IFR C560XL flight crew experienced TCAS RA with unrpted traffic near 5000 ft while on ZAU frequency.

Attributes
ACN	657208
Time
Date	200505
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : jot.vortac
State Reference	IL
Altitude	msl single value : 5000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zau.artcc
Operator	common carrier : air taxi
Make Model Name	Citation Excel
Operating Under FAR Part	Part 91
Flight Phase	descent : approach
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zau.artcc
Make Model Name	Cessna Single Piston Undifferentiated or Other Model
Operating Under FAR Part	Part 91
Person 1
Affiliation	company : air taxi
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 60 flight time total : 5500 flight time type : 2000
ASRS Report	657208
Person 2
Affiliation	company : air taxi
Function	flight crew : first officer
Events
Anomaly	conflict : nmac
Independent Detector	aircraft equipment : tcas other flight crewa
Resolutory Action	flight crew : took evasive action
Miss Distance	horizontal : 0 vertical : 200
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft ATC Human Performance
Primary Problem	Ambiguous

Narrative:

Under ZAU control; approximately 30 NM southeast of jot VOR; aircraft X received and complied with an RA alert at 5000 ft. Aircraft X was assigned 5000 ft and followed the TCAS command to approximately 4000 ft. We saw a C152 and C172 pass over us (approximately 500 ft) at our exact location. Failure to comply with the RA could have resulted in a midair. After getting a visual on the other aircraft; we returned to our assigned altitude of 5000 ft and notified ZAU of the RA and the altitude deviation.

Original NASA ASRS Text

Title: IFR C560XL FLT CREW EXPERIENCED TCAS RA WITH UNRPTED TFC NEAR 5000 FT WHILE ON ZAU FREQ.

Narrative: UNDER ZAU CTL; APPROX 30 NM SE OF JOT VOR; ACFT X RECEIVED AND COMPLIED WITH AN RA ALERT AT 5000 FT. ACFT X WAS ASSIGNED 5000 FT AND FOLLOWED THE TCAS COMMAND TO APPROX 4000 FT. WE SAW A C152 AND C172 PASS OVER US (APPROX 500 FT) AT OUR EXACT LOCATION. FAILURE TO COMPLY WITH THE RA COULD HAVE RESULTED IN A MIDAIR. AFTER GETTING A VISUAL ON THE OTHER ACFT; WE RETURNED TO OUR ASSIGNED ALT OF 5000 FT AND NOTIFIED ZAU OF THE RA AND THE ALTDEV.

Data retrieved from NASA's ASRS site as of January 2009 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.