Learjet 31
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 7
Situational Awareness; Distraction
Deviation - Altitude Overshoot; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Requested ATC Assistance / Clarification
Human Factors; Airspace Structure
I was working both arrival sectors combined. Aircraft X was on a vector for right downwind descending to 4;300 ft setting up for the ILS Approach. Aircraft X read back 4;300 ft correctly. I observed Aircraft X around 4;100 ft and started to issue an immediate climb to 4;300 ft. The aircraft continued its descent and asked for instructions again. I issued an immediate climb and a turn toward the lower MVA of 4;000 ft. The aircraft did not seem to climb. I issued a low altitude alert and told them the MVA in my area. I issued the altimeter. I did not issue the brasher warning. The aircraft advised they were in VFR conditions and had ground contact but could not see the field. Eventually I cleared them for a Visual Approach. They remained at 4;000 ft for a while before starting their descent. They landed without further incident. I filed a Mandatory Occurrence report.I should continue to scan even during slow periods and issue the Low Altitude alert immediately and altimeter.
GEG TRACON Controller reported an aircraft descended below its assigned altitude and below the MVA.