Date: 201912
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Light: Dusk
Aircraft: EMB ERJ 145 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Second aircraft: Small Aircraft; Low Wing; 1 Eng; Fixed Gear
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Human-Machine Interface; Distraction
Anomaly: Conflict Airborne Conflict
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Took Evasive Action
Contributing factors: Weather; Aircraft; Procedure
Primary problem: Aircraft

Narrative: While on descent into ZZZ we were issued an traffic advisory by ATC. It was a primary target only and we did not know its altitude. Our clearance was to descend to 3;000 feet and once passing 6;000 feet to fly direct to the outer marker. We were advised that the target was ten o'clock unverified while we were descending through 7;000 feet. We received a traffic alert; but neither myself nor the pilot flying could see the traffic. At 5;500 feet my First Officer saw the traffic at our altitude and closing. He disconnected the autopilot; brought the power to idle and initiated an evasive descent to guarantee our separation from the target. We advised ATC of the situation and continued towards ZZZ and landed. The traffic advisory by ATC; our TCAS system in our airplane and my First Officer's traffic scan which ultimately was the difference maker in being able to avoid the traffic. We performed an evasive descent to avoid the traffic conflict.This incident had a complex set of circumstances that I have never had to deal with before in over 6;500 hours of flying. I think it's important to explain the dynamics of what happened. We were issued a descent clearance which included a turn to a fix at a certain altitude. ATC had advised us of a primary target only that was not squawking; and an overcast layer of clouds below us at sunset. The turn to the outer marker passing 6;000 feet would have put us in direct line with the primary target which was displayed on our TCAS. I elected to stay on the heading so I wouldn't be flying directly towards the traffic. It wasn't until we were passing 5;000 feet that my First Officer had the traffic in sight and increased his descent rate to guarantee our separation. The aircraft passed over us by my best estimate of 400 feet vertically and less than a half a mile laterally. The other aircraft which was a low wing single engine piston was also turning as he must've had us in sight at this time as well. The fact that no Mode C was being reported was a bit unnerving and this was verified multiple times by ATC so no RA would have ever been issued. With the sun setting and a layer of clouds below us; it was very difficult to see any traffic and I'm glad my First Officer who was the Pilot Flying saw him when he did and initiated the evasive maneuver.




Synopsis: EMB-145 flight crew reported an airborne conflict with an aircraft with no transponder that resulted in an evasive maneuver.