14 CFR §25.1091

1. (a)The air induction system for each engine and auxiliary power unit must supply—
   1. (1)The air required by that engine and auxiliary power unit under each operating condition for which certification is requested; and
   2. (2)The air for proper fuel metering and mixture distribution with the induction system valves in any position.
2. (b)Each reciprocating engine must have an alternate air source that prevents the entry of rain, ice, or any other foreign matter.
3. (c)Air intakes may not open within the cowling, unless—
   1. (1)That part of the cowling is isolated from the engine accessory section by means of a fireproof diaphragm; or
   2. (2)For reciprocating engines, there are means to prevent the emergence of backfire flames.
4. (d)For turbine engine powered airplanes and airplanes incorporating auxiliary power units—
   1. (1)There must be means to prevent hazardous quantities of fuel leakage or overflow from drains, vents, or other components of flammable fluid systems from entering the engine or auxiliary power unit intake system; and
   2. (2)The airplane must be designed to prevent water or slush on the runway, taxiway, or other airport operating surfaces from being directed into the engine or auxiliary power unit air inlet ducts in hazardous quantities, and the air inlet ducts must be located or protected so as to minimize the ingestion of foreign matter during takeoff, landing, and taxiing.
5. (e)If the engine induction system contains parts or components that could be damaged by foreign objects entering the air inlet, it must be shown by tests or, if appropriate, by analysis that the induction system design can withstand the foreign object ingestion test conditions of §§ 33.76, 33.77 and 33.78(a)(1) of this chapter without failure of parts or components that could create a hazard.