Narrative:

I was the pilot monitoring (pm) during descent and initial approach into ccr. We were talking to travis TRACON. We were at 6;000 MSL and were given a decent to 4;000 MSL. The pilot flying (PF) initiated a 1;400 FPM decent. Approx 20-30 seconds later travis notified us of traffic at our 12:00 and told us that we needed to descend 'like he told us to.' we replied that we were already through 5;300 ft and descending at 1;400 FPM. We then immediately got a TCAS alert for traffic that was at our altitude within 1 mile of our current position followed by a descend RA with which we complied. Since we were in a descent to begin with; the traffic was initially under us and we could not see him. We descended and recovered at about 4;000 ft MSL. Very excessive force was required to prevent the collision. I would estimate the distance between the two aircraft at no more than 200 ft lateral. I asked the controller after landing if the traffic was IFR or VFR. It was reported as being VFR. Now; I know that the controller has only a responsibility for IFR traffic separation; however; he descended us directly into another aircraft with no adequate warning. It was dusk and difficult to see anything. In my opinion; this controller was responsible for this event; and responsibility or not he has an obligation to not descend us through an altitude that put us on a known collision with another aircraft and then give us no warning prior to the TA.since I had no visual on the other aircraft until after the incident; there was no way I could have seen and avoided this conflict. The controller needs to be more responsible with traffic separation.

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Original NASA ASRS Text

Title: A CE-560XL flight crew suffered an NMAC with an unknown VFR aircraft while descending as cleared by TRACON.

Narrative: I was the Pilot Monitoring (PM) during descent and initial approach into CCR. We were talking to Travis TRACON. We were at 6;000 MSL and were given a decent to 4;000 MSL. The Pilot Flying (PF) initiated a 1;400 FPM decent. Approx 20-30 seconds later Travis notified us of traffic at our 12:00 and told us that we needed to descend 'like he told us to.' We replied that we were already through 5;300 FT and descending at 1;400 FPM. We then immediately got a TCAS alert for traffic that was at our altitude within 1 mile of our current position followed by a DESCEND RA with which we complied. Since we were in a descent to begin with; the traffic was initially under us and we could not see him. We descended and recovered at about 4;000 FT MSL. Very excessive force was required to prevent the collision. I would estimate the distance between the two aircraft at no more than 200 FT lateral. I asked the Controller after landing if the traffic was IFR or VFR. It was reported as being VFR. Now; I know that the Controller has only a responsibility for IFR traffic separation; however; he descended us directly into another aircraft with no adequate warning. It was dusk and difficult to see anything. In my opinion; this Controller was responsible for this event; and responsibility or not he has an obligation to not descend us through an altitude that put us on a known collision with another aircraft and then give us no warning prior to the TA.Since I had no visual on the other aircraft until after the incident; there was no way I could have seen and avoided this conflict. The Controller needs to be more responsible with traffic separation.

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.