Air carrier captain reported an near midair collision with a light aircraft on approach to mdw.

Attributes
ACN	1473906
Time
Date	201708
Local Time Of Day	1801-2400
Place
Locale Reference	MDW.Airport
State Reference	IL
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Flight Plan	IFR
Aircraft 2
Make Model Name	Small Aircraft
Flight Phase	Cruise
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 404 Flight Crew Type 10000
Person 2
Function	Pilot Not Flying First Officer
Experience	Flight Crew Last 90 Days 265 Flight Crew Type 1234
Events
Anomaly	ATC Issue All Types Conflict NMAC
Miss Distance	Vertical 300

Narrative:

Flying to sailz at 3;000 ft on the RNAV Y 22L; a light aircraft caused an RA by descending into our flight path. ATC was too slow to offer de-conflicting vectors because she was too busy with multiple inbounds. The light single aircraft was behaving in a random manner; and when TCAS demanded a descent there was a lot of erroneous information on the radio. ATC finally called for a descent at the same time as the TCAS demanded avoiding action. The first officer was calling out the traffic; I saw him; disengaged the autopilot and throttles; and we ended up avoiding him by about 300 ft with negligible horizontal separation. I'm pretty sure I complied with the RA; although ATC called a descent at exactly the same time; and the TCAS voice was garbled as a result. I started the descent and went head down for TCAS guidance and the first officer said 'descend to 2500 ft'; which was a bit confusing because I assumed he knew I was now following TCAS guidance only. Any passengers who may have seen the light aircraft on the left side of the aircraft would have been very concerned.the pilot of the other aircraft needs to be warned of the dangers of flying in the approach airspace at mdw. He seriously jeopardized the safety of all onboard my aircraft.

Original NASA ASRS Text

Title: Air Carrier Captain reported an NMAC with a light aircraft on approach to MDW.

Narrative: Flying to SAILZ at 3;000 ft on the RNAV Y 22L; a light aircraft caused an RA by descending into our flight path. ATC was too slow to offer de-conflicting vectors because she was too busy with multiple inbounds. The light single aircraft was behaving in a random manner; and when TCAS demanded a descent there was a lot of erroneous information on the radio. ATC finally called for a descent at the same time as the TCAS demanded avoiding action. The First Officer was calling out the traffic; I saw him; disengaged the autopilot and throttles; and we ended up avoiding him by about 300 ft with negligible horizontal separation. I'm pretty sure I complied with the RA; although ATC called a descent at exactly the same time; and the TCAS voice was garbled as a result. I started the descent and went head down for TCAS guidance and the F/O said 'descend to 2500 ft'; which was a bit confusing because I assumed he knew I was now following TCAS guidance only. Any passengers who may have seen the light aircraft on the left side of the aircraft would have been very concerned.The pilot of the other aircraft needs to be warned of the dangers of flying in the approach airspace at MDW. He seriously jeopardized the safety of all onboard my aircraft.

Data retrieved from NASA's ASRS site 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.