FLC OF AN ACR MLG ACFT DEVIATED FROM ASSIGNED DSCNT ALT IN RESPONSE TO A TCASII RA.

Narrative

INTO DFW; ACFT WAS BEING VECTORED FOR A VISUAL APCH TO RWY 17L AT DFW. WX CONDITIONS WERE VFR; BUT SUN AND HAZE AT DUSK MADE VISIBILITY DIFFICULT. ATC FREQ WAS BUSY DUE TO NUMEROUS ACFT ARRIVING AT DFW. THE CAPT WAS FLYING AND BEGAN TO SLOW AND CONFIGURE THE ACFT AT 15 MI FROM DFW. ALT WAS 5000 FT MSL; SPD 250 KTS. ATC ASKED THE CREW IF THE ARPT WAS IN SIGHT. IT WAS NOT; DUE TO HAZE. THE ATC FREQ BECAME BUSY AND THE CREW CONTINUED ON A WESTERLY HDG. THE ARPT BECAME VISIBLE AS THE ACFT PASSED ABEAM THE APCH END OF RWY 17L. SIMULTANEOUSLY; ATC GAVE THE CREW A 90 DEG TURN S; WITH THE INSTRUCTIONS TO SLOW TO 170 KTS; AND CLRED THE VISUAL APCH TO RWY 17L. AS THE ACFT WAS IN A TIGHT TURN TO THE L; LEAVING 2800 FT MSL; TCASII ISSUED A MANDATORY 'CLB NOW' ALERT. DUE TO THE ACFT'S ATTITUDE; AND LOW VISIBILITY; THE CAPT IMMEDIATELY BEGAN TO CLB. ATC WAS NOTIFIED; AND THEY ACKNOWLEDGED THE RPT. ATC ASKED IF THE RWY WAS STILL IN SIGHT; THEN RECLRED THE CREW FOR THE VISUAL APCH TO RWY 17L. THE ACFT WAS NOW AT 3800 FT MSL; AND THE TCASII ISSUED THE 'CLR OF CONFLICT' ALERT. THE CAPT INTERCEPTED THE APCH PATH FOR RWY 17L; AND MADE A NORMAL LNDG. CAUSE OF INCIDENT: FACTORS INCLUDED LOW VISIBILITY; EXCESS SPD; AND TCASII ALERT. RECOMMENDATIONS: INCREASED TFC SEPARATION DURING MARGINAL VFR APCHS. ALL MAJOR ARPTS SHOULD STANDARDIZE ON SPD CTL FOR ACFT SEPARATION DURING APCH. TCASII PROCS SHOULD BE ADAPTED TO COMPENSATE FURTHER FOR DENSE TFC IN PARALLEL APCH ENVIRONMENT.