14 CFR § 29.859

Combustion heater fire zones. The following combustion heater fire zones must be protected against fire under the applicable provisions of §§ 29.1181 through 29.1191, and 29.1195 through 29.1203:

The region surrounding any heater, if that region contains any flammable fluid system components (including the heater fuel system), that could—

Be damaged by heater malfunctioning; or

Allow flammable fluids or vapors to reach the heater in case of leakage.

Each part of any ventilating air passage that—

Surrounds the combustion chamber; and

Would not contain (without damage to other rotorcraft components) any fire that may occur within the passage.

Ventilating air ducts. Each ventilating air duct passing through any fire zone must be fireproof. In addition—

Unless isolation is provided by fireproof valves or by equally effective means, the ventilating air duct downstream of each heater must be fireproof for a distance great enough to ensure that any fire originating in the heater can be contained in the duct; and

Each part of any ventilating duct passing through any region having a flammable fluid system must be so constructed or isolated from that system that the malfunctioning of any component of that system cannot introduce flammable fluids or vapors into the ventilating airstream.

Combustion air ducts. Each combustion air duct must be fireproof for a distance great enough to prevent damage from backfiring or reverse flame propagation. In addition—

No combustion air duct may communicate with the ventilating airstream unless flames from backfires or reverse burning cannot enter the ventilating airstream under any operating condition, including reverse flow or malfunction of the heater or its associated components; and

No combustion air duct may restrict the prompt relief of any backfire that, if so restricted, could cause heater failure.

Heater controls; general. There must be means to prevent the hazardous accumulation of water or ice on or in any heater control component, control system tubing, or safety control.

Heater safety controls. For each combustion heater, safety control means must be provided as follows:

Means independent of the components provided for the normal continuous control of air temperature, airflow, and fuel flow must be provided, for each heater, to automatically shut off the ignition and fuel supply of that heater at a point remote from that heater when any of the following occurs:

The heat exchanger temperature exceeds safe limits.

The ventilating air temperature exceeds safe limits.

The combustion airflow becomes inadequate for safe operation.

The ventilating airflow becomes inadequate for safe operation.

The means of complying with paragraph (e)(1) of this section for any individual heater must—

Be independent of components serving any other heater whose heat output is essential for safe operation; and

Keep the heater off until restarted by the crew.

There must be means to warn the crew when any heater whose heat output is essential for safe operation has been shut off by the automatic means prescribed in paragraph (e)(1) of this section.

Air intakes. Each combustion and ventilating air intake must be where no flammable fluids or vapors can enter the heater system under any operating condition—

During normal operation; or

As a result of the malfunction of any other component.

Heater exhaust. Each heater exhaust system must meet the requirements of §§ 29.1121 and 29.1123. In addition—

Each exhaust shroud must be sealed so that no flammable fluids or hazardous quantities of vapors can reach the exhaust systems through joints; and

No exhaust system may restrict the prompt relief of any backfire that, if so restricted, could cause heater failure.

Heater fuel systems. Each heater fuel system must meet the powerplant fuel system requirements affecting safe heater operation. Each heater fuel system component in the ventilating airstream must be protected by shrouds so that no leakage from those components can enter the ventilating airstream.

Drains. There must be means for safe drainage of any fuel that might accumulate in the combustion chamber or the heat exchanger. In addition—

Each part of any drain that operates at high temperatures must be protected in the same manner as heater exhausts; and

Each drain must be protected against hazardous ice accumulation under any operating condition.