They are seen as proper heirs to the throne who are also more modern royals, but some of that modern vibe requires Kate Middleton and Prince William to seemingly put their collective feet down when it comes to some of the more outdated royal rules.

According to Express UK, one rule, in particular, seems to be one the Duke and Duchess of Cambridge break consistently, but they have no intentions of changing their minds on it either.

The website reports that a royal family rule states that heirs to the throne cannot travel together by plane due to the risks of accidents or emergencies, which dates back to the earlier days of plane travel when it wasn’t deemed as safe. While planes are safer now, the rule is still technically on the books—and the royal couple has not followed it.

They not only broke it during a trip to Australia and New Zealand in 2014 with their eldest Prince George, whom the rule strictly forbids from traveling with his father, but they continue to do so now, with all three, as well as Princess Charlotte and Prince Louis, always traveling together on the same flights.

Most recently, the Cambridge family all boarded a flight together to Scotland, where others on the commercial airline didn’t even realize they were on board.

“The family were sat right at the front. I fly this route all the time and we were none the wiser. No one knew they were on the flight,” one passenger said at the time.

“Earlier in the flight, I’d had the feeling that someone had come on the plane that I’d recognized—but I thought it was just a footballer,” another said. “We landed, and people then saw two black Range Rovers on the tarmac. 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,” they added. “No one saw them get on the plane either. It was amazing how they did it—they must have slipped on quietly after people were already seated.”

The decision to continually flaunt breaking the rule likely comes from the couple’s desires to raise their children as normally as possible—something that is even more important to the Duchess.

“She wants to emulate her upbringing, living in the countryside with a close-knit family,” a source told People. “She desperately wants that normality for her own kids.”

Prince William, Kate Middleton and Kids
Royal Family arrive at 443 Maritime Helicopter Squadron on Sept. 24, 2016 in, Victoria, British Columbia. DON MACKINNON/AFP/Getty Images