Cos TRACON trainee reported they vectored an aircraft that had descended to avoid VFR traffic below ...

Attributes
ACN	1650536
Time
Date	201906
Local Time Of Day	1801-2400
Place
Locale Reference	COS.Airport
State Reference	CO
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Hercules (C-130)/L100/382
Operating Under FAR Part	Part 91
Flight Phase	Final Approach
Route In Use	Other Instrument Approach
Flight Plan	IFR
Person 1
Function	Approach Trainee
Qualification	Air Traffic Control Developmental
Events
Anomaly	ATC Issue All Types Conflict Airborne Conflict Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Published Material / Policy Inflight Event / Encounter CFTT / CFIT

Narrative:

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.

Original NASA ASRS Text

Title: COS TRACON Trainee reported they vectored an aircraft that had descended to avoid VFR traffic below the Minimum Vectoring Altitude.

Narrative: 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.

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.

Cos TRACON trainee reported they vectored an aircraft that had descended to avoid VFR traffic below the minimum vectoring altitude.