It has always been protocol in the British Royal Family that no direct heirs shall travel by plane together. (Exception was made in 2002 which allowed Charles, William and Harry to travel back from their skiing holiday after the death of The Queen Mum.) Little royal rebel Prince George was already breaking rules when he was just nine months by accompanying his Dad to Australia. Though his parents are becoming renowned for doing things their own way.
One Must Not Fly With One's Dad