The Earthing point for the Aircraft is its aluminum frame.
In any power system, Earthing or grounding is necessary so that power does not flow through a person who comes in contact with a live conductor. Earthing also helps identify faults quickly.
On the ground, the ground serves as the earthing point providing a low resistance path for current which may leak on to the body of a equipment. In Aircrafts and even automobiles, where contact with the ground is not possible, the frame itself serves as the grounding point.
See Also Static Electricity in Aircraft