answer....:
D = pillage improvements, it ends the units movement (bad for chopper as said above)
ground units automatically succeed and get to choose the improvement
Air units have a chance to succeed and get to chose the improvement
Artillery (only against a hex not occupied by a unit) have a chance to succeed and do not choose the improvement
Special warning: do not use artillery against one of your own improvement is you have another artillery unit adjacent to the square and within range of the the firing unit. You will fight an artillery duel between your own units.
I am not sure what factors go into the probability to hit with artillery or air units, but it appears to go up if the same unit attacks the same improvement it missed the prior turn.