B737-300 has a TCAS RA during approach to ord.

Attributes
ACN	703046
Time
Date	200607
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : c90.tracon
State Reference	IL
Altitude	msl single value : 10000
Environment
Flight Conditions	VMC
Aircraft 1
Controlling Facilities	tracon : c90.tracon
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Flight Phase	descent : approach
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tracon : c90.tracon
Operator	common carrier : air carrier
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	Part 121
Flight Phase	descent : vacating altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 200 flight time total : 13000 flight time type : 700
ASRS Report	703046
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : required legal separation
Independent Detector	aircraft equipment : tcas other flight crewa other flight crewb
Resolutory Action	controller : issued new clearance flight crew : returned to assigned altitude flight crew : took precautionary avoidance action
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Ambiguous

Narrative:

Level flight at 10000 ft; received TCAS climb RA and TCAS indicated traffic 500 ft below us. First officer was PF; and he followed proper RA procedures; and climbed to 11000 ft while advising ATC (ord approach). ATC advised us that he did not have any traffic at that altitude; but did have traffic below us at 4000 ft. After RA was resolved; we were cleared back direct ord and to 7000 ft. ATC asked flight Y; who was directly behind us; to be on lookout for TCAS traffic. I reported that their TCAS also indicated traffic at 4000 ft; and then it went to 9500 ft. We did not have visual contact with any traffic. Possibility of a faulty transponder from aircraft below us. No passenger or flight attendants were injured during this incident.

Original NASA ASRS Text

Title: B737-300 HAS A TCAS RA DURING APCH TO ORD.

Narrative: LEVEL FLT AT 10000 FT; RECEIVED TCAS CLB RA AND TCAS INDICATED TFC 500 FT BELOW US. FO WAS PF; AND HE FOLLOWED PROPER RA PROCS; AND CLBED TO 11000 FT WHILE ADVISING ATC (ORD APCH). ATC ADVISED US THAT HE DID NOT HAVE ANY TFC AT THAT ALT; BUT DID HAVE TFC BELOW US AT 4000 FT. AFTER RA WAS RESOLVED; WE WERE CLRED BACK DIRECT ORD AND TO 7000 FT. ATC ASKED FLT Y; WHO WAS DIRECTLY BEHIND US; TO BE ON LOOKOUT FOR TCAS TFC. I RPTED THAT THEIR TCAS ALSO INDICATED TFC AT 4000 FT; AND THEN IT WENT TO 9500 FT. WE DID NOT HAVE VISUAL CONTACT WITH ANY TFC. POSSIBILITY OF A FAULTY XPONDER FROM ACFT BELOW US. NO PAX OR FLT ATTENDANTS WERE INJURED DURING THIS INCIDENT.

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.