Crj flight crew reported descending below the ATC minimum vectoring altitude from their assigned alt...

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.

Original NASA ASRS Text

Title: 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.

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.

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.

Attributes
ACN	1735788
Time
Date	202003
Local Time Of Day	0601-1200
Place
Locale Reference	SCT.TRACON
State Reference	CA
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Regional Jet 700 ER/LR (CRJ700)
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Route In Use	Vectors
Flight Plan	IFR
Person 1
Function	Pilot Flying
Qualification	Flight Crew Multiengine Flight Crew Instrument Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Pilot Not Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument Flight Crew Multiengine
Events
Anomaly	Conflict Airborne Conflict Deviation - Altitude Excursion From Assigned Altitude Inflight Event / Encounter CFTT / CFIT

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.