Geg TRACON controller reported an aircraft descended below its assigned altitude and below the MVA.

Attributes
ACN	1505093
Time
Date	201712
Local Time Of Day	1801-2400
Place
Locale Reference	GEG.TRACON
State Reference	WA
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Learjet 31
Operating Under FAR Part	Part 91
Flight Phase	Descent
Route In Use	Vectors
Flight Plan	IFR
Person 1
Function	Approach
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 7
Events
Anomaly	Deviation - Altitude Overshoot Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy Inflight Event / Encounter CFTT / CFIT

Narrative:

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.

Original NASA ASRS Text

Title: GEG TRACON Controller reported an aircraft descended below its assigned altitude and below the MVA.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.