12/22/2006. Remarks by Bernhard Klein: "The plane was designed for the CEO of the Royal Dutch Shell Co., Mr. Kok, hence the registration PH-KOK. First flight 20, June 1938. At the outbreak of the Second World War, the PH-KOK was requisitioned by the Aviation Section of the Dutch Armed Forces. It was given the registration 978. It was destroyed at Ypenburg, nearThe Hague, on the first day of the German invasion, May 10, 1940.However the registration was not cancelled until April 14, 1942.

The F.K.57 had a fixed landing gear. Koolhoven also offered a military version with a retractable landing gear. He expected that in this configuration the plane would be excellently suitable for training multi-engine pilots. At that time, these training sessions still took place on an antiquated Fokker F.VIIb-3m. However, five Focke-Wulf 58 Weihe were bought for this purpose, so there was no further interest in the F.K.57."