The performance criterion for engine power for aircraft of the Skyvan type is related to the single engine climb gradient in the en route configuration. The required engine power (M.C.P.) was specified such as to give a payload capability of 3,500 lb. at an ambient condition of ISA plus 20 deg. C. By use of such a specification the effects of different engine bulk, weight and individual engine installation deficiencies were included in the assessment. The Garrett 331–201 engine was chosen, as far as aerodynamic considerations were concerned, because the required power was immediately available on then current development engines, substantiated by test bed data. In addition the installation was most suitable to the underslung position on Skyvan in that the nacelle lines, though severe, were acceptable, and the straight through exhaust system was preferred. The anti‐icing system of the Garrett gives no performance penalty in the ‘anti‐icing off’ configuration as related to certification performance, and was already certificated on the Volpar aircraft at that time.