MD80 flight crew has a TCAS RA during descent to ord.

Attributes
ACN	702156
Time
Date	200607
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : ord.airport
State Reference	IL
Altitude	msl single value : 14000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zau.artcc
Operator	common carrier : air carrier
Make Model Name	MD-83
Operating Under FAR Part	Part 121
Flight Phase	descent : intermediate altitude
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zau.artcc
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
ASRS Report	702156
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	conflict : airborne critical non adherence : required legal separation
Independent Detector	aircraft equipment : tcas other flight crewa other flight crewb
Resolutory Action	flight crew : returned to assigned altitude flight crew : took precautionary avoidance action
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 0 vertical : 1000
Supplementary
Problem Areas	ATC Human Performance Flight Crew Human Performance
Primary Problem	ATC Human Performance

Narrative:

Given descent clearance by ZAU to descend to 11000 ft. Instructed to maintain 2500 FPM rate of descent. During descent given left turn to heading 360 degrees. Shortly after rolling out on new heading and descending through approximately 14000 ft received traffic alert from TCAS system. Target aircraft on TCAS screen displayed 2000 ft below and in our flight path. Recognizing the ensuing conflict I stated to the captain that we would get an RA. The captain had recognized this concurrently and began to arrest descent. RA immediately followed with adjust vertical speed to an aggressive climb resolution. My TCAS display at this point was on 6 NM scale and showed conflict aircraft overlaid on our aircraft symbol. Vertical separation showed 1000 ft. Captain followed TCAS commands which instructed 2000 FPM rate of climb until clear of conflict was annunciated. Leveled at 17000 ft. During maneuver I instructed ATC that we were responding to RA and climbing. ATC rogered my transmission and after a pause made the statement 'I thought you said you were going to descend;' which I believe was directed at the conflicting aircraft. We then received clearance 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 aircraft. 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.

Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of January 2009 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.