Narrative:

I was descending on a 4 mile left base to runway 32L at buchanan field in concord; ca (ccr); in a low wing aircraft. I was doing so at the command of the FAA control tower at said airport. I started a descent at approximately 3000 ft MSL. During this descent I heard the tower clear a cessna aircraft for a touch and go on runway 32L. There were several other aircraft in the pattern on runway 32R. I heard no other communication from the tower concerning 32L. At a distance of approximately one (1) mile on base a cessna high wing aircraft passed left to right below my aircraft at a vertical distance of approximately fifty (50) feet. The white cessna aircraft appeared from below my left wing flying diagonally from my wing to fuselage. I clearly saw the two occupants in the front seats. They did not look up and did not appear to see my aircraft pass above them as they proceeded downwind in the pattern. At no time did the radar equipped tower warn of an impending collision. I was given permission to land and after landing I reported the near midair collision. I was told to call the tower after parking and engine shut down. When I did so and described the event I was told that their (the ccr tower) radar coverage was 'spotty' in that direction. The tower person also said I was asked to report a two (2) mile base. I did not receive that radio call from the tower; nor was I warned of the near midair collision.visibility at the time of this incident was approximately five (5) miles in haze. I think the tower controller might have been distracted by the relatively heavy traffic on the parallel runway 32R. It would be best if the radar coverage to the southwest was continuous at the ccr tower. This should be repaired or fixed immediately.

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Original NASA ASRS Text

Title: General aviation pilot on a visual approach to CCR experiences a NMAC with a Cessna on downwind not reported by the Tower. When queried after parking; the Tower indicates that their Radar is 'spotty' in the southwest quadrant.

Narrative: I was descending on a 4 mile left base to Runway 32L at Buchanan Field in Concord; CA (CCR); in a low wing aircraft. I was doing so at the command of the FAA Control Tower at said airport. I started a descent at approximately 3000 ft MSL. During this descent I heard the Tower clear a Cessna aircraft for a touch and go on runway 32L. There were several other aircraft in the pattern on Runway 32R. I heard no other communication from the Tower concerning 32L. At a distance of approximately one (1) mile on base a Cessna high wing aircraft passed left to right below my aircraft at a vertical distance of approximately fifty (50) feet. The white Cessna aircraft appeared from below my left wing flying diagonally from my wing to fuselage. I clearly saw the two occupants in the front seats. They did not look up and did not appear to see my aircraft pass above them as they proceeded downwind in the pattern. At no time did the radar equipped Tower warn of an impending collision. I was given permission to land and after landing I reported the near midair collision. I was told to call the Tower after parking and engine shut down. When I did so and described the event I was told that their (the CCR Tower) radar coverage was 'spotty' in that direction. The Tower person also said I was asked to report a two (2) mile base. I did not receive that radio call from the Tower; nor was I warned of the near midair collision.Visibility at the time of this incident was approximately five (5) miles in haze. I think the Tower Controller might have been distracted by the relatively heavy traffic on the parallel Runway 32R. It would be best if the radar coverage to the southwest was continuous at the CCR Tower. This should be repaired or fixed immediately.

Data retrieved from NASA's ASRS site 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.