Commercial Fixed Wing
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 28
Distraction; Situational Awareness
Conflict Airborne Conflict; Deviation - Track / Heading All Types
Air Traffic Control Provided Assistance; Air Traffic Control Issued New Clearance
Airspace Structure
Aircraft X was on ILS approach to Runway 4 at GSP when a VFR aircraft inbound to GMU went across final under Aircraft X. Traffic was called. GMU [Tower] was called. Aircraft X was below the clouds and stated he would execute missed approach if there was a RA. The pilot seemed to stay above glide slope to avoid the RA. Both aircraft eventually got each other in sight and Aircraft X landed.This is a known safety problem. The closest proximity of 2 aircraft in this situation is zero lateral and less than 100 feet vertical if the GSP arrival is on the glideslope and the VFR is in the GMU Delta airspace. However; it should not be taken for granted that aircraft on instrument and visual approaches will be at or above glideslope altitude. Since the VFR is under the GSP arrival; which is usually a jet; there are also wake turbulence issues.The Charlie and Delta airspace design is flawed. The FAA is either unwilling or unable to redesign the airspace to provide for the safety of the flying public.From my experience; the VFR aircraft involved are always inbound to GMU from SC72 (Chandelle). It is probably less than half a dozen aircraft that occasionally do this. When they depart GMU; they are instructed to fly over VPLOW when GSP Runway 4 is in use. If they were instructed to fly over VPLOW when inbound to GMU; the problem would be resolved. GMU Tower can instruct aircraft to remain outside the Delta and instruct aircraft where to enter the Delta; so there is no reason this solution would not work. It could be agreed to; briefed and implemented in 24 hours.This situation develops quickly and there are no guarantees that the controllers will see it before the RA or collision alert. When it is seen; or when the alarms go off; what does FAA management want the controllers to do?  Should we be breaking off the approach as a preventive measure?
GSP Tower Controller reported an airborne conflict over intersection VPLOW. Controller stated this is an ongoing issue between IFR and VFR aircraft near VPLOW intersection.