Narrative:

We had been talking to an approach controller prior to cgt inbound to mdw. Our position relative to ord was the 139 degree bearing at 39 DME. Our aircraft had been cleared to descend from 6;000 ft to 4;000 ft on a heading of 290 and to intercept the mdw runway 31C localizer. I was the non flying pilot. The localizer frequency was not being displayed on the primary flight display. As I began to insert the correct frequency and course into the localizer section of the navigation/rad page of the FMS; the first officer made an exclamation and began to take evasive action; first by disconnecting the autopilot and pushing the nose over. I looked up to see a piper saratoga pass overhead from left to right. The aircraft passed over us at approximately 100 ft. First officer recovered and continued a normal descent to 4;000 ft. We intercepted the 31C localizer and landed without incident. There were no reports of trauma or injury to passengers or other flight crew members. ATC did not give any traffic advisories prior to the incident. Suggestions; ATC was unaware of traffic in our area and failed to see any other targets as we reported the event. Our TCAS system failed to give a TA or RA.

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Original NASA ASRS Text

Title: Air Carrier on approach to MDW with C90 experienced a NMAC with an unreported general aviation aircraft descending from 6;000 to 4;000.

Narrative: We had been talking to an Approach Controller prior to CGT inbound to MDW. Our position relative to ORD was the 139 degree bearing at 39 DME. Our aircraft had been cleared to descend from 6;000 FT to 4;000 FT on a heading of 290 and to intercept the MDW Runway 31C localizer. I was the non flying pilot. The localizer frequency was not being displayed on the Primary Flight Display. As I began to insert the correct frequency and course into the localizer section of the NAV/RAD page of the FMS; the First Officer made an exclamation and began to take evasive action; first by disconnecting the autopilot and pushing the nose over. I looked up to see a Piper Saratoga pass overhead from left to right. The aircraft passed over us at approximately 100 FT. First Officer recovered and continued a normal descent to 4;000 FT. We intercepted the 31C localizer and landed without incident. There were no reports of trauma or injury to passengers or other flight crew members. ATC did not give any traffic advisories prior to the incident. Suggestions; ATC was unaware of traffic in our area and failed to see any other targets as we reported the event. Our TCAS system failed to give a TA or RA.

Data retrieved from NASA's ASRS site as of April 2012 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.