Air carrier on downwind to ord experienced TCASII RA with aircraft crossing behind.

Attributes
ACN	598347
Time
Date	200310
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : ord.airport
State Reference	IL
Altitude	msl single value : 6300
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : ord.tracon
Operator	common carrier : air carrier
Make Model Name	A320
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 27l
Flight Phase	descent : approach
Route In Use	approach : instrument precision
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tracon : c90.tracon
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	climbout : initial climbout : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 170 flight time total : 12000 flight time type : 1100
ASRS Report	598347
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	conflict : airborne less severe
Independent Detector	aircraft equipment : tcas other flight crewa
Resolutory Action	flight crew : took evasive action
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	ATC Human Performance Aircraft Navigational Facility
Primary Problem	Ambiguous

Narrative:

On downwind for an ILS runway 27L on assigned heading 090 degrees, we were cleared to descend from 7000 ft to 5000 ft, maintain 090 degree heading and 170 KTS. On descent at around 6300 ft, we got an RA showing traffic behind us about 3 mi and below about 1200 ft lower than us at the time, then the RA went to climb. I (the PF, captain) responded by going to toga, then to climb mode on the throttle and pitched up to the required vsi which was around 1500-2000 ft nose-up climb. After the RA went away we had to clean flaps up and then told 'ATC' what took place. He seemed surprised (as we) asked to call them on the ground. I spoke with the supervisor at the time. He explained that air carrier Y was on heading 160 degrees (90 degrees difference) and assigned a climb from 5000 ft to 13000 ft and was at around 5600 ft when we got our RA. I think even though the air carrier Y was behind us, it was the 'distance' that created the RA! We continued the radar vectors and landed on runway 27L without further excitement.

Original NASA ASRS Text

Title: ACR ON DOWNWIND TO ORD EXPERIENCED TCASII RA WITH ACFT XING BEHIND.

Narrative: ON DOWNWIND FOR AN ILS RWY 27L ON ASSIGNED HDG 090 DEGS, WE WERE CLRED TO DSND FROM 7000 FT TO 5000 FT, MAINTAIN 090 DEG HDG AND 170 KTS. ON DSCNT AT AROUND 6300 FT, WE GOT AN RA SHOWING TFC BEHIND US ABOUT 3 MI AND BELOW ABOUT 1200 FT LOWER THAN US AT THE TIME, THEN THE RA WENT TO CLB. I (THE PF, CAPT) RESPONDED BY GOING TO TOGA, THEN TO CLB MODE ON THE THROTTLE AND PITCHED UP TO THE REQUIRED VSI WHICH WAS AROUND 1500-2000 FT NOSE-UP CLB. AFTER THE RA WENT AWAY WE HAD TO CLEAN FLAPS UP AND THEN TOLD 'ATC' WHAT TOOK PLACE. HE SEEMED SURPRISED (AS WE) ASKED TO CALL THEM ON THE GND. I SPOKE WITH THE SUPVR AT THE TIME. HE EXPLAINED THAT ACR Y WAS ON HDG 160 DEGS (90 DEGS DIFFERENCE) AND ASSIGNED A CLB FROM 5000 FT TO 13000 FT AND WAS AT AROUND 5600 FT WHEN WE GOT OUR RA. I THINK EVEN THOUGH THE ACR Y WAS BEHIND US, IT WAS THE 'DISTANCE' THAT CREATED THE RA! WE CONTINUED THE RADAR VECTORS AND LANDED ON RWY 27L WITHOUT FURTHER EXCITEMENT.

Data retrieved from NASA's ASRS site as of July 2007 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.