MD80 FLT CREW HAS A TCAS RA DURING DSCNT TO ORD.

Narrative

GIVEN DSCNT CLRNC BY ZAU TO DSND TO 11000 FT. INSTRUCTED TO MAINTAIN 2500 FPM RATE OF DSCNT. DURING DSCNT GIVEN L TURN TO HDG 360 DEGS. SHORTLY AFTER ROLLING OUT ON NEW HDG AND DSNDING THROUGH APPROX 14000 FT RECEIVED TFC ALERT FROM TCAS SYS. TARGET ACFT ON TCAS SCREEN DISPLAYED 2000 FT BELOW AND IN OUR FLT PATH. RECOGNIZING THE ENSUING CONFLICT I STATED TO THE CAPT THAT WE WOULD GET AN RA. THE CAPT HAD RECOGNIZED THIS CONCURRENTLY AND BEGAN TO ARREST DSCNT. RA IMMEDIATELY FOLLOWED WITH ADJUST VERT SPD TO AN AGGRESSIVE CLB RESOLUTION. MY TCAS DISPLAY AT THIS POINT WAS ON 6 NM SCALE AND SHOWED CONFLICT ACFT OVERLAID ON OUR ACFT SYMBOL. VERT SEPARATION SHOWED 1000 FT. CAPT FOLLOWED TCAS COMMANDS WHICH INSTRUCTED 2000 FPM RATE OF CLB UNTIL CLR OF CONFLICT WAS ANNUNCIATED. LEVELED AT 17000 FT. DURING MANEUVER I INSTRUCTED ATC THAT WE WERE RESPONDING TO RA AND CLBING. ATC ROGERED MY XMISSION AND AFTER A PAUSE MADE THE STATEMENT 'I THOUGHT YOU SAID YOU WERE GOING TO DSND;' WHICH I BELIEVE WAS DIRECTED AT THE CONFLICTING ACFT. WE THEN RECEIVED CLRNC BACK DOWN TO 11000 FT AND DIRECT TO ORD. I STATED TO ATC THAT WE WOULD BE WRITING UP THE INCIDENT. NO ACKNOWLEDGEMENT WAS RECEIVED TO THIS STATEMENT OR ANY FURTHER XMISSIONS HEARD DIRECTED AT CONFLICT ACFT. IT IS MY OPINION THAT WITHOUT THE TCAS AND THE ACTIONS IT COMPELLED US TO COMMENCE WE COULD HAVE HAD A VERY BAD OUTCOME TO THIS EVENT.