267 

Federal Aviation Administration, DOT 

§ 23.859 

(3) The part of the ventilating air 

passage that surrounds the combustion 
chamber. 

(b) 

Ventilating air ducts. Each ven-

tilating air duct passing through any 
fire region must be fireproof. In addi-
tion— 

(1) Unless isolation is provided by 

fireproof valves or by equally effective 
means, the ventilating air duct down-
stream 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 

(2) Each part of any ventilating duct 

passing through any region having a 
flammable fluid system must be con-
structed or isolated from that system 
so that the malfunctioning of any com-
ponent of that system cannot intro-
duce flammable fluids or vapors into 
the ventilating airstream. 

(c) 

Combustion air ducts. Each com-

bustion air duct must be fireproof for a 
distance great enough to prevent dam-
age from backfiring or reverse flame 
propagation. In addition— 

(1) No combustion air duct may have 

a common opening with the ventilating 
airstream unless flames from backfires 
or reverse burning cannot enter the 
ventilating airstream under any oper-
ating condition, including reverse flow 
or malfunctioning of the heater or its 
associated components; and 

(2) No combustion air duct may re-

strict the prompt relief of any backfire 
that, if so restricted, could cause heat-
er failure. 

(d) 

Heater controls: general. Provision 

must be made to prevent the hazardous 
accumulation of water or ice on or in 
any heater control component, control 
system tubing, or safety control. 

(e) 

Heater safety controls. (1) Each 

combustion heater must have the fol-
lowing safety controls: 

(i) Means independent of the compo-

nents for the normal continuous con-
trol of air temperature, airflow, and 
fuel flow must be provided to auto-
matically shut off the ignition and fuel 
supply to that heater at a point remote 
from that heater when any of the fol-
lowing occurs: 

(A) The heater exchanger tempera-

ture exceeds safe limits. 

(B) The ventilating air temperature 

exceeds safe limits. 

(C) The combustion airflow becomes 

inadequate for safe operation. 

(D) The ventilating airflow becomes 

inadequate for safe operation. 

(ii) 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)(i) of this section. 

(2) The means for complying with 

paragraph (e)(1)(i) of this section for 
any individual heater must— 

(i) Be independent of components 

serving any other heater whose heat 
output is essential for safe operations; 
and 

(ii) Keep the heater off until re-

started by the crew. 

(f) 

Air intakes. Each combustion and 

ventilating air intake must be located 
so that no flammable fluids or vapors 
can enter the heater system under any 
operating condition— 

(1) During normal operation; or 
(2) As a result of the malfunctioning 

of any other component. 

(g) 

Heater exhaust. Heater exhaust 

systems must meet the provisions of 
§§ 23.1121 and 23.1123. In addition, there 
must be provisions in the design of the 
heater exhaust system to safely expel 
the products of combustion to prevent 
the occurrence of— 

(1) Fuel leakage from the exhaust to 

surrounding compartments; 

(2) Exhaust gas impingement on sur-

rounding equipment or structure; 

(3) Ignition of flammable fluids by 

the exhaust, if the exhaust is in a com-
partment containing flammable fluid 
lines; and 

(4) Restrictions in the exhaust sys-

tem to relieve backfires that, if so re-
stricted, could cause heater failure. 

(h) 

Heater fuel systems. Each heater 

fuel system must meet each power-
plant fuel system requirement affect-
ing safe heater operation. Each heater 
fuel system component within the ven-
tilating airstream must be protected 
by shrouds so that no leakage from 
those components can enter the ven-
tilating airstream. 

(i) 

Drains.  There must be means to 

safely drain fuel that might accumu-
late within the combustion chamber or 
the heater exchanger. In addition— 

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