14 CFR §23.2435

1. (a)The air induction system for each powerplant or auxiliary power unit and their accessories must—
   1. (1)Supply the air required by that powerplant or auxiliary power unit and its accessories under likely operating conditions;
   2. (2)Be designed to prevent likely hazards in the event of fire or backfire;
   3. (3)Minimize the ingestion of foreign matter; and
   4. (4)Provide an alternate intake if blockage of the primary intake is likely.
2. (b)The exhaust system, including exhaust heat exchangers for each powerplant or auxiliary power unit, must—
   1. (1)Provide a means to safely discharge potential harmful material; and
   2. (2)Be designed to prevent likely hazards from heat, corrosion, or blockage.