This from Wiki. Note 'revealed' in the second paragraph:
Willem-Alexander is an avid pilot and has said that if he had not been a royal, he would have liked to be an airline pilot so he could fly big planes such as the Boeing 747. In 1989, Willem-Alexander flew as a volunteer for the African Medical Research and Education Foundation (AMREF) in Kenya, and in 1991 he spent a month flying for the Kenya Wildlife Service. During the reign of his mother, he regularly flew the Dutch royal airplane on trips.
In May 2017, Willem-Alexander revealed that he had served as a co-pilot on KLM flights for 21 years, flying KLM Cityhopper's Fokker 70s twice a month, even after his ascension to the throne. With KLM's phased retirement of the Fokker 70, he began training to fly Boeing 737s. Willem-Alexander was rarely recognized while in the KLM uniform and wearing the KLM cap, though a few passengers had recognized his voice, even though he never gave his name and only welcomed passengers on behalf of the captain and crew.[