A lot of people asking what the helo was doing there. USCG helo pilot here who’s flown that route a thousand times:

DC has a whole network of helo routes and zones designed to organize helo traffic and route it under and around commercial traffic. Route 4 goes right down the east side of the Potomac, max altitude of 200 ft. It is not uncommon for helos to be flying under landing traffic once visual separation is established and with correct altitudes maintained.

From the ADSB data, it looks like the helo was southbound on Route 4, and the airliner was on final to rwy 33. Here’s one plausible scenario… just one that fits the facts we know right now, could be totally wrong: Landing on 33 is not as common as landing on rwy 1. Airliners are often not cleared/switched for RWY 33 until just a few miles south of the Wilson Bridge. Let’s say the H60 is southbound and is told to maintain visual separation with the landing CRJ. The 60 crew may not have caught that the CRJ in question was landing 33, which is less common. They look south and see lights of the next aircraft lined up for RWY 01, and they report “traffic in sight, will maintain visual separation.” Then they cruise south, looking south. Maybe the CRJ is a little low on their approach or the H60 is accidentally a little high on their route and fails to see the CRJ approaching from their 10 o’clock. The CRJ is focused on DCA which is surrounded by a sea of lights in the metro area. They don’t notice one small set of lights out of place at their 1-2 o’clock as they focus on the runway. The controller believes the helo will maintain visual separation so wouldn’t suspect a problem until too late to do anything. Bam.

EDIT: Updates…

I listened to the audio and can confirm that the CRJ was asked if they could switch from RWY 01 to RWY 33 just a few minutes before landing, which they agreed to do. Also, the H60 (PAT25) was asked to look for the CRJ a couple minutes before impact. They apparently reported the CRJ ‘in sight’ and agreed to maintain visual separation. They could have been looking at the correct aircraft, which was just beginning to circle east to line up for RWY 33, or they could have already been mistakenly looking at a different aircraft lining up for landing. There are a lot of lights out there at night. Then, when things are getting close, tower actually reconfirmed with PAT25 that they had the CRJ in sight, then directed PAT25 to pass behind the CRJ. To me, this indicates that tower might have seen that it was going to be a close pass and wanted to be sure that PAT25 wasn’t trying to cross right in front of the CRJ. Unfortunately, if PAT25 was mistaken on which aircraft they were watching, this wouldn’t help.

Common question: what about Night Vision Goggles (NVGs)? – I’m in the USCG, but I assume this Army crew likely had NVGs. But goggles are not a panacea… they don’t show color, they dramatically limit your peripheral view, and in bright, urban environments, they can get oversaturated aka washed out. Flying through DC, it can change minute by minute as to whether you are better off “aided” (goggles down in front of your eyes) or “unaided” (goggles flipped up out of the way on your helmet). Sometimes it even varies depending on which side of the aircraft you’re on. Just because they had goggles doesn’t mean they were more likely to see the airliner. The airliner has a lot of bright lights on already, and the same goggles that help them avoid trees and power lines could also have reduced their peripheral vision at key moments.

LAST EDIT: Another FAQ, then I have to sleep….

What about TCAS? – TCAS is great but speaking for the systems I’m familiar with, they’re not primarily designed for a dense airport environment like that… its accuracy at short range is not great, and with so many aircraft so close to you, including those that are sitting on the ground at DCA, you generally have to mute or inhibit the alerts because it would go off constantly and drown out your communications with your crew and ATC. Think about a ring doorbell camera: it’s great for alerting you when a suspicious person shows up unexpected at 1 AM, but it’s not much good while you’re having a house party at 7pm… you probably muted it because you KNOW there are dozens of people there and you’re okay with it. I have no idea what kind of system the CRJ or H60 have or what their procedures are, but it’s possible that TCAS could have been saturated/muted while flying that close to DCA, and even if it wasn’t, they may not have been able to distinguish the alert for the CRJ from another aircraft until too late.