For most people, travelling essentials include passport, wallet, phone and other personal items, but for royals flying onboard it means one extra item, and it's for a grim reason.

Princess Kate, as well as other royals embarking on a flight, will need to carry a black dress or other formal wear with them. This isn't just so they have formal attire but in case any other royal dies during the journey.

The protocol was established after the late Queen Elizabeth II had been on an emergency flight back to the UK in 1952 when King Goerge VI had died.

The then-princess Elizabeth was not in the UK when she became the Queen and was instead at the beginning of a Commonwealth tour in Kenya when her father died suddenly at Sandringham House in Norfolk from lung cancer aged 56.

As King George VI's death was unexpected, the Queen had not packed a black dress to take with her on her official tour.

The Queen's plane landed at London Airport the day after the king died, on February 7, and a suitable dress was brought aboard for her to change into before she disembarked.

Ever since then, it has become a custom for royals to travel with a black dress or appropriate mourning attire in the case that someone should die while they are abroad.

This wasn't the only protocol senior royals followed. Queen Elizabeth also had a doctor on board as well as a blood supply, lest any medical emergencies should occur during the journey.

It's now believed that heirs to the throne, King Charles and Prince William, both follow these steps. It is also reported that two heirs can't travel on the same plane together.

This rule can be broken with the monarch's permission – and Prince William has travelled with Prince George – but once a royal child turns 12 it is recommended they don't travel with another heir in case a plane crash occurs and the future of the monarchy is left unstable.

There have been other members of the Royal Family to die in air accidents over the years, including in 1937 when Prince Philip’s sister, Princess Cecile, died in a crash along with six other passengers and the plane crew.

Cecile was eight months pregnant at the time and had given birth mid flight, as a newborn baby who had died was found among the wreckage.

During World War Two, the Queen’s uncle, Prince George, Duke of Kent, died in 1942 while the Queen's cousin, Prince William of Gloucester, died while competing in the Goodyear International Air Trophy on August 28, 1972.