King Willem-Alexander, the reigning monarch of the Netherlands disclosed Wednesday during an interview with the Dutch newspaper, De Telegraaf he was moonlighting as a co-pilot for a KLM airlines affiliate for over two decades.

The monarch called the part-time role a “hobby” and said he co-pilots two KLM Cityhopper flights per month and has been doing it for over 21 years. Prior to that he used to pilot planes for Dutch carrier Martinair. He told the newspaper currently he has been training to fly Boeing 737s for KLM itself, which are also known as Dutch Royal Airlines.

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The interview revealed the king frequently took charge of the co-pilot’s chair for short-haul flights around Europe.

“For me the most important thing is that I have a hobby for which I need to concentrate completely,” he said.

“You have an aircraft, passengers and crew. You have responsibility for them. 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 added.

According to information provided by the Royal Dutch household to CNN, King Willem-Alexander piloted his first aircraft over 30 years ago while he was still a student. During the late ‘80s, passion for his “hobby” took him to Kenya, where he worked as a volunteer pilot, first for the medical aid organization, African Medical Research & Education Foundation (AMREF), and later went to work for the Kenya Wildlife Service.

He has been flying the KLM Cityhopper’s Fokker 70 fleet even after he acceded to the throne in 2013. The monarch had served in the Royal Netherlands Air Force before becoming king and had a pilot’s license. But it was not disclosed earlier how frequently he went incognito into cockpits of regular commercial flights.

Fokker 70 was a narrow-body regional airliner, and now the monarch is being retrained to fly a Boeing 737. He told De Telegraaf it “seemed nice to fly to other destinations one day, with more passengers and bigger distances.”

He also told the newspaper he was hardly recognized in his second job, especially since 9/11. 

Prior to the Sept. 11 terror attacks, he said, when the cockpit doors used to remain open, he would sometimes be spotted by people.

"People regularly came to have a look and thought it was nice or surprising that I was sitting there," he said.

“After September 11, the cockpit door is closed and there is much less contact between the cabin and the cockpit,” he added. “But yes, some people recognize my voice during the flight.”

Willem-Alexander acceded to the throne four years ago after his mother, Queen Beatrix, abdicated the throne to him, following a 33 year rule.

However, the Dutch monarch is not the only royal with a history in the cockpit. Brunei's Sultan Hassanal Bolkiah also reportedly takes charge of the cockpit sometimes when he flies on state visits.

Even Britain's heir to the throne, Prince Charles, is a qualified pilot while both his sons, Prince William and Prince Harry, have flown helicopters in their previous military careers.

William was said to have flown a search-and-rescue chopper and Harry, Apache helicopters in Afghanistan.