Watching the most clear of images I have seen so far, at this link, just makes it hard to understand how the copter did not know. Understanding the plane is above it and descending as they get closer, the lights on the plane are super bright and one would think even night vision would have caught something in their angle of vision at least in the last 10-15 seconds. If they can't see that plane in that position, visual separation should never be an allowed tactic. https://www.yahoo.com/news/dca-surveillance-cameras-catch-clearest-171807768.html