Narrative:

On vectors from phl approach right base to final, runway 27R descending from 3000 to 2100 ft. We got TCASII TA, 2 targets 900 ft below us. I asked approach if he had any traffic in our area. He replied negative. Shortly, another TA, approach said he had a pop-up target ahead of us, issued new heading for intercept of ILS. I think the controller said 'maintain 3000' but I wasn't sure it was for us. I tried to verify the clearance, meanwhile more TCASII alerts and picked up traffic about 12:30, just below us. At first both first officer, who was flying, and I thought it was a mid sized commuter airplane a few mi ahead on a parallel base leg. We looked, listened to TCASII and tried to talk to approach (a condition verging on 'data overload') then realized the traffic was an small transport much closer to us. I instructed the first officer to 'take it up' raising my hand, palm up several times. He smoothly applied nose up and some power (we were flaps 2 degree/190 KTS) and I could see that he had the traffic in sight on his side as it went out of my sight, below and to the right. I looked at the TCASII screen, the red square said -01, the first officer said he saw .01 NM on the data line, upper right, for lateral separation. We both heard the TCASII say, I believe, 'descend, descend' for an initial resolution and then 'monitor vertical speed'. The lower arc of the first officer's vertical speed indicator was red as the traffic passed below. The traffic was illegally flying through the TCA at 2100 ft (phl field visibility reported as 1 1/2 mi). Without a clearance, I rarely have TCASII on board and don't get a lot of practice with it. However, it alerted us to a real threat -- fortunately squawking mode C -- and helped prevent a likely midair collision.

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Original NASA ASRS Text

Title: FLC OF LGT RECEIVED TCASII RA. CLBED TO AVOID. SAW SMT TFC PASS TO R 600 FT, APPROX 2100.

Narrative: ON VECTORS FROM PHL APCH R BASE TO FINAL, RWY 27R DSNDING FROM 3000 TO 2100 FT. WE GOT TCASII TA, 2 TARGETS 900 FT BELOW US. I ASKED APCH IF HE HAD ANY TFC IN OUR AREA. HE REPLIED NEGATIVE. SHORTLY, ANOTHER TA, APCH SAID HE HAD A POP-UP TARGET AHEAD OF US, ISSUED NEW HDG FOR INTERCEPT OF ILS. I THINK THE CTLR SAID 'MAINTAIN 3000' BUT I WASN'T SURE IT WAS FOR US. I TRIED TO VERIFY THE CLRNC, MEANWHILE MORE TCASII ALERTS AND PICKED UP TFC ABOUT 12:30, JUST BELOW US. AT FIRST BOTH FO, WHO WAS FLYING, AND I THOUGHT IT WAS A MID SIZED COMMUTER AIRPLANE A FEW MI AHEAD ON A PARALLEL BASE LEG. WE LOOKED, LISTENED TO TCASII AND TRIED TO TALK TO APCH (A CONDITION VERGING ON 'DATA OVERLOAD') THEN REALIZED THE TFC WAS AN SMT MUCH CLOSER TO US. I INSTRUCTED THE FO TO 'TAKE IT UP' RAISING MY HAND, PALM UP SEVERAL TIMES. HE SMOOTHLY APPLIED NOSE UP AND SOME PWR (WE WERE FLAPS 2 DEG/190 KTS) AND I COULD SEE THAT HE HAD THE TFC IN SIGHT ON HIS SIDE AS IT WENT OUT OF MY SIGHT, BELOW AND TO THE R. I LOOKED AT THE TCASII SCREEN, THE RED SQUARE SAID -01, THE FO SAID HE SAW .01 NM ON THE DATA LINE, UPPER R, FOR LATERAL SEPARATION. WE BOTH HEARD THE TCASII SAY, I BELIEVE, 'DSND, DSND' FOR AN INITIAL RESOLUTION AND THEN 'MONITOR VERT SPD'. THE LOWER ARC OF THE FO'S VERT SPD INDICATOR WAS RED AS THE TFC PASSED BELOW. THE TFC WAS ILLEGALLY FLYING THROUGH THE TCA AT 2100 FT (PHL FIELD VISIBILITY RPTED AS 1 1/2 MI). WITHOUT A CLRNC, I RARELY HAVE TCASII ON BOARD AND DON'T GET A LOT OF PRACTICE WITH IT. HOWEVER, IT ALERTED US TO A REAL THREAT -- FORTUNATELY SQUAWKING MODE C -- AND HELPED PREVENT A LIKELY MIDAIR COLLISION.

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.