The early morning CLT-IAD (UAX E175) that goes anywhere between 6am and 615am is like this about 5 days of the week. Sometimes at start of boarding First will only be 3/12 and Economy Plus just a handful. Even other airline crew non-revs make First. Regular coach is usually fairly full. But it connects to the daytime IAD-LHR at leaves Dulles around 830am, and I've been told by a couple of agents that a fair number of coach passengers make that connection daily. Also, I fly the reverse of this flight (arrives CLT from IAD after 11pm) and that's normally full.