For 21 years, the voice from the cockpit never gave the show away. The familiar announcements about the weather on arrival or expected landing time on certain KLM flights, came from a qualified but far from ordinary pilot.
As the Dutch learned this week, the announcements concealed a royal secret - they were made by King Willem-Alexander, who for two decades had carried unsuspecting passengers around European skies.
The 50-year-old father of three talked about his flying duties in an interview to celebrate his wife Queen Maxima's birthday, describing how his "hobby", as he put it, allowed him the freedom to leave his earthbound royal duties and turn his attention to something else.
So far his skills have been deployed on Fokker F-70 aircraft. KLM is replacing the narrow bodied twin engined jets in favour of Boeing 737s. The monarch intends to retrain for the larger aircraft.
The Dutch king is not the only royal with a appetite for flying. Prince Charles and his two sons are qualified pilots, as is Jordan's King Abdullah. The Sultan of Brunei is known to fly his own Boeing 747, while in New Zealand the closest we have seem to have come to royalty at the joystick is the sight of former All Black skipper Richie McCaw at the controls of a helicopter.
King Willem-Alexander said he was rarely recognised in his pilot's uniform, though before the 9/11 attacks, when cockpit doors were open and flying was more relaxed, passengers would regularly check the flight deck to see the king at the controls. He sat in the co-pilot's seat because he flew as a regular guest pilot, and welcomed customers on board "on behalf of the captain and crew" - rather than revealing his true identity.
As the co-pilot he did not need to divulge his name and passengers seemed none the wiser. As he pointed out most travellers didn't listen to the intercom. Now that his story has been told, KLM customers might start to pay a little more attention.