Air Traffic Control Developmental
Situational Awareness; Distraction; Training / Qualification
ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance; Flight Crew Took Evasive Action
Airspace Structure; Chart Or Publication; Human Factors; Procedure
I turned Aircraft X on a 200 heading to intercept the localizer. As they're in the turn there's a VFR data block crossing the final 3 miles ahead of them. I give them traffic. They say they have traffic in sight. They began to descend 200 feet even they were not cleared on the approach. I keyed up to give the approach clearance. I said their call sign but my instructor told me to give a low altitude alert; so I gave them a low altitude alert and advised they climb. They said they should've told me they were descending for the traffic they had in sight and began their climb. I cleared them on the approach below the MVA (Minimum Vectoring Altitude) when I should've waited until they were at least at 8900 feet. I think we should extend the Class Charlie airspace out to a point where we can vector aircraft to final for an instrument approach without having to run into conflict like this because of VFRs crossing over the final at that exact point.
COS TRACON Trainee reported they vectored an aircraft that had descended to avoid VFR traffic below the Minimum Vectoring Altitude.