A controller reported an aircraft descending below their assigned altitude and the minimum vectoring...

Attributes
ACN	1454918
Time
Date	201706
Local Time Of Day	0001-0600
Place
Locale Reference	FAI.TRACON
State Reference	AK
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	Small Aircraft Low Wing 1 Eng Fixed Gear
Operating Under FAR Part	Part 91
Flight Phase	Final Approach
Route In Use	Other Instrument Approach
Flight Plan	IFR
Aircraft 2
Make Model Name	Single Engine Turboprop Undifferentiated
Operating Under FAR Part	Part 121
Flight Phase	Final Approach
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) 2.0
Events
Anomaly	ATC Issue All Types Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy Inflight Event / Encounter CFTT / CFIT

Narrative:

I took the radar position from the previous controller. Right before I accepted the position; he cleared aircraft Y for the visual approach. As aircraft Y got closer to the field I realized the aircraft he had already instructed to contact tower; aircraft X; was on an instrument approach (being afforded IFR separation). By that time aircraft Y was 5-6 miles in trail of aircraft X doing three times the speed.I cancelled his visual approach clearance and told him to maintain 3000 ft and fly heading 110 to square him off to final. Their mode C indicated they were at 3500 ft and descending in a 2900 ft MVA (minimum vectoring altitude). He asked for clarification on the altitude then read it back. I addressed other traffic and when I went back to him I saw he had descended to 2000 ft. I asked him to verify he was maintaining 3000 ft. He said he was already through 3000 ft when it was issued so he was climbing back up. I subsequently cleared him for the visual approach without losing IFR separation between him and the preceding aircraft X. Deconfliction of traffic before passing the position to the next controller or a more thorough briefing.

Original NASA ASRS Text

Title: A Controller reported an aircraft descending below their assigned altitude and the Minimum Vectoring Altitude; while the Controller was trying to de-conflict traffic already on approach.

Narrative: I took the radar position from the previous controller. Right before I accepted the position; he cleared Aircraft Y for the Visual Approach. As Aircraft Y got closer to the field I realized the aircraft he had already instructed to contact tower; Aircraft X; was on an instrument approach (being afforded IFR separation). By that time Aircraft Y was 5-6 miles in trail of Aircraft X doing three times the speed.I cancelled his Visual Approach clearance and told him to maintain 3000 ft and fly heading 110 to square him off to final. Their mode C indicated they were at 3500 ft and descending in a 2900 ft MVA (Minimum Vectoring Altitude). He asked for clarification on the altitude then read it back. I addressed other traffic and when I went back to him I saw he had descended to 2000 ft. I asked him to verify he was maintaining 3000 ft. He said he was already through 3000 ft when it was issued so he was climbing back up. I subsequently cleared him for the Visual Approach without losing IFR separation between him and the preceding Aircraft X. Deconfliction of traffic before passing the position to the next controller or a more thorough briefing.

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.

A controller reported an aircraft descending below their assigned altitude and the minimum vectoring altitude; while the controller was trying to de-conflict traffic already on approach.