Emb-145 captain reported a near midair collision in the vicinity of bwi airport.

Attributes
ACN	1568845
Time
Date	201808
Local Time Of Day	1201-1800
Place
Locale Reference	PCT.TRACON
State Reference	VA
Environment
Flight Conditions	Marginal
Light	Daylight
Aircraft 1
Make Model Name	EMB ERJ 145 ER/LR
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Aircraft 2
Make Model Name	Skyhawk 172/Cutlass 172
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument Flight Crew Multiengine
Events
Anomaly	Conflict NMAC Deviation - Altitude Excursion From Assigned Altitude

Narrative:

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.

Original NASA ASRS Text

Title: EMB-145 Captain reported a NMAC in the vicinity of BWI airport.

Narrative: 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.

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.