Air Carrier flight crew reported a NMAC with a small aircraft during arrival descent. Flight crew visually acquired the aircraft and took evasive action to avoid a collision.
Synopsis
Air Carrier flight crew reported a NMAC with a small aircraft during arrival descent. Flight crew visually acquired the aircraft and took evasive action to avoid a collision.
Narrative
FO was Pilot Flying and I was Pilot Monitoring.We were cleared to 6000 from 8000 while east of JIIMS intersection on the JIIMS 4 ARRIVAL and were directed to turn to heading 010 at JIIMS. The controller initially advised us of anonymous traffic to our left (11 o'clock) at 3 miles and 7400 feet. He then advised us to make an immediate right turn in order to avoid a conflict. We subsequently got a momentary TCAS amber TA -300 that then disappeared. Hard right turn was initiated by the FO and then I made visual contact with traffic and directed a more aggressive right turn. FO disconnected AP (Auto Pilot)/AT (Auto Throttles) and manually flew the maneuver resulting in Bank Angle" aural. "Monitor Vertical Speed" aural was heard but red visual RA was not observed on the ND (Navigation Display).The Aircraft Y subsequently passed off to our left at approximately 200 feet. ATC notified.The other aircraft; Aircraft Y; was well within his right; not to be talking to Approach Control since he was above the TMA (terminal Maneuvering Area) altitude of 7000 feet MSL.I was astonished when the TA alert on the TCAS disappeared when we were so close to the Aircraft Y. I was expecting it to transition to a visual red RA. We were very fortunate to have visually acquired the aircraft in order to avoid a collision.Cause: Thinking back on it; we normally cross JIIMS at 8000 MSL. Since we were in radar contact and talking to Approach Control and the altitude is not published; we were subconsciously lulled into a false sense of security with respect to other traffic. Recommend JIIMS have a published hard crossing altitude of 8000 feet. That would certainly would have mitigated this situation."
Second reporter narrative
I was Pilot Flying. We were on the JIIMS 4 arrival just prior to the JIIMS intersection and cleared down to 6;000 feet and a right turn to 010 at JIIMS. At approximately 8;000 feet; Approach Control called traffic at 11 o'clock and three miles at 7;400 feet. They were not in radio contact with the aircraft. The traffic popped up on TCAS showing 300 feet below us; stayed on the screened briefly; and then disappeared. We got a monitor vertical speed" annunciation before it disappeared. Approach advised an immediate turn to the right to avoid the traffic but didn't come across with any urgency. I selected heading on the MCP (Mode Control Panel) and set a heading of 360. The Captain gained visual contact with the aircraft; an Aircraft Y. Based on his calls we were getting too close to the traffic so I disengaged the autopilot and increased angle of bank while briefly leveling. I continued my descent once we were cleared of the traffic. The lack of any further info from the TCAS after the "monitor vertical speed" and the late turn call from the controller resulted in an unsafe separation with the Aircraft Y. Cause: The Approach Controller should've given us a right turn well before the point we wound up turning. The Aircraft Y was on a constant course and level at 7;500 feet well before the incident."
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.