Narrative:

We were descending via the EAGLZ2 RNAV arrival. As we were approaching eaglz intersection; socal approach advised us of traffic 10 or 11 o'clock; northwest bound at 8;500 ft and type of aircraft unknown. And; that he was not talking to that aircraft. As this aircraft was converging towards us; I was not able to see it visually. Near stomn intersection; I acquired the aircraft on TCAS. He was at about 10 miles and rapidly converging on our position. I informed the captain. Then socal approach told us to descend to 8;000 ft. The captain did so without delay to get below the other aircraft's altitude.at this point; the other aircraft was at about 7 miles from us and still converging rapidly. Then; within 5 miles; the other aircraft begins a descent and the 'traffic traffic' annunciations begin to alarm. Moments later; just as socal approach was trying to give us a control instruction to descend; the TCAS RA alarmed. TCAS voice annunciations and pitch commands for 'climb; climb' were sounding off and being displayed. I replied to socal approach's instruction by stating 'unable; we are responding to a RA'. Moments later; the TCAS annunciated to us 'level off'. We cleared the conflict; were vectored across the final for our descent and made a safe landing. After a moment of reflection; I'm left wondering why socal had us descend thru his altitude? Especially if they were not 'talking to him'? I don't think that was a good call on his part. I don't think there was anything else we could have done. Had I realized his rate of closure sooner; I would not have accepted the clearance to descend to 8;000 ft. Perhaps a VFR corridor for general aviation aircraft crossing over the mountains would be appropriate. Or; convert ont area into a class B airspace and protect the IFR arrival corridors.

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Original NASA ASRS Text

Title: Air Carrier flight crew reported responding to an RA that was contrary to ATC instructions.

Narrative: We were descending via the EAGLZ2 RNAV arrival. As we were approaching EAGLZ intersection; SOCAL approach advised us of traffic 10 or 11 o'clock; northwest bound at 8;500 ft and type of aircraft unknown. And; that he was not talking to that aircraft. As this aircraft was converging towards us; I was not able to see it visually. Near STOMN intersection; I acquired the aircraft on TCAS. He was at about 10 miles and rapidly converging on our position. I informed the Captain. Then SOCAL approach told us to descend to 8;000 ft. The captain did so without delay to get below the other aircraft's altitude.At this point; the other aircraft was at about 7 miles from us and still converging rapidly. Then; within 5 miles; the other aircraft begins a descent and the 'TRAFFIC TRAFFIC' Annunciations begin to alarm. Moments later; just as SOCAL approach was trying to give us a control instruction to descend; the TCAS RA alarmed. TCAS voice annunciations and pitch commands for 'CLIMB; CLIMB' were sounding off and being displayed. I replied to SOCAL approach's instruction by stating 'Unable; we are responding to a RA'. Moments later; the TCAS annunciated to us 'LEVEL OFF'. We cleared the conflict; were vectored across the final for our descent and made a safe landing. After a moment of reflection; I'm left wondering why SOCAL had us descend thru his altitude? Especially if they were not 'talking to him'? I don't think that was a good call on his part. I don't think there was anything else we could have done. Had I realized his rate of closure sooner; I would not have accepted the clearance to descend to 8;000 ft. Perhaps a VFR corridor for General Aviation aircraft crossing over the mountains would be appropriate. Or; convert ONT area into a class B airspace and protect the IFR arrival corridors.

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.