Narrative:

While holding for dtw at the polar intersection; I had three aircraft issued holding instructions. While issuing holding instructions; I briefly noticed a VFR limited data block near the holding pattern that only registered for one or two apparent hits that seemed level at 115. When issuing lower to a DC95; I cleared the aircraft to 130; but then noticed the target again apparently level at 130. I told the DC95 to maintain 140 then called the traffic. Again; the limited data block information disappeared and I began issuing known and relevant traffic information to the DC95. When the target appeared again; the aircraft was now at 138 as was the DC95. I re-issued the cleared altitude of 140 to the DC95 and began calling traffic which appeared 1-2 miles from the DC95. I then noticed the limited data block appeared to be climbing into the DC95's altitude and told the DC95 immediately. At this time the DC95 said they were getting an RA and I allowed the DC95 to perform maneuvers while also telling them of additional traffic in the pattern. When they completed the maneuver; I cleared them to 130 which was now available. I informed management early on that I had what appeared to be a VFR aircraft in the holding with a faulty or misused transponder before the RA; also because of the nearby tfr. The unknown aircraft continued giving different transponder readings; switching from VFR; to primary only; to discreet code until they left my airspace near phn airport. I don't know if the unknown aircraft did anything 'illegal' but this was a very dangerous event and the DC95 pilot informed me that if they had not followed the RA they would have been co located at the same altitude. There have been several less dangerous situations over the years I have been here involving VFR aircraft and a loss of separation. I'm not sure what can be done to make GA pilots more aware of peak traffic areas that they should take into consideration and avoid; but clearly; I believe this could have resulted in a loss of life if not for the TCAS advisory.

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Original NASA ASRS Text

Title: ZOB Controller described a TCAS RA event during holding operations at POLAR Intersection; apparently activated by an unknown VFR aircraft maneuvering in and/or near the holding pattern airspace.

Narrative: While holding for DTW at the POLAR Intersection; I had three aircraft issued holding instructions. While issuing holding instructions; I briefly noticed a VFR limited Data Block near the holding pattern that only registered for one or two apparent hits that seemed level at 115. When issuing lower to a DC95; I cleared the aircraft to 130; but then noticed the target again apparently level at 130. I told the DC95 to maintain 140 then called the traffic. Again; the limited Data Block information disappeared and I began issuing known and relevant traffic information to the DC95. When the target appeared again; the aircraft was now at 138 as was the DC95. I re-issued the cleared altitude of 140 to the DC95 and began calling traffic which appeared 1-2 miles from the DC95. I then noticed the limited Data Block appeared to be climbing into the DC95's altitude and told the DC95 immediately. At this time the DC95 said they were getting an RA and I allowed the DC95 to perform maneuvers while also telling them of additional traffic in the pattern. When they completed the maneuver; I cleared them to 130 which was now available. I informed management early on that I had what appeared to be a VFR aircraft in the holding with a faulty or misused transponder before the RA; also because of the nearby TFR. The unknown aircraft continued giving different transponder readings; switching from VFR; to primary only; to discreet code until they left my airspace near PHN airport. I don't know if the unknown aircraft did anything 'illegal' but this was a very dangerous event and the DC95 pilot informed me that if they had not followed the RA they would have been co located at the same altitude. There have been several less dangerous situations over the years I have been here involving VFR aircraft and a loss of separation. I'm not sure what can be done to make GA pilots more aware of peak traffic areas that they should take into consideration and avoid; but clearly; I believe this could have resulted in a loss of life if not for the TCAS advisory.

Data retrieved from NASA's ASRS site as of July 2013 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.