Air carrier X possible less than standard separation from air carrier Y. Possible system error.

Attributes
ACN	174804
Time
Date	199104
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : ord
State Reference	IL
Altitude	msl bound lower : 4000 msl bound upper : 4500
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : ord tower : lnk
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Flight Phase	cruise other
Route In Use	enroute : on vectors
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other
Flight Phase	cruise other
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 : 85 flight time total : 15000 flight time type : 550
ASRS Report	174804
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : clearance non adherence : required legal separation
Independent Detector	other flight crewa
Resolutory Action	flight crew : took evasive action
Consequence	Other
Miss Distance	horizontal : 5900 vertical : 500
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	other

Narrative:

Air carrier X being vectored for a runway 14L ILS approach, turning onto the localizer at 4000'. The so saw traffic to our right. We increased our left turn and descended below our cleared altitude. Air carrier Y to our right was an medium large transport, our estimate of 500' less than 1 mi. Air carrier Y was in a right turn, probably being vectored for runway 14R.

Original NASA ASRS Text

Title: ACR X POSSIBLE LESS THAN STANDARD SEPARATION FROM ACR Y. POSSIBLE SYSTEM ERROR.

Narrative: ACR X BEING VECTORED FOR A RWY 14L ILS APCH, TURNING ONTO THE LOC AT 4000'. THE SO SAW TFC TO OUR RIGHT. WE INCREASED OUR LEFT TURN AND DSNDED BELOW OUR CLRED ALT. ACR Y TO OUR RIGHT WAS AN MLG, OUR ESTIMATE OF 500' LESS THAN 1 MI. ACR Y WAS IN A RIGHT TURN, PROBABLY BEING VECTORED FOR RWY 14R.

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.