14 CFR 29.1103 - Induction systems ducts and air duct systems.
(a) Each induction system duct upstream of the first stage of the engine supercharger and of the auxiliary power unit compressor must have a drain to prevent the hazardous accumulation of fuel and moisture in the ground attitude. No drain may discharge where it might cause a fire hazard.
(b) Each duct must be strong enough to prevent induction system failure from normal backfire conditions.
(c) Each duct connected to components between which relative motion could exist must have means for flexibility.
(d) Each duct within any fire zone for which a fire-extinguishing system is required must be at least -
(1) Fireproof, if it passes through any firewall; or
(e) Each auxiliary power unit induction system duct must be fireproof for a sufficient distance upstream of the auxiliary power unit compartment to prevent hot gas reverse flow from burning through auxiliary power unit ducts and entering any other compartment or area of the rotorcraft in which a hazard would be created resulting from the entry of hot gases. The materials used to form the remainder of the induction system duct and plenum chamber of the auxiliary power unit must be capable of resisting the maximum heat conditions likely to occur.
(f) Each auxiliary power unit induction system duct must be constructed of materials that will not absorb or trap hazardous quantities of flammable fluids that could be ignited in the event of a surge or reverse flow condition.