Narrative:

We were at 3000 ft MSL, cleared for ILS runway 36L. We were tracking the localizer in IMC conditions with a ceiling of 900 ft overcast. Approximately over grand prairie airport, we were forced to respond to TCASII RA. The aircraft ahead of us on the approach had questioned the controller about a non-descript TCASII indication. The traffic we avoided appeared very suddenly about 1 mi in front of us, 400 ft below and climbing into our path. Fortunately, the preceding airplane had said something, so we were looking for the traffic. The other aircraft continued to climb to within 200 ft of our altitude, 2800 ft. As the 2 aircraft symbols on the TCASII started to merge, TCASII commanded a climb RA with over 2000 FPM climb rate. I was hand flying the airplane and responded immediately. The first officer notified ATC. We quickly cleared the conflicting traffic and returned to a normal flight path and landed without further incident. At the gate, I called the approach control facility to discuss the incident. I was told that a helicopter had penetrated the class B airspace without clearance. 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 system, 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 equipment in our airplanes. The system really did its job.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

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.