King Willem-Alexander of the Netherlands
King Willem-Alexander of the Netherlands wears a traditional Australian farmer's hat during a speech at an event on sustainable food production on Cockatoo Island on Sydney Harbour, Australia, November 3, 2016. Reuters/Jason Reed

King Willem-Alexander of the Netherlands is not just the country’s head of state, he was also an airline cockpit’s co-captain. The 50-year-old monarch has been secretly moonlighting as a co-pilot for the Dutch airline KLM for 21 years.

He admitted to Dutch newspaper De Telegraaf that he had been flying twice a month on KLM’s Fokker 70 planes. He usually flew with Captian Maarten Putman while their passengers were none the wiser they were being flown by the king himself.

He called flying a commercial airplane a “hobby” that let him focus on something else other than his royal duties while he was up in the air. “You can’t take your problems from the ground into the skies. You can completely disengage and concentrate on something else. That, for me, is the most relaxing part of flying.”

He never used his name when addressing his passengers. He was also rarely recognised when wearing his uniform and KLM cap. Some passengers, however, could recognise his voice.

“The advantage is that I can always say that I warmly welcome passengers on behalf of the captain and crew,” the king said. “Then I don’t have to give my name.”

Willem-Alexander has ended his run as co-pilot to Fokker 70 planes, which are being phased out by KLM. He plans to retain and learn to fly the bigger Boeing 737s in the next few months.

The Dutch king, who ascended to the throne in 2013 following his mother Beatrix’s abdication, isn’t the only flying monarch. The UK’s Prince Charles and his sons Prince William and Prince Harry are qualified pilots. William has recently given up his role as an RAF air ambulance pilot earlier this year to become a full-time working royal, while Harry served as an Apache helicopter pilot in Afghanistan a few years before.