An large transport had conflict with an unknown aircraft in the high altitude structure.

Attributes
ACN	221359
Time
Date	199209
Day	Wed
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : mzv
State Reference	IL
Altitude	msl bound lower : 29000 msl bound upper : 31000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zau
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
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : instrument pilot : flight engineer pilot : commercial pilot : cfi pilot : atp
Experience	flight time last 90 days : 200 flight time total : 12000 flight time type : 1800
ASRS Report	221359
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	conflict : airborne less severe non adherence : required legal separation other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other controllera
Resolutory Action	flight crew : took evasive action other
Consequence	Other
Miss Distance	horizontal : 3000 vertical : 1000
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Cruise at 31000 ft. TCASII gave caution advisory (traffic at 1-2 O'clock). Simultaneously ATC instructed us to immediately descend to 29000 ft and RA of TCASII went off. (TCASII RA instructed to also descend). Descent was immediate. Aircraft passed over us (on TCASII slightly to our right) at 29000 ft (seconds later) we received (clear of conflict) from TCASII. Avoidance was maintained. We never had visual contact with traffic. Good catch by ATC. TCASII is a great aid to aviation and collision avoidance.

Original NASA ASRS Text

Title: AN LGT HAD CONFLICT WITH AN UNKNOWN ACFT IN THE HIGH ALT STRUCTURE.

Narrative: CRUISE AT 31000 FT. TCASII GAVE CAUTION ADVISORY (TFC AT 1-2 O'CLOCK). SIMULTANEOUSLY ATC INSTRUCTED US TO IMMEDIATELY DSND TO 29000 FT AND RA OF TCASII WENT OFF. (TCASII RA INSTRUCTED TO ALSO DSND). DSCNT WAS IMMEDIATE. ACFT PASSED OVER US (ON TCASII SLIGHTLY TO OUR R) AT 29000 FT (SECONDS LATER) WE RECEIVED (CLR OF CONFLICT) FROM TCASII. AVOIDANCE WAS MAINTAINED. WE NEVER HAD VISUAL CONTACT WITH TFC. GOOD CATCH BY ATC. TCASII IS A GREAT AID TO AVIATION AND COLLISION AVOIDANCE.

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.