Hello Alan,
While a bond to the aluminum has been practiced in the past, for heavy loads the earth , ground or return circuit is carried on its' own wire.
With DC power, the earth or ground circuit is the negative return, while the 3 phase AC power really does not have a return, but may be grounded for safety. Most 3 phase motors do not have a ground return.
In several common circuits, the safety ground , RF (radio frequency) ground or point of zero potential is often the skin and structure of the aircraft.
Welcome to the forum !
vap