Narrative:

On the arrival into phl; philly approach control issued us a descent to 7;000 ft and a heading of 090. We were about 20 miles from the airport and then ATC vectored us onto a 070 heading. Rolling out on the 070 heading; ATC issued us a traffic alert for a single engine VFR aircraft at 12 o'clock at an altitude of 6;500 ft (500 ft below us). We informed ATC that we were searching but did not have the traffic in sight. The VFR aircraft then reported us in sight. We did not have that traffic on our TCAS at that time; however other aircraft appeared on the TCAS just fine. As we approached the traffic; he finally showed up on our TCAS and we immediately got a TA followed by an RA. At this point I picked up the traffic and the flying pilot followed the climbing RA. When the aircraft finally showed up on our TCAS; he was 400 ft below us and appeared to be gradually climbing. We had the current ATC issued local altimeter setting in. He passed directly underneath us as we climbed up to a little above 7;300 ft. I informed ATC we were climbing to follow an RA; and when clear of the conflict we descended back to 7;000 ft and continued without further incident. There were several factors related to this incident. The biggest was the fact that philly approach vectored us directly over a VFR aircraft at only 500 ft above when we told them we didn't have the aircraft in sight. The next factor was that small single engine piston aircraft not showing up on our TCAS until the last second. We had flown that plane all day and had no trouble with our TCAS. Also; on that flight we had all other targets showing up on our TCAS. This led us to believe there was an issue with the small plane's equipment. Finally; the VFR aircraft being closer then the 500 ft ATC anticipated and likely gradually climbing was the final factor that led to the RA event. I believe that ATC should not vector aircraft directly over each other with fewer than 1;000 ft of vertical separation. If the controller would have turned us away from the traffic when we said we did not have the traffic in sight then we would not have gotten an RA. I think there is a lack of communication and possibly some controllers might not know how little deviation it takes from that small separation to quickly become a TCAS event.

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Original NASA ASRS Text

Title: EMB145 First Officer reports a TCAS RA generated by a VFR aircraft 500 FT below and reported by ATC. The VFR traffic appeared to be climbing slightly as it passed under with the Captain responding to the RA.

Narrative: On the arrival into PHL; Philly Approach Control issued us a descent to 7;000 FT and a heading of 090. We were about 20 miles from the airport and then ATC vectored us onto a 070 heading. Rolling out on the 070 heading; ATC issued us a traffic alert for a single engine VFR aircraft at 12 o'clock at an altitude of 6;500 FT (500 FT below us). We informed ATC that we were searching but did not have the traffic in sight. The VFR aircraft then reported us in sight. We did not have that traffic on our TCAS at that time; however other aircraft appeared on the TCAS just fine. As we approached the traffic; he finally showed up on our TCAS and we immediately got a TA followed by an RA. At this point I picked up the traffic and the flying pilot followed the climbing RA. When the aircraft finally showed up on our TCAS; he was 400 FT below us and appeared to be gradually climbing. We had the current ATC issued local altimeter setting in. He passed directly underneath us as we climbed up to a little above 7;300 FT. I informed ATC we were climbing to follow an RA; and when clear of the conflict we descended back to 7;000 FT and continued without further incident. There were several factors related to this incident. The biggest was the fact that Philly Approach vectored us directly over a VFR aircraft at only 500 FT above when we told them we didn't have the aircraft in sight. The next factor was that small single engine piston aircraft not showing up on our TCAS until the last second. We had flown that plane all day and had no trouble with our TCAS. Also; on that flight we had all other targets showing up on our TCAS. This led us to believe there was an issue with the small plane's equipment. Finally; the VFR aircraft being closer then the 500 FT ATC anticipated and likely gradually climbing was the final factor that led to the RA event. I believe that ATC should not vector aircraft directly over each other with fewer than 1;000 FT of vertical separation. If the Controller would have turned us away from the traffic when we said we did not have the traffic in sight then we would not have gotten an RA. I think there is a lack of communication and possibly some Controllers might not know how little deviation it takes from that small separation to quickly become a TCAS event.

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.