Large Transport; Low Wing; 2 Turbojet Eng
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 172; Flight Crew Type 8500
Communication Breakdown; Situational Awareness
ATC Issue All Types; Conflict NMAC
Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Human Factors; Airspace Structure
During descent into the Austin Airport on a sunny morning; ATC cleared us to 'Descend via the WLEEE 3 RNAV Arrival; landing north.' The First Officer was flying and I was the pilot monitoring. The FO flew the aircraft with the autopilot engaged and in LNAV and VNAV with the autothrottle engaged and 4000 ft selected in the MCP altitude window (per the descend via clearance). Upon passing approximately 6000 ft; the Austin Approach Controller cleared us to maintain 5000 ft. The FO reset the altitude in the MCP Altitude Window to 5000 ft. Then selected Level Change and set the speed in the MCP speed window to 210 knots. Just before leveling off at 5000 ft; the FO shouted 'WHOA!' As I looked out the window I could see what appeared to be a small aircraft flying on an easterly heading at approximately 5200 ft moving from our 11:30 position to our 5 o'clock position. The small aircraft appeared to be so close I could see the rivets on its fuselage and the aircraft completely filled the FO's number one cockpit window from my perspective. It went by our right side and slightly above. It appeared to have maroon trim on white paint. No TCAS alerts were issued; nor were any 'primary targets' depicted on the Navigation Display representing the small aircraft. No evasive flight control maneuvering was made since the FO spotted the aircraft earlier than I did and was quickly able to determine that it would pass by at a higher altitude then we were. I reported the aircraft to ATC and sternly explained just how close that aircraft came to us. The Controller seemed concerned and explained that he saw a primary target only; and stated that was why he leveled us off at 5000 ft. Neither the FO nor I recall being told the reason for the 5000 ft level off or being given any Traffic Advisory. Before being handed off to Tower frequency; the AUS Approach Controller stated he would be making a note of the traffic event. We continued the approach to a normal landing without further incident. Upon completion of the Parking Checklist; the FO and I assessed events and I contacted Dispatch. I advised our Dispatcher of what had happened and he felt it was important enough to bring his Supervisor into the conversation. I explained the event to the Supervisor and he stated he would be forwarding the information of the event to the ATC desk of Dispatch. He then stated we should also speak with the Chief Pilot and attempted to connect us. However; after a few minutes of waiting; he stated the Chief Pilot was handling another issue and relayed on his behalf. He asked if we both felt OK to continue flying. The FO and I talked it over; and we both felt mentally fit to continue the day as scheduled. My review of the airspace approximately three miles east of the BOWTZ intersection at 5000 ft (the area where the incident occurred) is depicted as Class E Airspace. In the future I will spend more time looking out the window for traffic instead of fixating on the aircraft automation and watching the descent profile. Additionally; I believe all aircraft in Class E Airspace should be required to have a transponder with Mode C capability in working order at all times. This requirement should be mandated by the FAA ASAP.
Air carrier Captain reported a NMAC with a small aircraft while on descent to AUS on the WLEEE3 Arrival in Class E airspace.