Each engine, auxiliary power unit, fuel-burning heater, or other item of combusting equipment that is intended for operation in flight must be isolated from the rest of the airplane by means of firewalls or shrouds, or by other equivalent means.
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14 CFR · PART 125
§ 125.143 Firewalls.
Official text · Current as of Oct 19, 2026 · from eCFR
Not legal advice. This is the official text as published in the eCFR, which is updated daily but is not the legal edition. For legal purposes, use the annual Code of Federal Regulations or ask an aviation attorney.