Until recently at IAD, BA operated 3 flights a 6pm, 8pm and 10pm.

All 3 consistently had different aircraft!

6pm= 747

8pm= 767

10pm= 777

This was the case for quite a while and you always knew whatever time you chose, what aircraft you were getting. I always quite liked to leave at around 8pm but that was the worst option in terms of actual plane.

They eliminated that flight and now fly an A380 at 6pm and a 777 at 10pm.