstream IV / G350 / G450
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Situational Awareness; Workload; Distraction
Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance; Flight Crew Became Reoriented
Human Factors; Aircraft
Upon being vectored for the ILS approach to Runway XX at ZZZ; ATC cleared us to descend from 3;000 MSL to 2000 MSL. The PNF was distracted while trying to control the cabin pressure that had been wildly uncontrollable since the beginning of descent from FL410. As the PIC and PF; I set the Altitude Alerter to 2;000 and vertical speed to 700 FPM descent; with autopilot engaged. During the descent to 2;000 MSL; I briefly looked up at the Pressurization Control Panel to monitor the PNF progress in controlling the cabin pressure; I noted the cabin pressure was finally close to the aircraft's actual altitude. I directed the PNF to select 'RAM AIR' ON.  That action stabilized the cabin for landing. ATC called to confirm our 2;000 feet clearance limit and I realized that the aircraft did not level off at 2;000 feet and the autopilot was not engaged. I acknowledged our clearance to ATC; manually flew the plane from 1;700 feet to 2;000 feet and reengaged to Autopilot. Since the cabin pressure was finally under control; an uneventful approach and landing were completed. Due to the distraction of the cabin pressure control issues; as PF I briefly failed to monitor the aircraft flight profile. Since I had tasked the PNF to attempt to stabilize the cabin pressure; I take full responsibility for failing to continuously monitor the aircraft flight profile.No matter what cockpit distractions occur; it is incumbent upon the PF to continuously monitor the aircraft flight profile. And if using the Autopilot; PF should continuously monitor that it functions as programed and does not inadvertently disconnect.
The flight went well up to the time of descent when the cabin pressure started to malfunction. All attempts were made to control the pressurization using ADC 1 & 2 and Manual control. As preparations were made for landing; air traffic control issued further descent to 2;000 feet and the flying pilot was in control of the aircraft with the auto pilot engaged. The aircraft did not capture the 2;000 foot altitude because the autopilot was inadvertently disconnected and the aircraft continued descent to 1;600 feet at which time the deviation was noted and immediately corrected.  The aircraft was subsequently vectored onto the final course and the rest of the approach and landing was uneventful.  The pilot monitoring should have noticed the altitude deviation as a result of the auto pilot not being engaged while manually controlling the pressurization and the pilot flying should have been aware that the auto pilot was not engaged. It is imperative that the pilot flying concentrates on flying the aircraft while the other attends to the malfunction and refers to the checklists.
GIV flight crew reported an altitude deviation occurred when they became distracted by a cabin pressure issue and did not notice that the autopilot had disconnected.