On each side two parallel, airfoil-section struts braced the wing from the lower fuselage to the spars, each pair only about 1,200 mm (47 in) from the centreline.

[1] The FK.42 was powered by an air-cooled, 110 hp (82 kW) Cirrus Hermes four-cylinder inline engine, mounted upright in the nose.

The fuselage, like the wings, was all-wood; spruce longerons and ply skin gave it flat sides and bottom but there was rounded upper decking.

[1][3] The trainer was flown solo from the rear seat; the pupil's forward position had deployable dual controls.

Both the FK.42 and the larger, three-seat FK.41 were at the International Light Plane Meeting held in Rotterdam between 27 and 30 June 1929,[2] where Frederick Koolhoven certainly flew the FK.41.