B737-300 experiences possibly false TCAS RA with a phantom aircraft.

Attributes
ACN	739150
Time
Date	200705
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : mdw.airport
State Reference	IL
Altitude	msl single value : 6000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : c90.tracon tower : zzz.tower
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
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Plan	VFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 252 flight time type : 14000
ASRS Report	739150
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne critical inflight encounter other
Independent Detector	other flight crewa
Resolutory Action	flight crew : took evasive action
Miss Distance	vertical : 100
Supplementary
Problem Areas	Aircraft ATC Human Performance Flight Crew Human Performance
Primary Problem	Ambiguous

Narrative:

Level at 6000 ft MSL over minok inbound to mdw; we received an RA and immediately descended without using zero G's. TCAS showed an aircraft +100 ft within the 3 mi circle. 10 seconds later; it is gone. I alerted approach control that we were responding to an RA and descending. They told another aircraft about a VFR aircraft at 6000; but not us. I am guessing they saw the aircraft we avoided if it was even there. Possibly a false warning. I called dispatch and reported the event; but did not tell approach control that I was going to file a report. I should have told them but; at the time; I was unsure if there was really an airplane there. We never saw one. No ideas since aircraft appeared at the last second and disappeared just as fast.

Original NASA ASRS Text

Title: B737-300 EXPERIENCES POSSIBLY FALSE TCAS RA WITH A PHANTOM ACFT.

Narrative: LEVEL AT 6000 FT MSL OVER MINOK INBOUND TO MDW; WE RECEIVED AN RA AND IMMEDIATELY DSNDED WITHOUT USING ZERO G'S. TCAS SHOWED AN ACFT +100 FT WITHIN THE 3 MI CIRCLE. 10 SECONDS LATER; IT IS GONE. I ALERTED APCH CTL THAT WE WERE RESPONDING TO AN RA AND DSNDING. THEY TOLD ANOTHER ACFT ABOUT A VFR ACFT AT 6000; BUT NOT US. I AM GUESSING THEY SAW THE ACFT WE AVOIDED IF IT WAS EVEN THERE. POSSIBLY A FALSE WARNING. I CALLED DISPATCH AND RPTED THE EVENT; BUT DID NOT TELL APCH CTL THAT I WAS GOING TO FILE A RPT. I SHOULD HAVE TOLD THEM BUT; AT THE TIME; I WAS UNSURE IF THERE WAS REALLY AN AIRPLANE THERE. WE NEVER SAW ONE. NO IDEAS SINCE ACFT APPEARED AT THE LAST SECOND AND DISAPPEARED JUST AS FAST.

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.