In a downline the plane should be accelerating at least at 1G - probably more if the engine is running and turning the prop (OK you 3d guys - I know about the wide props and propeller breaking). If you are reaching terminal velocity with your planes in a downline (from 400 or so feet in most cases?)- your planes are either built a lot stronger than mine or have a lot more drag than any of mine and you are probably holding your downlines a lot longer than I am brave enough to do.