Caravan Undifferentiated
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 12
Distraction; Workload; Confusion; Communication Breakdown
ATC Issue All Types; Deviation - Altitude Overshoot; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT; Inflight Event / Encounter Loss Of Aircraft Control
Air Traffic Control Provided Assistance; Air Traffic Control Issued New Clearance; Flight Crew Regained Aircraft Control; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Overcame Equipment Problem
Aircraft; Weather; Software and Automation; Human Factors; Airspace Structure
Aircraft X was level at 10;000 feet westbound to ZZZ. The pilot asked for a block 10;000 to 12;000 feet; which I issued. Lots of aircraft were taking that same block due to weather in the area. Shortly after the minimum safe altitude warning sounded; and when I looked; Aircraft X was at 7;100 feet in a 5;000 feet minimum IFR altitude. Confused; I continued to watch and realized it was sounding because the aircraft was dropping so rapidly. Knowing something was definitely wrong I reached out to the pilot; but no response. After the aircraft went from 7;100 feet to 6;400 feet to 2;400 feet; I reached out two more times with no response.Finally; the pilot responded and requested descent to 1;000 feet and I informed them they were in a 5;000 feet minimum IFR altitude. They then canceled IFR but never descended and actually climbed to 2;800 feet. Eventually they reached 5;900 feet and requested to return to ZZZ1. A clearance was issued direct and assigned 6;000 feet which they had requested but then climbed to 7;000 feet; so I just issued 7;000 feet. The pilot was very shaken up. It sounds like a software issue? I recommend fixing that or grounding the C208's until it is fixed.
A Center Controller reported a C208 rapidly descended below its assigned altitude; was not responding to ATC calls; and flew below the Minimum IFR Altitude; later the pilot established communications; climbed to a safe altitude and returned to its point of departure.