EMB ERJ 145 ER/LR
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Situational Awareness
Conflict NMAC; Deviation - Altitude Excursion From Assigned Altitude
Flight Crew Took Evasive Action; Flight Crew FLC complied w / Automation / Advisory
Human Factors
While in the BWI terminal [area] at 6;000 feet MSL we received a TCAS TA (Traffic Advisory) followed quickly by an RA (Resolution Advisory) to descend. I disconnected the auto pilot and began a descent to comply with the RA. Meanwhile the PM (pilot monitoring) notified ATC of our RA descent. We spotted the traffic (C172); passing nearly overhead at approximately 400 feet above us. ATC advised us that the traffic was supposed to be at 6;500 feet and seemed unsure what the parameters were for RA activation. Once clear of the conflict we returned to our assigned altitude and the flight concluded without any further problems.I believe we performed a correct TCAS RA procedure. However we discussed how quickly the TA became an RA in this scenario and how we must always be prepared to react.
EMB-145 Captain reported a NMAC in the vicinity of BWI airport.