"This is your captain speaking..." is something you hear on almost every flight.

But for some flying Dutch airline KLM, this voice may have seemed familiar.

Their King Willem Alexander has revealed he has flown twice-monthly flights with the airline's Cityhopper fleet on short-haul routes for the past 21 years.

"For me the most important thing is that I have a hobby for which I need to concentrate completely," he told the De Telegraaf newspaper.

"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 said.

He intends to carry on as co-pilot, but will spend the coming months learning how to fly Boeing 737s.

'This is your Royal Highness speaking' pic.twitter.com/koDBlHWqa8

— Royal Dutch Airlines (@KLM) May 17, 2017

King Willem Alexander first took to the skies over 30 years ago while still a student.

The Dutch royal household says the King obtained a supplementary licence to fly multi-engine jet aircraft in 1989, and earned his Airline Transport Pilot Licence in 2001.

It says he "occasionally flies as a guest pilot" for KLM Cityhopper to keep in practice.

Until mid-2017 he flew either the government airplane, or a Fokker 70 from the regular fleet.

It adds that he is to receive training on the Boeing 737 in order to continue flying as a guest pilot with the airline.

"This decision was taken before the contract award procedure for replacing the government aeroplane was launched and is unrelated to it", the household says.

KLM Cityhopper recently announced it will no longer fly Fokker 70s (FK70) after 2017.