Super King Air 350
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 4
Situational Awareness; Communication Breakdown; Training / Qualification
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
ATC Equipment / Nav Facility / Buildings; Airspace Structure; Human Factors
While conducting OJT for my trainee; I ensured separation between Aircraft X and a preceding aircraft using vertical separation. Aircraft X entered a known non-radar area and the trainee asked the preceding aircraft his altitude and they responded with level at 7;000 ft. The trainee then issued Aircraft X 8;000 ft. to which the replay was that he was already cleared to 4;000 ft. We asked his altitude and he told us 4;200 ft. The minimum IFR altitude in that area is 4;600 ft. and Aircraft X was climbed to 6;000 ft. but no warning was issued. We did not issue this altitude to this aircraft and confirmed it by reviewing the audio recording after our training session. I do not believe longitudinal separation was lost between Aircraft X and the preceding aircraft due to routings and issued holding instructions. Overall I feel that this was pilot error in a non-radar environment. Having and ADSB-GBT or WAM (Wide Area Multi-lateration) in the area would have given us enough radar coverage to have noticed the early descent out of 15;000 ft. and we could have corrected it before the aircraft descended below the minimum IFR altitude.
Center Controller reported that an aircraft descended below the minimum IFR altitude in a non-radar area; resulting in CFTT.