TCAS ra’s are inhibited below 1000ft AGL. ADS-B in is overrated and I doubt an airliner would have that function since crews are not allowed to maneuver based on that info alone. There are so many things that will have to be investigated by the NTSB that I will not comment further on this accident. What I will say is that this accident would have never happened if the Secret Service had been allowed to close DCA permanently years ago. The only, ONLY reason it remains open now is for the convenience of Congress so that Senators and Representatives don’t have to go to Dulles to airline home. The fact that general aviation is severely restricted to fly there, very few non governmental airplanes do. The aviation taxes that I pay for to an airport I can’t use would be better spent elsewhere. RIP to those who perished and condolences to their families.