Narrative:

C172 was on an assigned heading of 240 to have him pass 15 miles east of cmh at 45; so he would be below the glide slope for the 28L ILS which was the only one in use as runway 28R was closed. Emb-145 was on an assigned heading of 310 to intercept the ILS 28L localizer and level at 50. I exchanged traffic at approximately 10 miles separation and both pilots acknowledged they would look for traffic but said it was hazy. I issued traffic again at approximately 2 miles and neither one reported the other in sight. C172's mode C altitude had indicated 46 when I first issued traffic at 10 miles and I gave him the altimeter setting and he verified he was level at 45. The required separation standard between IFR and VFR aircraft in class C outer area airspace is 500 feet. Emb-145 advised that his TCAS had issued a resolution advisory and that they were climbing when the aircraft were within 1/2 mile of each other. I advised them that there was no other traffic and asked them to advise when they were able to return to 50. Emb-145 appeared to climb to 57 before returning to 50 about 1 1/2 miles past C172. At that point; I cleared emb-145 for the ILS 28L. I advised them that they appeared to be above the glide slope; but they said they wished to continue with the approach. I acknowledged them and told them to contact cmh tower. Recommendation; I probably should have turned C172 so that emb-145 could have intercepted the glide slope without being the minimum altitude above him. I was trying to best accommodate both aircraft while using minimum separation standards. TCAS doesn't seem to agree with our standards.

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Original NASA ASRS Text

Title: CMH Controller described a TCAS RA event between IFR traffic level at 5;000 FT and VFR traffic reported/observed at 4;500 FT; the reporter questioned TCAS software vs. ATC separation standards.

Narrative: C172 was on an assigned heading of 240 to have him pass 15 miles East of CMH at 45; so he would be below the glide slope for the 28L ILS which was the only one in use as Runway 28R was closed. EMB-145 was on an assigned heading of 310 to intercept the ILS 28L localizer and level at 50. I exchanged traffic at approximately 10 miles separation and both pilots acknowledged they would look for traffic but said it was hazy. I issued traffic again at approximately 2 miles and neither one reported the other in sight. C172's Mode C altitude had indicated 46 when I first issued traffic at 10 miles and I gave him the altimeter setting and he verified he was level at 45. The required separation standard between IFR and VFR aircraft in Class C outer area airspace is 500 feet. EMB-145 advised that his TCAS had issued a resolution advisory and that they were climbing when the aircraft were within 1/2 mile of each other. I advised them that there was no other traffic and asked them to advise when they were able to return to 50. EMB-145 appeared to climb to 57 before returning to 50 about 1 1/2 miles past C172. At that point; I cleared EMB-145 for the ILS 28L. I advised them that they appeared to be above the glide slope; but they said they wished to continue with the approach. I acknowledged them and told them to contact CMH tower. Recommendation; I probably should have turned C172 so that EMB-145 could have intercepted the glide slope without being the minimum altitude above him. I was trying to best accommodate both aircraft while using minimum separation standards. TCAS doesn't seem to agree with our standards.

Data retrieved from NASA's ASRS site as of April 2012 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.