Narrative:

Aircraft X was cleared for the visual approach into runway 4 at gsp about 12 miles south of the airport. 20 seconds later an unknown aircraft Y departed gmu airport which is just a few miles from gsp. Aircraft Y was on a 1200 code not talking to approach control and was departing out the gap in the class C to the southeast that lets planes fly under the gsp approach at 1800 feet. Aircraft Y was indicating 1900 feet. This happens all the time. I advised aircraft X that a plane was departing gmu and would be at 1900 feet over mulde (the final approach fix) on the approach. Aircraft X acknowledged the traffic; but never said they had the traffic in sight. The two got within 300 feet vertical causing the captain (ca) to go off in the TRACON before both continued safely to their destinations. We have been [reporting] this exact scenario for months in hopes that the class C will get changed. The fact that we cannot clear an aircraft into the airport the class C is meant to protect without getting a ca or RA completely defeats the purpose of having the class C. The class C is supposed to protect the main airport. It does not. Allowing aircraft we are not talking with to fly between 300-500 feet of aircraft on an ILS approach let alone a visual approach is dangerous. If both aircrafts altimeters are off there is a risk of a collison. If the aircraft going into gsp is a heavy there is a major risk of a wake turbulence upset of the plane beneath with no altitude to recover. I am getting tired of breaking IFR planes out to keep them safe from VFR aircraft that have found a loophole in the design of the class C.

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Original NASA ASRS Text

Title: A TRACON Controller reported an unidentified VFR aircraft climbed to an altitude below Class C airspace; but was still in confliction with another aircraft on an approach to the Class C airport.

Narrative: Aircraft X was cleared for the Visual Approach into Runway 4 at GSP about 12 miles south of the airport. 20 seconds later an unknown Aircraft Y departed GMU airport which is just a few miles from GSP. Aircraft Y was on a 1200 code not talking to approach control and was departing out the gap in the Class C to the southeast that lets planes fly under the GSP approach at 1800 feet. Aircraft Y was indicating 1900 feet. This happens all the time. I advised Aircraft X that a plane was departing GMU and would be at 1900 feet over MULDE (the final approach fix) on the approach. Aircraft X acknowledged the traffic; but never said they had the traffic in sight. The two got within 300 feet vertical causing the Captain (CA) to go off in the TRACON before both continued safely to their destinations. We have been [reporting] this exact scenario for months in hopes that the Class C will get changed. The fact that we cannot clear an aircraft into the airport the Class C is meant to protect without getting a CA or RA completely defeats the purpose of having the Class C. The Class C is supposed to protect the main airport. It does not. Allowing aircraft we are not talking with to fly between 300-500 feet of aircraft on an ILS approach let alone a Visual Approach is dangerous. If both aircrafts altimeters are off there is a risk of a Collison. If the aircraft going into GSP is a heavy there is a major risk of a wake turbulence upset of the plane beneath with no altitude to recover. I am getting tired of breaking IFR planes out to keep them safe from VFR aircraft that have found a loophole in the design of the Class C.

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.