Narrative:

While on final approach to runway 4R inside rekks (marker beacon) at approximately 1400 ft AGL the PNF switched to tower frequency for landing clearance. The tower was talking to traffic landing on runway 9. Next the TCASII gave us a TA and a white high wing single engine aircraft was spotted directly below and just slightly to the left of our aircraft flying a heading approximately 10-20 degree right of our heading. We were on a descend on final and it appeared that he would pass under us. The TA warning gave us an audit 'traffic, traffic' and a yellow DOT on the radar screen but could not read an altitude or climb or descend arrow because he was directly below us. He did not pass under us so we leveled off and advised the tower. The tower said they would switch their radar and picked him up. By this time we started a go around and advised the tower of same. We were vectored around and completed another approach and landing. We contacted the tower by telephone and was advised that they would pass us on to TRACON who had him on radar. TRACON advised us that they had a light aircraft at 1500 ft VFR outside the marker. This evidently was not our traffic and we could not get any further information. The TCASII system was in the TA selected position because we were on final approach.

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Original NASA ASRS Text

Title: CLOSE PROX ACR-WDB GA-SMA IN TFC PATTERN AT ORD.

Narrative: WHILE ON FINAL APCH TO RWY 4R INSIDE REKKS (MARKER BEACON) AT APPROX 1400 FT AGL THE PNF SWITCHED TO TWR FREQ FOR LNDG CLRNC. THE TWR WAS TALKING TO TFC LNDG ON RWY 9. NEXT THE TCASII GAVE US A TA AND A WHITE HIGH WING SINGLE ENG ACFT WAS SPOTTED DIRECTLY BELOW AND JUST SLIGHTLY TO THE L OF OUR ACFT FLYING A HDG APPROX 10-20 DEG R OF OUR HDG. WE WERE ON A DSND ON FINAL AND IT APPEARED THAT HE WOULD PASS UNDER US. THE TA WARNING GAVE US AN AUDIT 'TFC, TFC' AND A YELLOW DOT ON THE RADAR SCREEN BUT COULD NOT READ AN ALT OR CLB OR DSND ARROW BECAUSE HE WAS DIRECTLY BELOW US. HE DID NOT PASS UNDER US SO WE LEVELED OFF AND ADVISED THE TWR. THE TWR SAID THEY WOULD SWITCH THEIR RADAR AND PICKED HIM UP. BY THIS TIME WE STARTED A GAR AND ADVISED THE TWR OF SAME. WE WERE VECTORED AROUND AND COMPLETED ANOTHER APCH AND LNDG. WE CONTACTED THE TWR BY TELEPHONE AND WAS ADVISED THAT THEY WOULD PASS US ON TO TRACON WHO HAD HIM ON RADAR. TRACON ADVISED US THAT THEY HAD A LIGHT ACFT AT 1500 FT VFR OUTSIDE THE MARKER. THIS EVIDENTLY WAS NOT OUR TFC AND WE COULD NOT GET ANY FURTHER INFO. THE TCASII SYS WAS IN THE TA SELECTED POS BECAUSE WE WERE ON FINAL APCH.

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.