It's all down to an unwritten rule that two royal heirs should never fly on the same flight together. The rule, although not official, has typically been followed over the years by the family in order to protect their royal lineage should anything happen to the plane.

In the British Royal Family, there is a long-standing unofficial protocol that prevents high-ranking heirs to the throne from flying on the same aircraft together. This rule is designed to protect the line of succession; in the tragic event of a plane crash, the monarchy must ensure that at least one direct heir survives to maintain the continuity of the crown. Historically, this meant that the Monarch and the Prince of Wales, or the Prince of Wales and his first-born son, would travel on separate flights. However, this rule is often relaxed for younger heirs with the written permission of the Sovereign. For example, Prince William and Catherine have frequently flown with their children on the same plane for international tours, though it is widely reported that once Prince George reaches the age of 12 (which occurs in July 2025), he will be required to fly separately from his father on official business. While modern aviation is exceptionally safe, the protocol remains a symbolic and practical safeguard for the stability of the institution of the monarchy.