Kate Middleton and Prince William oftentimes follow the different royal rules that they need to adhere to. But there’s reportedly one strict rule that they’ve been repeatedly breaking for the sake of their three children.

In 2014, Prince William and Kate traveled on the same plane with their eldest child, Prince George. The royal rule dictates that heirs to the throne are not allowed to travel on the same plane together.

In case of an emergency, it’s important that at least one heir to the throne will survive. But since Prince George was just a few months old when William and Kate went on royal tours to New Zealand and Australia, his parents were forced to travel with him on the same plane.

Since then, Prince William and Kate have always traveled with their kids on the same plane. But this is quite problematic because all of Prince William’s children are heirs to the throne. However, it’s only Prince George who will become king one day.

And just last month, the Cambridges took a commercial flight to Scotland for their short vacation. An onlooker said that Prince William, Kate and their three children went straight to the front row of the plane.

“I fly this route all the time and we were none the wiser,” the source told Daily Mail. “No one knew they were on the flight,” the source added.

Another onlooker told The Sun that he initially thought that Prince William was a football player because he didn’t recognize him immediately. But he knew that someone famous was on the same flight with him.

“We landed, and people then saw two black Range Rovers on the tarmac,” the source said. “Suddenly, Will, Kate, and the kids all got up and left. They weren’t harassed by anyone at all as they got off the plane. They just seemed like a family traveling together, to be honest,” the source added.

As of late, there’s still no confirmation on where Kate, Prince William and their kids will be headed next. But it’s guaranteed that they will continue to ride the same plane until Prince George, Princess Charlotte and Prince Louis get older.

Prince William, Prince George, Kate Middleton and Princess Charlotte Photo of Prince William, Prince George, Kate Middleton and Princess Charlotte. Chris Jackson - Pool/Getty Images