Air Carrier flight crew reported NMAC during initial approach to land at MIA airport.
Synopsis
Air Carrier flight crew reported NMAC during initial approach to land at MIA airport.
Narrative
I was the Pilot Flying (PF) on Flight XXXX from ZZZ1 to ZZZ2 on DATE. During the descent on SNDBR2 arrival for Runway 9; we encountered a near-missed midair collision just before LLEGG at 4000 ft. I had to perform an aggressive maneuver to avoid the collision. The descent on the arrival was all normal before reaching LLEGG. Just as we were approaching LLEGG; we observed a traffic in the opposite direction on TCAS. ATC made no notification of it. Our FMS was programmed to be at 4000 ft. to cross LLEGG and we were on profile. As we continued to observe the traffic. It appeared to be heading to LLEGG fix. Judging from TCAS altitude difference; the traffic was at 4000 ft. also which I thought it must an IFR traffic under ATC control. As we were about 4100 ft. just before LLEGG; the traffic indicated to be -100 ft. and also about to be in LLEGG. There was no ATC warning and no TCAS warning. I felt unsafe to approach LLEGG and immediately bank right and slight pitch up for climb to avoid the traffic. After I put in the maneuver; the TCAS responded 'Traffic; traffic'. The Captain queried to the ATC if he was talking to the other traffic. The ATC said that he was not talking to that traffic. After we avoided the traffic; we maneuvered back to intercept the final course for Runway 9 after passed LLEGG for normal stabilized landing. We were less than 100 ft. in vertical separation; and the traffic was too close to the airplane horizontally on ND in 10 nm range-scale with both of us heading towards LLEGG. The traffic was a VFR traffic at IFR altitude. The traffic was in Class B airspace and was not in communication with the ATC. ATC made no attempt to notify us of the traffic until after we had already made evasive maneuver to avoid it and told him about the traffic. ATC should have monitored the other traffic more closely and should have made attempt to separate us from that traffic which was not in communication with him before I need to make to input myself.
Second reporter narrative
While in the descent on the SNDBR 2 arrival; some point after the HASLM intersection; traffic popped up on the TCAS very suddenly. I asked ATC if they were talking to that traffic to which he replied no. The traffic was heading for us and was very close. We could now visually see the traffic. We decided to take turn away from the traffic because he was heading for us and was now at the same altitude. At point of turning away from the traffic; at the same time we got a TA. Eventually; the traffic must have passed behind us after we turned away. (We were heading NNE and the traffic was heading SSE). We were given vectors by ATC and continued to a landing into ZZZ2 and I again asked ATC if he was in touch with the traffic to which he again responded; no. Normally; ATC advise us of the traffic; in this case however; it was not done; but the TCAS is a good safety layer. Perhaps a bigger buffer zone in the Class B airspace.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.