Widebody transport crew received a TCASII TA at 4000 ft starting base leg.

Attributes
ACN	501363
Time
Date	200102
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : ord.airport
State Reference	IL
Altitude	msl bound lower : 3300 msl bound upper : 4000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : c90.tracon
Operator	common carrier : air carrier
Make Model Name	Widebody, Low Wing, 2 Turbojet Eng
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	descent : approach
Route In Use	approach : visual
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tracon : c90.tracon
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	climbout : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time type : 56
ASRS Report	501363
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : instrument pilot : commercial
Events
Anomaly	conflict : nmac non adherence : clearance
Independent Detector	aircraft equipment : tcas other controllera other flight crewa
Consequence	faa : reviewed incident with flight crew
Miss Distance	vertical : 500
Supplementary
Problem Areas	ATC Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

We were downwind west at 4000 ft for runway 32L flying at 200 KTS. We were just issued a base leg turn to heading 040 degrees. At about this time, we got a TCASII TA 'traffic' at about 12 O'clock at about 3000 ft. Then TRACON issued an urgent command to climb immediately. I disconnected the autoplt, the first officer stated that a descent looked better to him. I concurred and initiated a gentle slow descent watching the intruder carefully. He was gradually creeping up on the windshield. I was concerned with what he would do next. We got down to about 3300 ft MSL. The intruder went right over the top. The first officer estimated the distance at 300-500 ft. It was night VMC. I could not estimate the distance. We returned to near 4000 ft and continued the approach and landing on runway 32L uneventfully. At the layover hotel, we called the TRACON watch supervisor. He told us that the intruder was an mdw departure stopped below us at 3000 ft. However, he got a climb RA and climbed into us. He also reported seeing us.

Original NASA ASRS Text

Title: WDB CREW RECEIVED A TCASII TA AT 4000 FT STARTING BASE LEG.

Narrative: WE WERE DOWNWIND W AT 4000 FT FOR RWY 32L FLYING AT 200 KTS. WE WERE JUST ISSUED A BASE LEG TURN TO HDG 040 DEGS. AT ABOUT THIS TIME, WE GOT A TCASII TA 'TFC' AT ABOUT 12 O'CLOCK AT ABOUT 3000 FT. THEN TRACON ISSUED AN URGENT COMMAND TO CLB IMMEDIATELY. I DISCONNECTED THE AUTOPLT, THE FO STATED THAT A DSCNT LOOKED BETTER TO HIM. I CONCURRED AND INITIATED A GENTLE SLOW DSCNT WATCHING THE INTRUDER CAREFULLY. HE WAS GRADUALLY CREEPING UP ON THE WINDSHIELD. I WAS CONCERNED WITH WHAT HE WOULD DO NEXT. WE GOT DOWN TO ABOUT 3300 FT MSL. THE INTRUDER WENT RIGHT OVER THE TOP. THE FO ESTIMATED THE DISTANCE AT 300-500 FT. IT WAS NIGHT VMC. I COULD NOT ESTIMATE THE DISTANCE. WE RETURNED TO NEAR 4000 FT AND CONTINUED THE APCH AND LNDG ON RWY 32L UNEVENTFULLY. AT THE LAYOVER HOTEL, WE CALLED THE TRACON WATCH SUPVR. HE TOLD US THAT THE INTRUDER WAS AN MDW DEP STOPPED BELOW US AT 3000 FT. HOWEVER, HE GOT A CLB RA AND CLBED INTO US. HE ALSO RPTED SEEING US.

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.