Date: 202003
Local time of day: 0601-1200
Location: SCT.TRACON, CA
Flight conditions: VMC
Light: Daylight
Aircraft: Regional Jet 700 ER/LR (CRJ700)
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B LAX
ATC / advisory: TRACON SCT
Reporter organization: Air Carrier
Reporter function: Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Communication Breakdown; Human-Machine Interface
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter CFTT / CFIT
Detector: Automation Aircraft RA; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Human Factors; Procedure; Company Policy
Primary problem: Procedure

Narrative: Arriving on the RNAV arrival; we were given a descent by ATC to 2;600 feet while on downwind. A few seconds after leveling off at 2;600 feet; we received a TCAS RA to descend. I disconnected the autopilot and began a descent that would comply with the RA instructions and told my First Officer to contact ATC and tell them we were descending for an RA. He complied and ATC responded by telling us the Minimum Vectoring Altitude was 2;600 feet. TCAS was still commanding a descent so I continued down to comply. ATC then issued a low altitude alert and told us to climb but I still had a descent command from the TCAS and so I continued to descend and my First Officer again stated that we were descending for an RA. The TCAS then issued a level off command and so I leveled off and then TCAS said were clear and so I began a climb back to our assigned altitude.About this time; my First Officer saw the traffic go by our right side. ATC then issued a turn inbound for the Visual Approach and the flight continued without incident. As we were taxiing in; we were given a number to call to speak with ATC. Once at the gate after shutdown; I called ATC and spoke with them. They just wanted to clarify that we descended for an RA and were confused because they did not have any traffic on radar. I told them that it was an RA for traffic at about one o'clock and 2;500 feet and was an airplane and not a helicopter according to my First Officer. I asked if an aircraft at that altitude and position needed to be in contact with them and he said no. He thanked me for calling and said no other follow up was necessary. An aircraft was near an arrival course and not talking to ATC and ATC did not have radar coverage on them. Improve radar coverage in this area and educate pilots to always be in contact with ATC around high density airports.


Second narrative: While descending on the RNAV ARRIVAL; we were given the instructions to descend and maintain 2;600 feet. After leveling off we received a Traffic RA command 'Descend'. The Flying Pilot executed the Traffic RA escape maneuver and I informed Approach Control that we were responding to a Traffic RA and I continued with my scan. Once we received the 'Clear of Conflict' we immediately returned back to our assigned altitude. I believe the cause of this event was due to a lost of separation. I'm not sure of any suggestions for avoiding recurrence of this event but I do believe [Company's] escape maneuver work well.


Synopsis: CRJ flight crew reported descending below the ATC Minimum Vectoring Altitude from their assigned altitude for an RA of traffic that ATC did not report to them.