Abstract: The invention relates to an improvement to an installation for preventing the ingress of super-cooled water, ice, snow and other debris as taught in U.S. Pat. No. 3,329,377. In the patent, an inertial separator effect is used to prevent ingress of super cooled water, ice, snow and other debris to an aircraft engine. The effect is obtained by an installation including a duct formed in a sub-nacelle of the engine to extend generally longitudinally of the engine. An air deflecting surface in the duct confines the incoming air to a passage of reduced cross section, and an opening in the duct wall downstream of the deflecting surface is formed between the trailing edge of the deflecting surface and the leading edge of a rearward portion of the duct, the opening in the duct wall communicating with the engine air intake. In accordance with the present invention, any or all of the following improvements are made to the above installation:A.