Recently, on their royal German and Polish tour, the Duke and Duchess of Cambridge failed to follow an important protocol.
During the royal tour, as the plane landed and the doors opened, Prince William, Kate Middleton, Prince George and Princess Charlotte made their grand entrance. Adorable family pictures were taken, as the crowd watched in excitement. However, this should not have happened if the most important royal rule when it comes to travelling, had been followed.
Royal protocol dictates that Prince William is not supposed to be on the same plane as Prince George and Princess Charlotte. This is because two royal heirs to the throne must never fly on the same flight together. Therefore, Prince George and Princess Charlotte must be on different flights as they are the third and fourth in line, respectively, for the throne.
Though there is no written rule regarding the protocol, it has been observed throughout the royal family. In case of a fatal crash that could potentially destroy the lineage, The Crown prefers that the heirs to the throne do not fly together. However heirs have travelled together before and are required to seek permission from the Queen who will make the final decision.
A royal spokesperson told the BBC back in 2014: "While there is no official rule on this, and royal heirs have travelled together in the past, it is something that the Queen has the final say on."
When Prince George was just nine months old, the Queen granted permission for Prince William to bend the rules and fly with his parents on a tour of Australia and New Zealand back in 2014. Last September, the royal family of four traveller to Canada on the same flight, and in July this year, the family toured Poland and Germany together.
Thus we can assume that Prince William has been given permission (after the first time) to travel with both his children who are heirs to the throne, together on the same flight