For 21 years, the king of the Netherlands has flown twice a month as co-pilot while his passengers were in the dark, he has told a Dutch newspaper.
Willem-Alexander acceded to the throne in 2013 but his other role, in the cockpit, has continued.
"I find flying simply fantastic," he told De Telegraaf newspaper.
He intends to carry on as co-pilot but will spend the coming months learning how to fly Boeing 737s.
Until now Willem-Alexander has worked behind the joystick of a Fokker and it was already known that he had appeared as a "guest pilot" before being crowned king, in order to maintain his pilot's licence.
What was not clear was that he was co-piloting passenger flights incognito, twice a month as king, often with KLM Captain Maarten Putman.
The Dutch government said last month that he had flown Fokker 70 aircraft for both the government and KLM Cityhopper service, and that the plane was being replaced this year with a 737. Cityhopper flights are aimed mainly at business travellers in dozens of European destinations, particularly in the UK, Germany and Norway.
Rarely recognised on board
Willem-Alexander once said that if he had not been born in a palace, his dream would have been to fly a big passenger plane such as a Boeing 747, so it is no surprise that he intends to retrain for the updated plane.
He told De Telegraaf that he never used his name when addressing passengers and was rarely recognised in uniform and wearing his KLM cap. However, he admitted that some passengers had recognised his voice.
"The advantage is that I can always say that I warmly welcome passengers on behalf of the captain and crew," he said. "Then I don't have to give my name."
Passengers were more likely to realise they were being flown by royalty before the 11 September attacks on the US in 2001 as the cockpit door was unlocked. But there was now less contact with the cabin, he said.
Not the only flying royal
The Dutch king's appetite for flying was apparently encouraged by his mother, Beatrix, who abdicated as queen in 2013. And he is on a long list of royal pilots:
- The Sultan of Brunei is known to fly his own Boeing 747
- Prince Charles is a qualified pilot as are both his sons
- Prince William gives up his role as RAF air ambulance helicopter pilot this year
- Prince Harry served in Afghanistan as an Apache helicopter co-pilot
- Jordan's King Abdullah is a trained pilot
Leaving problems on the ground
In his interview, King Willem-Alexander appeared enthusiastic about his future prospects as a co-pilot.
"It also seemed nice to fly to other destinations one day, with more passengers and bigger distances. That was the real motive for training on the 737," he said.
The king explained that the most important thing for him was to have a hobby he could fully concentrate on and that flying was his biggest way of relaxing.
"You have a plane, passengers and crew and you are responsible for them. You can't take your problems with you off the ground. You can completely switch off for a while and focus on something else."