Narrative:

Air carrier X southeast of phl approximately 8 mi on left downwind heading 090 degrees for runway 27R level at 4000 ft MSL. Aircraft was on autoplt and captain was PF. Captain observed traffic on TCAS at approximately 1 O'clock position and level; then visually acquired same. Captain instructed first officer to get a left turn immediately. First officer requested turn and phl approach responded with a turn to north and advisement of traffic position. Almost at same time TA turned into an RA requiring a minimum of 1800 FPM climb. Aircraft cleared traffic by 300 ft in a climbing left turn and then observed the light twin on left side proceeding north at 4000 ft MSL. Phl approach issued flight clearance to 5000 ft and advised that the traffic was VFR not talking with them. Flight continued approach and landing normally.

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Original NASA ASRS Text

Title: ACR PLT DESCRIBED TCAS RA EVENT WITH UNREPORTED TFC AT 4000 WHILE ON DOWNWIND TO PHL FOR RWY 27L.

Narrative: ACR X SE OF PHL APPROX 8 MI ON L DOWNWIND HDG 090 DEGS FOR RWY 27R LEVEL AT 4000 FT MSL. ACFT WAS ON AUTOPLT AND CAPT WAS PF. CAPT OBSERVED TFC ON TCAS AT APPROX 1 O'CLOCK POS AND LEVEL; THEN VISUALLY ACQUIRED SAME. CAPT INSTRUCTED FO TO GET A L TURN IMMEDIATELY. FO REQUESTED TURN AND PHL APCH RESPONDED WITH A TURN TO N AND ADVISEMENT OF TFC POS. ALMOST AT SAME TIME TA TURNED INTO AN RA REQUIRING A MINIMUM OF 1800 FPM CLB. ACFT CLRED TFC BY 300 FT IN A CLBING L TURN AND THEN OBSERVED THE LIGHT TWIN ON L SIDE PROCEEDING N AT 4000 FT MSL. PHL APCH ISSUED FLT CLRNC TO 5000 FT AND ADVISED THAT THE TFC WAS VFR NOT TALKING WITH THEM. FLT CONTINUED APCH AND LNDG NORMALLY.

Data retrieved from NASA's ASRS site as of January 2009 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.