14 CFR 23.1103 - Induction system ducts.
(a) Each induction system duct must have a drain to prevent the accumulation of fuel or moisture in the normal ground and flight attitudes. No drain may discharge where it will cause a fire hazard.
(b) Each duct connected to components between which relative motion could exist must have means for flexibility.
(c) Each flexible induction system duct must be capable of withstanding the effects of temperature extremes, fuel, oil, water, and solvents to which it is expected to be exposed in service and maintenance without hazardous deterioration or delamination.
(d) For reciprocating engine installations, each induction system duct must be -
(1) Strong enough to prevent induction system failures resulting from normal backfire conditions; and
(2) Fire resistant in any compartment for which a fire extinguishing system is required.
(e) Each inlet system duct for an auxiliary power unit must be -
(1) Fireproof within the auxiliary power unit compartment;
(2) Fireproof for a sufficient distance upstream of the auxiliary power unit compartment to prevent hot gas reverse flow from burning through the duct and entering any other compartment of the airplane in which a hazard would be created by the entry of the hot gases;
(4) Constructed of materials that will not absorb or trap hazardous quantities of flammable fluids that could be ignited by a surge or reverse-flow condition.
(f) Induction system ducts that supply air to a cabin pressurization system must be suitably constructed of material that will not produce hazardous quantities of toxic gases or isolated to prevent hazardous quantities of toxic gases from entering the cabin during a powerplant fire.