Dutch King Willem-Alexander has revealed that he has been secretly flying commercial aircraft for the last four years.

The king told the Dutch newspaper De Telegraaf that he was stopping flying the Fokker 70 for KLM (Royal Dutch Airways). The Dutch-made aircraft is being retired from service to be replaced by the Boeing 737.

While it was no secret that the king had a pilot's license, it was not known that he was flying as co-pilot on KLM short-haul services around Europe twice a month. He often addressed passengers but avoided giving his name. Some passengers did, he said, recognize 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 volunteered to fly for the African Medical Research and Education Foundation and the Kenya Wildlife Service in the 1990s. He often flew the Dutch royal airplane during the reign of his mother.

The 50-year-old father of three said flying was his hobby and it allowed him to leave his royal duties on the ground.

"You have an aircraft, passengers and crew. You have responsibility for them," he told De Telegraaf.

"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."

The Fokker 70 is a twin engine jet that can carry 72-85 passengers with two pilots. As of last year there were 45 in service with eight airlines and two governments - the Netherlands and Kenya.

Willem-Alexander became the Prince of Orange in 1980, the heir to the Dutch throne, and king in 2013 after the death of Queen Beatrix.

The Dutch royal family are sometimes described as the bicycling monarchs because of their relaxed style, which they share with the Scandinavian monarchies.

Willem-Alexander said that he plans to retrain to fly KLM's Boeing 737s so passengers will have many more opportunities to be flown by royalty.

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