TCASII COMMANDED AN ACR 727 TO DECREASE ITS SEPARATION FROM A CLBING DC-9.

Narrative

WHILE MAKING A DSCNT TO LAND AT ORD; OUR ACFT WAS ASSIGNED A LEVEL OFF AT 7000 FT. APCHING APPROX 7500 FT; WE RECEIVED A TCASII TA FOR AN ACFT AHEAD; BELOW US; AND CLBING TO 6000 FT. NO TFC HAD BEEN ISSUED TO US BY ATC. THIS ACFT WAS VISUALLY ACQUIRED; MOMENTARILY; IN BTWN CLOUD LAYERS. AT APPROX 7100 FT OUR TCASII REACTED WITH AN RA TO 'MONITOR VERT SPD;' INDICATING A 1500 FPM DSCNT. THE FO WAS FLYING AND HESITATED BEFORE REACTING. THE RA CONTINUED; NOW COMMANDING 'DSND NOW.' A DSCNT WAS INITIATED. WE WERE REASONABLY SURE THE ACFT PREVIOUSLY SEEN WAS NOT A FACTOR; BUT WERE UNABLE TO CONTACT ATC DUE TO RADIO CONGESTION. WE FOLLOWED THE RA COMMANDS. WX CONDITIONS WERE INTERMITTENT IMC. AFTER BEGINNING OUR DSCNT; THE RA CHANGED AGAIN TO 'INCREASE VERT SPD' TO 2500 FPM. THE RA STOPPED AT APPROX 6500 FT AND OUR ACFT LEVELED OFF AT 6400 FT. ATC WAS NOTIFIED IMMEDIATELY. AFTER THE FACT; ATC SUPVRS INDICATED THAT SEPARATION BTWN OUR ACFT HAD BEEN SIGNIFICANTLY COMPROMISED. THE OTHER ACFT ALSO RECEIVED A TCASII ALERT TO CLB! I FEEL THE TCASII PROGRAMMING IN THIS SIT WAS IN ERROR AND CONTRIBUTED TO A DEV IN ATC SEPARATION. I ALSO BELIEVE THAT IF TFC HAD BEEN ISSUED TO US; WE MAY NOT HAVE REACTED IN THE SAME MANNER. SUPPLEMENTAL INFO FROM ACN 267337: TCASII DOES NOT KNOW THAT BOTH ACFT WILL LEVEL OFF WITHIN SEPARATION REQUIREMENTS. IMC PREVAILED AT THE TIME AND WE WERE NOT ABLE TO ESTABLISH OTHER ACFT VISUALLY.