Meanwhile, her husband Prince William tends to wear suits.
“When members of the Royal Family disembark on an official trip they are greeted by official representatives from the nation’s government, along with the international press. These are official events,” continued Ms Chertoff.
However, behind closed doors, things may be a little different.
“They may wear more relaxed clothing for long flights, but when exiting a plane they’re dressed in a suit or dress,” Ms Chertoff said.
Royal Family transport costs (Image: DX)
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When the Royal Family members travel for leisure purposes, it is also likely they do not follow such strict rules.
Express.co.uk spoke to Anne Chertoff, Chief Operating Officer of Beaumont Etiquette which runs a Duchess-themed etiquette course.
Chertoff shared her insight into Prince William and Kate s travels - can they do what they like when out of the public eye?
While the royal pair are obviously afforded a level of privacy on these trips, the reality, said the etiquette expert, is that they actually have very little time to themselves.
In short, there is not much opportunity to break away from the strict schedule.
Kate Middleton has certain rules she has to follow when on state visits (Image: Getty Images)
It s important she exits the aircraft displaying the correct etiquette.
The royals, particularly the women, must keep their chin level to the stairs as they leave the aircraft.
This etiquette rule means the woman in question not only has good posture but also allows them to appear engaged with what is in front of them.
Kate Middleton: It s understood some guidelines only apply to the female members of the clan (Image: Getty Images)
Kate Middleton: There is one rule the Duchess has to follow when travelling by plane (Image: Getty Images)
However, they are permitted to hold onto the rail to steady themselves.
Prince William and Kate flying with British Airways (Image: Getty Images)
In 2015, Prince William was spotted onboard a Ryanair flight alongside his bodyguard.
The future king was spotted in an economy cabin, flying to Scotland from Norfolk as part of his training with East Anglian Air Ambulance.
Passengers onboard spotted the prince as he was exiting the aircraft.
One traveller told The Sun: “I never expected to see him on Ryanair, but its refreshing to see a member of the royal family take a cheap flight.”
In 2012, repots circulated suggesting Prince William and the Duchess of Cambridge flew home from a romantic skiing trip in the French Alps with easyJet.