Commercial Fixed Wing
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 13
Situational Awareness
ATC Issue All Types; Conflict NMAC; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance
Procedure; Human Factors
Aircraft X checked in 14;000 [ft.] descending via the Throne arrival. I issued converging traffic 12 NM NW descending Aircraft Y leaving 13;000 for 12;000 restricted above Aircraft X on the arrival. Approximately 4 NM converging when Aircraft X was level 11;000 on arrival I saw Aircraft Y start a descent out of 12;000. I issued a traffic alert and told Aircraft X to turn right immediately to heading 280. I reissued the traffic and told Aircraft X to descend to 8;000. I Told the Hemet controller to stop the descent of Aircraft Y but separation was already lost. Aircraft X reported Aircraft Y in sight was then instructed to maintain visual separation and proceed direct CRCUS to rejoin the Throne arrival when able. The Throne arrival crossing altitude at 11;000 when aircraft are descending to 12;000 on the Disney STAR leaves very little room for error on a descent and is a collision hazard for aircraft not adhering to the arrival restrictions.
Aircraft X; was descending via the Disney Arrival for SNA. I had traffic from the east also descending via the Disney arrival that was going to be a tie. I vectored Aircraft X off the arrival on heading 170 and descended them to 8;000 feet to get them underneath unidentified traffic that was tracking the Disney arrival at 12;500 [ft.] and descending. Aircraft Y was descending via the THRNE arrival at 11;000. I handed Aircraft Y off to NORR and switched them to NORR. The aircraft were converging and Aircraft X was descending out of 12;000 when the NORR controller alerted me to the conflict and they called traffic to Aircraft X. I instructed Aircraft X to stop their descent and issued traffic. The aircraft crossed paths at EROCK with approximately 500 feet vertical. Neither aircraft responded to a TCAS RA. I should have stopped Aircraft X at 12;000 [feet[ when I gave them the vector to ensure they stayed above Aircraft Y on the THRNE at 11;000 [feet].
SoCal TRACON Controllers reported an NMAC between two SNA arrivals.