14 CFR § 23.2440

A powerplant, auxiliary power unit, or combustion heater that includes a flammable fluid and an ignition source for that fluid must be installed in a designated fire zone.

Each designated fire zone must provide a means to isolate and mitigate hazards to the airplane in the event of fire or overheat within the zone.

Each component, line, fitting, and control subject to fire conditions must—

Be designed and located to prevent hazards resulting from a fire, including any located adjacent to a designated fire zone that may be affected by fire within that zone;

Be fire-resistant if carrying flammable fluid, gas or air, or is required to operate in the event of a fire; and

Be fireproof or enclosed by a fire proof shield if storing concentrated flammable fluids.

The applicant must provide a means to prevent hazardous quantities of flammable fluids from flowing into, within or through each designated fire zone. This means must—

Not restrict flow or limit operation of any remaining powerplant or auxiliary power unit, or equipment necessary for safety;

Prevent inadvertent operation; and

Be located outside the fire zone unless an equal degree of safety is provided with a means inside the fire zone.

A means to ensure the prompt detection of fire must be provided for each designated fire zone—

On a multiengine airplane where detection will mitigate likely hazards to the airplane; or

That contains a fire extinguisher.

A means to extinguish fire within a fire zone, except a combustion heater fire zone, must be provided for—

Any fire zone located outside the pilot's view;

Any fire zone embedded within the fuselage, which must also include a redundant means to extinguish fire; and

Any fire zone on a level 4 airplane.