14 CFR 23.1191 - Firewalls.
(a) Each engine, auxiliary power unit, fuel burning heater, and other combustion equipment, must be isolated from the rest of the airplane by firewalls, shrouds, or equivalent means.
(b) Each firewall or shroud must be constructed so that no hazardous quantity of liquid, gas, or flame can pass from the compartment created by the firewall or shroud to other parts of the airplane.
(c) Each opening in the firewall or shroud must be sealed with close fitting, fireproof grommets, bushings, or firewall fittings.
(e) Each firewall and shroud must be fireproof and protected against corrosion.
(f) Compliance with the criteria for fireproof materials or components must be shown as follows:
(1) The flame to which the materials or components are subjected must be 2,000 ±150 °F.
(2) Sheet materials approximately 10 inches square must be subjected to the flame from a suitable burner.
(3) The flame must be large enough to maintain the required test temperature over an area approximately five inches square.
(g) Firewall materials and fittings must resist flame penetration for at least 15 minutes.
(h) The following materials may be used in firewalls or shrouds without being tested as required by this section:
(1) Stainless steel sheet, 0.015 inch thick.
(2) Mild steel sheet (coated with aluminum or otherwise protected against corrosion) 0.018 inch thick.
(3) Terne plate, 0.018 inch thick.
(4) Monel metal, 0.018 inch thick.
(5) Steel or copper base alloy firewall fittings.
(6) Titanium sheet, 0.016 inch thick.