Any Unknown or Unlisted Aircraft Manufacturer
Flight Crew Air Transport Pilot (ATP)
Distraction; Situational Awareness; Workload
Conflict NMAC
Air Traffic Control Issued Advisory / Alert; Flight Crew Requested ATC Assistance / Clarification
Airspace Structure; Human Factors
On approach to Runway 11 in STL; at approximately 2;500 feet crossing just inside HIXOB; we encountered a single engine aircraft that crossed our flight path estimated at 300-500 feet above our altitude. The aircraft was not displaying on our TCAS as it appears he did not have a transponder on. We reported the traffic to ATC on STL Tower 118.5 and inquired about the traffic. The controller stated he had just started intermittently picking up a primary target. Another controller came on and inquired if we knew what type of aircraft it was. Pilot Monitoring (PM) stated it appeared to be a [small aircraft]. No TCAS RA was encountered due to the traffic not being displayed. It appears that the aircraft was near the boarder of; or possibly within the class B airspace; however; ATC was not aware of the aircraft in time to notify us of the potential conflict. The flight continued with no further incident.The VFR aircraft was not displaying a transponder or in communication with ATC.
While conducting an approach the flight crew encountered unknown VFR traffic in close proximity which was not displaying on TCAS; possibly due to no or inoperative transponder. ATC was queried and requested help in identifying the aircraft type. No RA was generated by this event.