Date: 201708
Local time of day: 1801-2400
Location: MDW.Airport, IL
Flight conditions: Mixed
Weather elements / visibility: 3
Light: Daylight
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class E C90
ATC / advisory: TRACON C90
Second aircraft: Small Aircraft
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: ATC Issue All Types; Conflict NMAC
Detector: Automation Aircraft RA; Person Flight Crew
When detected: In-flight
Result: Flight Crew FLC complied w / Automation / Advisory; Flight Crew Took Evasive Action
Contributing factors: Human Factors; Procedure
Primary problem: Procedure

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.


Second narrative: We were cleared off PANGG 3 arrival direct to SAILZ intersection at 3;000 ft as cleared by Chicago Approach. The RA event occurred at approximately 10 to 15 miles south/southeast of SAILZ. Preceding company aircraft was provided an avoidance vector by ATC to the east to avoid GA aircraft (Cessna 172 or equivalent) in vicinity of SAILZ at 3;000 ft and proceeding across approach corridor heading south. We were not given avoidance vectors but told to continue toward SAILZ at 3;000 ft. Chicago Approach advised us that GA aircraft was cleared to 3;500 ft. Moments later we received an RA to descend which the Captain (PF) executed while Chicago Approach simultaneously directed us to descend to 2;500 ft. At that instant we saw the GA aircraft break out of the ragged ceiling approximately 300 ft above and to the left of our aircraft. We leveled at 2;500 ft and reported the RA to ATC and proceeded on the RNAV (RNP) Y Runway 22L and landed.


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