CRITICAL AIRBORNE CONFLICT OCCURS AT 5000 FT BTWN A B737 AND A DHC8 ON APCH FOR ILS APCHS TO THE S WHILE 11 NM N OF CVG.

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THE FLT DEPARTED MSY NORMALLY WITH NO AUTOTHROTTLES AND THE FO WAS FLYING. APCHING CVG; THERE WAS A STORM ABOUT 15-20 MI N OF CVG. WE WERE VECTORED TO A DOWNWIND TO RWY 18R AT 7000 FT. LOOKING AT THE WX AND HEARING RPTS OF MODERATE TURB OF AN ACFT NEAR THE WX; I ASKED FOR A L 270 DEG TURN FROM DOWNWIND TOWARDS THE MARKER FOR RWY 18R; ATC CLRED US FOR A R TURN TO 090 DEGS AND DSCNT TO 5000 FT AND WE DID SO. ON THAT HEADING WE ENTERED THE CLOUDS WITH MODERATE TURB. I BEGAN SETTING UP FOR RWY 18L. WE SAW TFC ON THE TCASII AT 11 O'CLOCK POS AND ABOUT 4 MI. I ASKED ATC ABOUT THE TFC AT 11 O'CLOCK POS AT OUR ALT. ATC ASKED THE DHC8 ABOUT THEIR HEADING; THEN SAID 'ACR Y; DSND IMMEDIATELY.' THE FO HEARD A CLB RA AND DISCONNECTED THE AUTOPLT AND STARTED A CLB BEFORE ATC ISSUED US A CLB IMMEDIATELY CALL. I REPLIED TO ATC WITH THE CLB INSTRUCTIONS. WITH OUR NOSE PITCHED UP ABOUT 15-20 DEGS IN THE CLB; THE TCASII NOW SHOWED THE OTHER ACFT AT -01 0R -02 OFF TO OUR L SIDE TO THE N. WITH THE HVY MODERATE TURB SHAKING THE ACFT; IT WAS DIFFICULT FOR THE FO IN THE CLB. THE TURB MADE IT UNCOMFORTABLE TO CONCENTRATE ON ALMOST ANYTHING EXCEPT THE ALT AND PWR SETTING AND DIFFICULT TO SEE THE INSTS AT TIMES. I CHKED THE RADAR/TCASII AND SAW THE OTHER ACFT STILL BELOW US. I WAS DIVIDING MY TIME BTWN MAKING SURE THE ACFT REMAINED STABILIZED AND CHKING THE TCASII TFC. UPON CHKING THE ACFT ATTITUDE AND PWR SETTINGS AND CONFIDENT WE WERE CLBING SATISFACTORILY; I LOOKED AT THE VSI EXPECTING TO SEE A CLB RA; BUT SAW A COMMANDED DSCNT OF ABOUT 4000 FPM. I SAW THE ALT STILL AT -01 ON THE TCASII; WITH IT VERY CLOSE TO OUR ACFT; APPEARING TO BE WITHIN 1 MI. I DID NOT HEAR A DSCNT RA COMMAND AT ANY TIME DURING THE FLT. WITH THE OTHER ACFT BELOW US AND CLOSE; WITHIN 1 MI; I ELECTED TO INCREASE OUR PITCH SLIGHTLY HIGHER AT THAT POINT. THIS WAS THE FIRST TIME I SAW THAT THE TCASII WAS COMMANDING A DSCNT. AT THAT SPLIT SECOND AND MOMENT IN TIME; I FELT IT WAS NOT THE SAFEST COURSE OF ACTION TO TELL THE FO TO REVERSE OUR APPROX 20 DEG PITCH ATTITUDE TO A COMMANDED 4000 FPM DSCNT WITH THE OTHER ACFT APPROX 100 FT BELOW OURS AND MORE THAN LIKELY WITHIN 1 MI OF OUR ACFT. SHORTLY AFTER WE INCREASED THE PITCH SLIGHTLY; WE WERE CLR OF THE OTHER ACFT. WE LEVELED OFF AT ABOUT 6500 FT AND AFTER VECTORS WE LANDED ON RWY 36R IN CVG. I MADE A LOGBOOK ENTRY ABOUT