Date: 201710
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Dusk
Aircraft: A320
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Second aircraft: Skyhawk 172/Cutlass 172
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown
Second reporter function: First Officer; Pilot Not Flying
Anomaly: ATC Issue All Types; Conflict NMAC
Detector: Automation Aircraft RA; Automation Aircraft TA; Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Clearance; Flight Crew Took Evasive Action
Contributing factors: Environment - Non Weather Related; Airspace Structure
Primary problem: Environment - Non Weather Related

Narrative: Inbound to [our destination] at 2000 MSL on a 050 heading and 210 KIAS assigned for radar vectors to the visual approach to runway 11. ATC called; 'Traffic 10 o`clock at 1800; 2 to 3 miles' I looked for traffic. No Joy. Traffic on TCAS showed as an amber target. ATC called; 'Traffic 12 o`clock less than a mile.' I Pilot Flying (PF); said to First Officer; 'I don`t have him.' TCAS issued a Descent RA of 1600 FPM; the autopilot was disconnected; and the RA was followed.I looked up in time to see a light brown and white C-172 at 12 o`clock pass approximately 300 feet above our airplane. ATC asked; 'What are you doing?' Pilot Not Flying (PNF) responded; 'Complying with TCAS RA.' ATC responded; 'Ok; continue.' TCAS issued a reduce descent command. ATC said; 'When able; turn to 060. I turned to 060. TCAS issued; 'Clear of conflict.' I leveled off at 1400 MSL.We asked ATC what altitude he wanted us at; and we were told 1800 MSL. I climbed back to 1800 MSL. We were then turned inbound and cleared for a visual approach to runway 11 and completed a normal visual approach to and landing to that runway. CRM and cockpit communication worked very well in a busy cockpit; which contributed to a safe outcome to this event.


Second narrative: We were on a heading of 050 and an altitude of 2000 FT MSL being vectored by ATC for the ILS 11 approach when this incident occurred. ATC informed us about a traffic at 10 o'clock within 2 to 3 miles. I immediately started looking for the traffic and informed the captain that I did not see the traffic and he also stated not in sight. I told ATC traffic not in sight.After that ATC stated traffic less than a mile 12 o'clock 1800 FT MSL. Without delay; I started looking for traffic; at that very moment the TCAS system instructed us to descend. The captain promptly turn off the automation system and executed the descend. While still looking outside; I saw a Cessna 172 with white and brown colors directly ahead of us and 300 FT above our altitude. The Captain descended to 1400 FT MSL. ATC asked what are you doing? I stated responding to a TCAS RA and the controller said continue. After we cleared the traffic; I queried about our altitude. ATC instructed us to climb to 1800 FT. MSL the minimum vectoring altitude and also stated when able turn to a heading of 060.


Synopsis: Airbus 320 flight crew reported a NMAC during descent; and descending to comply with an TCAS RA.