An large transport had a TCASII descend command towards an aircraft below him in the WX at ord.

Attributes
ACN	219485
Time
Date	199208
Day	Wed
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : ord
State Reference	IL
Altitude	msl bound lower : 4000 msl bound upper : 4000
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : ord
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Flight Phase	descent : approach
Route In Use	arrival other enroute : on vectors
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	descent : approach
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 250 flight time total : 10000 flight time type : 3000
ASRS Report	219485
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	other anomaly other
Independent Detector	atc equipment other atc equipment : unspecified aircraft equipment other aircraft equipment : unspecified other controllera
Resolutory Action	controller : issued new clearance flight crew : took evasive action other
Consequence	Other
Miss Distance	vertical : 1000
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

While being vectored for approach to 9L at ord. We were at 4000 ft on a 230 degree heading when we noticed another aircraft on TCASII at level altitude being vectored on an apparent collision course. The controller gave us a turn to 310 degree as we tightened up the turn and received a 'TA' and 'descend' from the TCASII. I had heard the controller give a descent to 3000 ft to the other aircraft so I directed the first officer to maintain altitude and tighten the turn as much as possible. The TCASII quickly changed to a climb as the aircraft passed directly below us.

Original NASA ASRS Text

Title: AN LGT HAD A TCASII DSND COMMAND TOWARDS AN ACFT BELOW HIM IN THE WX AT ORD.

Narrative: WHILE BEING VECTORED FOR APCH TO 9L AT ORD. WE WERE AT 4000 FT ON A 230 DEG HDG WHEN WE NOTICED ANOTHER ACFT ON TCASII AT LEVEL ALT BEING VECTORED ON AN APPARENT COLLISION COURSE. THE CTLR GAVE US A TURN TO 310 DEG AS WE TIGHTENED UP THE TURN AND RECEIVED A 'TA' AND 'DSND' FROM THE TCASII. I HAD HEARD THE CTLR GIVE A DSCNT TO 3000 FT TO THE OTHER ACFT SO I DIRECTED THE FO TO MAINTAIN ALT AND TIGHTEN THE TURN AS MUCH AS POSSIBLE. THE TCASII QUICKLY CHANGED TO A CLB AS THE ACFT PASSED DIRECTLY BELOW 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.