AN UNKNOWN PWR SETTING ON THE ENGS. I CONTACTED FLT CTL; THE DUTY PLT; AND CONTACTED THE CTLR AT THE TRACON AFTER THE FLT. THE CTLR SAID ACFT Y HAD A CLRNC OF 4000 FT; AND A HEADING OF 270 DEGS; BUT DECIDED TO LEVEL AT 5000 FT AND KEEP A SOUTHWESTERLY HEADING INSTEAD OF THE ASSIGNED HEADING ALT. ADHERING TO ATC CLRNCS IS OBVIOUSLY IMPORTANT AND DEVS FROM THOSE CLRNCS NEED TO BE RELATED TO ATC IMMEDIATELY. AVOIDANCE OF UNSTABLE WX WOULD HAVE HELPED. IF CONDITIONS PERMIT; A QUICKER XCHK OF ALL AVAILABLE INSTS. SUPPLEMENTAL INFO FROM ACN 587205: IN ADDITION ON FUTURE SIMULATION TRAINING; IT WOULD BE BENEFICIAL TO HAVE DIFFICULT CONDITIONS: IE; ACFT EMERS OR WX DURING A TCASII ENCOUNTER. ALSO SEEING A TCASII RA REVERSAL WOULD BE BENEFICIAL. SUPPLEMENTAL INFO FROM ACN 587204: I ASKED THE CAPT TO CALL APCH AND INQUIRE TO THE PRESENCE OF THE TFC. THE ACFT WAS SLIGHTLY ABOVE OUR ALT (200-300 FT) AND DSNDING. WE THEN GOT A TA; AND A YELLOW SQUARE ON THE TCASII. SHORTLY AFTERWARDS; APCH CAME UP AND TOLD ACR Y TO DSND IMMEDIATELY TO 4000 FT. I HAD EVERYTHING I COULD DO TO MAINTAIN POSITIVE CTL OF THE ACFT DURING THE CLB AND I CONCENTRATED ON ATTITUDE AND AIRSPD. AT SOME POINT; THE CAPT TOLD ME TO GO TO FIREWALL THRUST. I ADVANCED THE THROTTLES AND KEPT THE CLB COMING UP. WE THEN GOT AN RA BEHIND; THEN SHORTLY AFTERWARDS; A 'CLR OF CONFLICT.' ON THE DEBRIEF; THE CAPT SAID HE SAW THE TCASII GUIDANCE GIVE A REVERSAL (WHILE WE WERE ESTABLISHED IN THE CLB); BUT FELT WE WERE TOO CLOSE TO EXECUTE IT (HE SAW ALT READOUT OF 0-100 FT) AND THAT'S WHEN HE CALLED FOR FIREWALL THRUST. WE WERE ON ALT AND HEADING AS INSTRUCTED. ACR Y; ON THE OTHER HAND; LEFT HIS ASSIGNED ALT AND HEADING AND NEVER TOLD ANYONE -- THIS CAUSED THE CLOSE CALL. ACR Y ALSO CLBED AT SOME POINT AFTER BEING INSTRUCTED TO DSND TO 4000 FT BY APCH CTL. I BELIEVE THAT APCH CTL WAS SATURATED BY CALLS COMING IN COMPLAINING ABOUT MODERATE TO SEVERE TURB AND DID NOT SEE ACR Y LEAVING HIS ASSIGNED ALT/HEADING. THE CAPT NEVER HEARD AN RA REVERSAL -- NOR DID I; BUT HE DID SEE THE TA/VSI NEEDLE SHOWING A DSCNT. SUPPLEMENTAL INFO FROM ACN 586880: VECTOR FOR APCH TO CVG. AT 360 DEG HDG AND 6000 FT EXPERIENCED HVY TURB. REQUESTED IMMEDIATE TURN. ASSIGNED 270 DEGS (L TURN). CLRED FROM 6000 FT TO 4000 FT. INITIATED DSCNT AND ABOUT 5000 FT WE GOT AN RA; 'CLB; CLB NOW.' CLB WAS INITIATED IMMEDIATELY; AS ATC WAS INFORMED. THE OTHER ACFT WAS ABOUT AT THE SAME ALT. DURING THIS EPISODE WE WERE IN HVY TURB AND IT WAS DIFFICULT TO MAINTAIN HEADING AND ATTITUDE.