MD80 FLT CREW RESPONDS TO TCASII ON APCH TO DFW WITH D10 AND IS ADVISED THAT A VFR HELI PENETRATED CLASS B WITHOUT CLRNC. WX A FACTOR.

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WE WERE AT 3000 FT MSL; CLRED FOR ILS RWY 36L. WE WERE TRACKING THE LOC IN IMC CONDITIONS WITH A CEILING OF 900 FT OVCST. APPROX OVER GRAND PRAIRIE ARPT; WE WERE FORCED TO RESPOND TO TCASII RA. THE ACFT AHEAD OF US ON THE APCH HAD QUESTIONED THE CTLR ABOUT A NON-DESCRIPT TCASII INDICATION. THE TFC WE AVOIDED APPEARED VERY SUDDENLY ABOUT 1 MI IN FRONT OF US; 400 FT BELOW AND CLBING INTO OUR PATH. FORTUNATELY; THE PRECEDING AIRPLANE HAD SAID SOMETHING; SO WE WERE LOOKING FOR THE TFC. THE OTHER ACFT CONTINUED TO CLB TO WITHIN 200 FT OF OUR ALT; 2800 FT. AS THE 2 ACFT SYMBOLS ON THE TCASII STARTED TO MERGE; TCASII COMMANDED A CLB RA WITH OVER 2000 FPM CLB RATE. I WAS HAND FLYING THE AIRPLANE AND RESPONDED IMMEDIATELY. THE FO NOTIFIED ATC. WE QUICKLY CLRED THE CONFLICTING TFC AND RETURNED TO A NORMAL FLT PATH AND LANDED WITHOUT FURTHER INCIDENT. AT THE GATE; I CALLED THE APCH CTL FACILITY TO DISCUSS THE INCIDENT. I WAS TOLD THAT A HELI HAD PENETRATED THE CLASS B AIRSPACE WITHOUT CLRNC. I ADDED THAT HE WAS ALSO OPERATING IN IMC CONDITIONS AS WE DIDN'T BREAK OUT UNTIL 900 FT AGL. IF IT HAD NOT BEEN FOR THE TCASII SYS; IT IS VERY LIKELY THAT I WOULD NOT BE HERE. THIS WAS THE CLOSEST NEAR MIDAIR I'VE EXPERIENCED SINCE THE INSTALLATION OF THE TCASII EQUIP IN OUR AIRPLANES. THE SYS REALLY DID ITS JOB.