A car isn't earthed either, but thats not why you don't get electrocuted if it's hit by lightning. If a car/plane is hit by lightning the lightning won't stop there, it keeps going till it finds earth. What stops you from getting fried is the fact you have metal all around you, this allows the electricity to travel around you and down to earth, some planes have metal plates on the wings to direct the lighning off it.If you were touching the frame of a car or plane the same time it was hit you could get electrocuted, but in general planes aren't affected. if you look up planes hit by lightning on www.google.com you'll find some stories about it.