Small Aircraft; Low Wing; 1 Eng; Fixed Gear
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 2.0
Situational Awareness
ATC Issue All Types; 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 Separated Traffic; Flight Crew Requested ATC Assistance / Clarification; General Flight Cancelled / Delayed
Human Factors; Airspace Structure; Procedure
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.
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.