For two decades, the king of the Netherlands has been putting the royal in KLM Royal Dutch Airlines.

King Willem-Alexander said in an interview published on Wednesday that for the last 21 years, he has flown twice a month as a commercial airline co-pilot for KLM's Cityhopper subsidiary, a regional carrier that flies among European cities.

Dutch King Willem-Alexander regularly pilots for KLM. Credit:Peter Dejong

He was seldom recognised in uniform, particularly after the September 11 terrorist attacks, when safety protocols were introduced to limit access to cockpits, he said.

The king would often greet passengers over the intercom, but not readily identify himself.