I don't think you can blame the mid-air collision on DEI. ATC repeatedly ask PAT25 if they had the CRJ in sight and to maintain visual separation, which takes it out of the controller's hands. the only mistake i can see right now is the helicopter was flying at 300-350 feet which is above the 200 feet fight ceiling for that part of the helicopter corridor.
What likely happened is the Helo pilots either:
- mistook another plane for the CRJ (DC is incredibly busy airspace at any hour day or night)
- mistook ground lights as the CRJ
- Saw the correct CRJ but due to NVG's messing with depth perception mis read the situation
it also has to be mentioned that picking out the landing lights and marker lights of a aircraft against the ground clutter and lights of a city is very hard. especially if the plane is not moving against the background in the horizontal. at the time of collision the plane would have appeared stationary to the helo pilots because it was heading almost directly towards them. the pilots of the CRJ could not see the helo at all because it would have been below them and to their right.
I think the real problem here is having a helicopter flight corridor in the final descent pattern of a major airport, and with the ever increasing number of aircraft in the skies across the whole world this was a disaster waiting to happen. not even 24 hours before this incident a similar one happened in the exact same place, but because the airliner was still above 1000 feet the TCAS (traffic collision avoidance system) was still engaged and alerted the airliner to perform evasive manuevers.
below 1000 feet TCAS is disabled because it would give constant warnings from tall buildings and trees etc. the collision happened at 350-400 feet.