Narrative:

We were given a southerly heading and a descent from 11;000 down to 4;000. While descending through 10;000 we observed a TCAS target without altitude readout located in close proximity to two nearby airports. In the large majority of cases when a TCAS target is shown near an airport without an altitude readout it is assumed that the target is either on the ground or in the traffic pattern. Initially this was the assumption that I made; however we kept our eyes out ahead of us just in case. As we continued our descent and came close to the TCAS target we both spotted an aircraft in the direction of the target but could not positively identify it as the aircraft in question. As we descended through roughly 8;000 the TCAS target changed to amber and we received a 'traffic; traffic' advisory. At about the same time we both realized that the aircraft we had spotted was indeed the aircraft we were being warned of. Realizing that without an altitude readout on the target we would receive no RA instructions and that the aircraft had little or no movement in our windscreen (aside from growing) I began to decrease the rate of descent of the aircraft. Soon after that I decided to disengage the autopilot and decreased our descent rate even more. I may have begun a slight climb as well but I do not remember as I was only focused on airspeed and visually avoiding the aircraft. As the aircraft passed underneath us I saw that the aircraft was propeller driven with military-like markings on the wings (white and black stripes akin to old P-51 markings). At almost the same time as the aircraft passed below us ATC issued us another clearance. I do not recall the nature of the clearance as my concentration was directed at avoiding the aircraft. The first officer responded to the clearance and told ATC that we were 'climbing slightly to avoid traffic that was passing 2-300 feet below us.' ATC responded that they showed no traffic near us and when able continue descent. After passing the traffic I re-engaged the autopilot and continued the flight without further incident. There was no visible evasive action taken by the other aircraft. The event occurred because an aircraft that apparently was not on ATC's radar was flying eastbound directly in the path of our descent. As he was not visible on ATC radar; and as the aircraft either did not have mode C or it was turned off/inop we were given no RA instructions regarding the aircraft along with a much reduced awareness time.ultimately; good old 'see and avoid' saved the day but our TCAS system was very instrumental in allowing our flight to continue safely. Without it; a simple cockpit distraction could have lead us down a path that; at the very least; would have required a very aggressive avoidance maneuver. Because of that; and due to the nature of our flying being largely to smaller airports with limited ATC control; we should change our policy to not allow the TCAS to be MEL'd when operating in certain areas. TCAS is just too vital to safety given the nature of our express flying; especially into the smaller airports and those with high volumes of training activity.

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Original NASA ASRS Text

Title: A CRJ experienced a TCAS advised traffic conflict while being vectored for an approach in the vicinity of multiple uncontrolled airports. ATC did not have the traffic on radar.

Narrative: We were given a southerly heading and a descent from 11;000 down to 4;000. While descending through 10;000 we observed a TCAS target without altitude readout located in close proximity to two nearby airports. In the large majority of cases when a TCAS target is shown near an airport without an altitude readout it is assumed that the target is either on the ground or in the traffic pattern. Initially this was the assumption that I made; however we kept our eyes out ahead of us just in case. As we continued our descent and came close to the TCAS target we both spotted an aircraft in the direction of the target but could not positively identify it as the aircraft in question. As we descended through roughly 8;000 the TCAS target changed to amber and we received a 'TRAFFIC; TRAFFIC' advisory. At about the same time we both realized that the aircraft we had spotted was indeed the aircraft we were being warned of. Realizing that without an altitude readout on the target we would receive no RA instructions and that the aircraft had little or no movement in our windscreen (aside from growing) I began to decrease the rate of descent of the aircraft. Soon after that I decided to disengage the autopilot and decreased our descent rate even more. I may have begun a slight climb as well but I do not remember as I was only focused on airspeed and visually avoiding the aircraft. As the aircraft passed underneath us I saw that the aircraft was propeller driven with military-like markings on the wings (white and black stripes akin to old P-51 markings). At almost the same time as the aircraft passed below us ATC issued us another clearance. I do not recall the nature of the clearance as my concentration was directed at avoiding the aircraft. The First Officer responded to the clearance and told ATC that we were 'climbing slightly to avoid traffic that was passing 2-300 feet below us.' ATC responded that they showed no traffic near us and when able continue descent. After passing the traffic I re-engaged the autopilot and continued the flight without further incident. There was no visible evasive action taken by the other aircraft. The event occurred because an aircraft that apparently was not on ATC's radar was flying eastbound directly in the path of our descent. As he was not visible on ATC radar; and as the aircraft either did not have Mode C or it was turned off/inop we were given no RA instructions regarding the aircraft along with a much reduced awareness time.Ultimately; good old 'see and avoid' saved the day but our TCAS system was very instrumental in allowing our flight to continue safely. Without it; a simple cockpit distraction could have lead us down a path that; at the very least; would have required a very aggressive avoidance maneuver. Because of that; and due to the nature of our flying being largely to smaller airports with limited ATC control; we should change our policy to not allow the TCAS to be MEL'd when operating in certain areas. TCAS is just too vital to safety given the nature of our express flying; especially into the smaller airports and those with high volumes of training activity.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.