Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 7
Human-Machine Interface; Distraction; Communication Breakdown; Workload
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Airspace Structure; Human Factors; Procedure
Aircraft X was descending for an arrival into a non towered airport. I had elected to vector Aircraft X north of the IAF for a straight in approach. Aircraft X descended with the terrain as instructed until entering a 13;000 ft. MIA (Minimum IFR Altitude) box. At that point Aircraft X was cleared direct the IAF. After Aircraft X was cleared direct the IAF; I cleared the aircraft to cross the initial approach fix at or above 13;000 ft. for the straight in RNAV Approach. The Pilot read back the clearance and left out the straight in command. I restated the straight in portion of the clearance to the Pilot and he read back the straight in portion of the clearance correctly.I attended to a few other duties before switching the aircraft to CTAF. As I returned to switch Aircraft X to the CTAF I noticed the aircraft had descended below 13;000 ft. to 11;800 ft. I restated the clearance to verify that the Pilot was to cross the IAF at 13;000 ft.; then issued a low altitude alert for the area MIA the Pilot was in. At that point the Pilot was in the lowest MIA in that area of 12;000 ft. The aircraft had descended to 11;200 ft. before I read the low altitude alert because it took me a few seconds to bring up my MIA chart and verify that he wasn't on the approach already as well as in the 12;000 ft. MIA box. When advising the aircraft of the low altitude alert the Pilot responded that they were currently on a visual approach. I verified with the Pilot of Aircraft X that the airport was in sight. The pilot confirmed airport in sight; and that is when I cleared the Pilot for the visual approach. I advised my Supervisor of the incident immediately after the visual approach clearance was given.
A Center Controller reported an aircraft descended below its assigned altitude and below the Minimim IFR Altitude.