“After that, he had to have a separate aircraft and we could only fly all four together when they were young with the written permission of Her Majesty. When William became 12, he would fly normally in a 125 from Northolt and we would fly the 146 out with the other three on.”

It’s not the only travel rule the royals follow. According to Dickie Arbiter, former press secretary to Queen Elizabeth II, the royal planes usually fly with blood matched to the royal’s blood type.

“[You’ve] got to make sure that you’re covering every eventuality,” he explained on Hello’s A Right Royal Podcast , “and the aircraft would carry blood in case there was blood transfusion. There’s never any guarantee you are going to get the right type of blood at your destination.”

Blood isn’t the only medical consideration. In 2016, the Telegraph ’s Gordon Rayner reported that the Queen would also travel alongside a Royal Navy doctor whenever she went on a world tour, who would research information about local hospitals.

“In countries where a reliable blood supply is questionable, the Queen and the Prince of Wales both travel with their own personal packs of blood following in their convoy wherever they go,” he added. “Their doctor is never more than a few paces away, carrying a bulky medical bag containing a mobile defibrillator and all manner of emergency medicine.”