FK.51 intermediate trainers of NEI army aviation over Java, wearing orange triangle markings introduced in late 1939. Serials are K-102, K-104, K-105, K-106, K-107, K-110, K-113, K-115 and K-120. The numbers were upped by 100 during 1940, so K-105 is the same airframe as K-5. Photo from: Nationaal Archief (Netherlands)

Entered service on 9 February 1938, destroyed at Haamstede in May 1940 (likely the location of this photo). The FK.51 was developed in early 1935 as a trainer and ultimately received 10 orders (serials 1 - 10) from the LVA. A second order for 15 aircraft (serials 11 - 25) followed in 1937. These were painted in light blue, whereas the first batch of 10 was painted dark brown. Later, another 29 aircraft (serials 400 - 428) were ordered as light scout aircraft, these were painted dark brown again. Photo from W. Muntinga

A new FK.51 observation aircraft, really just an intermediate trainer. The big man is Frits Koolhoven. Photo from: Nationaal Archief (Netherlands)

Koolhoven FK.51 trainers, Dutch army s/n 2 and 21. The first ten had rounded tailfins and a brown paint job with curly numbers. Photo from: Nationaal Archief (Netherlands)

FK.51 intermediate trainers 15 and 18 in the same formation, no doubt, as in photo 236129. Dutch army serial number 18 crashed at Eindhoven on 14 August 1939 and was a write-off. Photo from: Nationaal Archief (Netherlands)

The first FK.51 for the Dutch army air force was allocated serial number 1 in the low range assigned to trainer aircraft. Early FK.51s had rounded tailfins. Photo from: Nederlands Instituut voor Militaire Historie

FK.51 intermediate trainers of the NEI army air force, probably photographed at Kalidjati (Kalijati), Java. Photo from: Leiden University Libraries

FK.51 intermediate trainers, army air force serial numbers 10, 15 and 18 through 21, in flight over Holland. The first batch, s/n 1 through 10, were painted brown; the second batch were pale blue and the typeface of the serials was also changed. The pale blue machines here were all delivered on 9 February 1938. A fatal collision involving s/n 20 dates the photograph before 15 July of the same year. Photo from: Nederlands Instituut voor Militaire Historie

Pratt & Whitney R-985 Wasp Junior engines powered the forty-five Royal Netherlands East Indies Army FK.51 intermediate trainers. Detail from slide showing K-9 on the right. Photo from: Wereldmuseum

A colour slide from Andir, showing the mud colour applied to KNIL (Royal Netherlands East Indies Army) aircraft. The left Koolhoven trainer is K-5. Note the pilot on the right. Photo from: Wereldmuseum

The Dutch army air force's intermediate flying training school was based at Haamstede airfield in Zeeland province in 1939. Photo from: Nederlands Instituut voor Militaire Historie

One of the first batch of ten FK.51 intermediate trainers of the Dutch army air force. Photo from: Bodo Sandberg collection

This replica of the Koolhoven FK-51 had recently been added to the Kamp van Zeist Air Force museum. Frits Koolhoven was the other great Dutch aircraft builder.

First flown in 1935, the FK.51 intermediate trainer was numerically the most succesful aircraft of the Koolhoven firm. Over 100 FK.51s were built for the Dutch forces, including some as artillery spotters, but they were not used operationally in the 1940 May war. Another batch was sold to Republican Spain.