Poor animation system in the engine.
You could (perhaps) bake such a bendy-bone-based animation...
But I guess you want it to work ingame.
Also, see "inverse kinematics" - it might be what you want.
Same here, you can bake it, but having it ingame - there are some plugins maybe, but about their quality I don't know.
EDIT: another idea: use drones that fly to and from the fuel tanks (to = full drone, from = empty drone)
EDIT: or go the long and painful way: make a chain of bones and implement your own "bendy bones"