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§ 25.1193
Cowling and nacelle skin.
(a) Each cowling must be constructed and supported so that it can resist any vibration, inertia, and air load to which it may be subjected in operation.
(b) Cowling must meet the drainage and ventilation requirements of § 25.1187.
(c) On airplanes with a diaphragm isolating the engine power section from the engine accessory section, each part of the accessory section cowling subject to flame in case of fire in the engine power section of the powerplant must—
(1) Be fireproof; and
(2) Meet the requirements of § 25.1191.
(d) Each part of the cowling subject to high temperatures due to its nearness to exhaust system parts or exhaust gas impingement must be fireproof.
(e) Each airplane must—
(1) Be designed and constructed so that no fire originating in any fire zone can enter, either through openings or by burning through external skin, any other zone or region where it would create additional hazards;
(2) Meet paragraph (e)(1) of this section with the landing gear retracted (if applicable); and
(3) Have fireproof skin in areas subject to flame if a fire starts in the engine power or accessory sections.