Narrative:

TCASII RA. We were on the dupont arrival to phl at 15000 ft when TCASII traffic alert popped up at our 12 O'clock position at 10 mi. It was at our altitude. At 5 mi, we had an RA to descend. It passed overhead at approximately 1000 ft. ATC issued a descent clearance to 10000 ft about the same time that we started our descent, so I don't believe that an altitude deviation was a problem. We told ATC of the RA, but I did not use the terminology of an near midair collision. He stated that he had previous traffic in that area that popped up intermittently at 15500 ft and that now it showed behind us a that altitude. We believed that the aircraft was a small bizjet at 15000 ft and that we would have had a collision with it had we not responded to the RA. The first officer saw the aircraft out of his eyebrow window. He was also PF and responded immediately to the RA.

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Original NASA ASRS Text

Title: MD80 AND A BIZJET HAD LESS THAN COMFORTABLE SEPARATION IN PHL CLASS E AIRSPACE.

Narrative: TCASII RA. WE WERE ON THE DUPONT ARR TO PHL AT 15000 FT WHEN TCASII TFC ALERT POPPED UP AT OUR 12 O'CLOCK POS AT 10 MI. IT WAS AT OUR ALT. AT 5 MI, WE HAD AN RA TO DSND. IT PASSED OVERHEAD AT APPROX 1000 FT. ATC ISSUED A DSCNT CLRNC TO 10000 FT ABOUT THE SAME TIME THAT WE STARTED OUR DSCNT, SO I DON'T BELIEVE THAT AN ALTDEV WAS A PROB. WE TOLD ATC OF THE RA, BUT I DID NOT USE THE TERMINOLOGY OF AN NMAC. HE STATED THAT HE HAD PREVIOUS TFC IN THAT AREA THAT POPPED UP INTERMITTENTLY AT 15500 FT AND THAT NOW IT SHOWED BEHIND US A THAT ALT. WE BELIEVED THAT THE ACFT WAS A SMALL BIZJET AT 15000 FT AND THAT WE WOULD HAVE HAD A COLLISION WITH IT HAD WE NOT RESPONDED TO THE RA. THE FO SAW THE ACFT OUT OF HIS EYEBROW WINDOW. HE WAS ALSO PF AND RESPONDED IMMEDIATELY TO THE RA.

Data retrieved from NASA's ASRS site as of July 2007 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.