EMB145 LNDG AT ORD RWY 27L EXPERIENCED TCASII RA AND EXECUTED GAR; TWR INDICATED TFC WAS A HELI 500 FT BELOW.

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WHILE ON FINAL APCH FOR RWY 27L TO ORD; WE HAD JUST PASSED THE FAF INBOUND AND SWITCHED TO TWR FREQ. WHEN WE RECEIVED A TCAS AURAL WARNING 'TFC; TFC;' CLOSELY FOLLOWED BY A 'CLB; CLB' RA SHOWING A RECOMMENDED CLB RATE OF AT LEAST 1500 FPM; WHICH I COMPLIED WITH. AS A RESULT OF COMPLYING WITH THE RA AND OUR CLOSE PROX TO THE LNDG RWY; WE WERE NO LONGER IN A POS FOR A NORMAL LNDG AND I INFORMED THE FO THAT I WAS ABANDONING THE APCH AND TO INFORM TWR THAT WE WERE GOING AROUND DUE TO AN RA; WHICH HE DID. WHEN TWR GOT THE CALL ABOUT OUR GAR; THEY REPLIED THAT THE TFC WAS A HELI AND THAT WE EASILY HAD 500 FT ON HIM. NEITHER MYSELF NOR MY FO EVER SAW THE TFC; ALTHOUGH WE LOOKED FRANTICALLY FOR IT DURING THE WHOLE EVENT. AS WE WERE SWITCHED TO APCH FREQ FOR VECTORS FOR ANOTHER APCH TO RWY 27L; I INFORMED THE TWR CTLR THAT OUR RA COMMANDED A 1500 FPM CLB; AND SOME WORDS TO THE EFFECT THAT I DOUBTED WE HAD THE NEEDED CLRNC. I BASE THIS ON THE FACTS THAT WE WERE FULLY STABILIZED AT A FAIRLY LOW AIRSPD ON THE APCH AND THAT WE WERE INSIDE THE FAF; AND OUR BAROMETRIC RATE OF DSCNT WAS AT OR BELOW 1000 FPM. I SPOKE LATER VIA LANDLINE TO THE TWR SUPVR AND HE STATED THAT '2 QUALIFIED CTLRS HAD THE HELI TFC IN SIGHT.' I MENTIONED TO HIM THAT MY TCAS SHOULDN'T GO OFF IF I HAD THE NECESSARY SEPARATION; AND ALSO THAT I THINK THAT THEY SHOULD HAVE BEEN PROVIDING TA'S TO BOTH PLTS OF BOTH ACFT IF THEY WERE GOING TO BE THAT CLOSE. I FEEL THAT THE CHICAGO TWR CTLRS WERE LAX IN PROVIDING PROPER SEPARATION BTWN ACFT; ESPECIALLY SO LOW TO THE GND INSIDE THE FAF; AND ALSO FAILED TO PROVIDE TIMELY AND ACCURATE TA'S TO THE PLTS OF OUR ACFT AND THE HELI TFC. CONTRIBUTING TO THE SITUATION WAS OUR INABILITY TO SEE AND AVOID AN ACFT BELOW OUR GLIDE PATH ON A CAVU DAY IN THE NORMAL GND CLUTTER; ESPECIALLY CONSIDERING THAT IT WAS A HELI THAT MAY OR MAY NOT HAVE BEEN MOVING FAST ENOUGH FOR US TO VISUALLY ACQUIRE AND AVOID THIS LOSS OF SEPARATION. BEING CLRED FOR THE VISUAL APCH HAS ITS RESPONSIBILITIES; BUT FOR THE TWR TO THEN PLACE A HELI AT THE PLACE THAT IT DID IN THIS INSTANCE; I THINK CONSTITUTES POOR JUDGEMENT ON THE PART OF THE PARTICIPATING CTLRS. I AM NOT SURE IF THE HELI PLT WAS AT HIS ASSIGNED ALT AND POS; SINCE WE NEVER SAW HIM; BUT THIS POSSIBILITY COULD ALSO HAVE LED TO THIS EVENT AS WELL.