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14 CFR Ch. I (1–1–14 Edition) 

§ 25.859 

provisions is requested, the following 
must be met for each cargo or baggage 
compartment with those provisions: 

(a) The detection system must pro-

vide a visual indication to the flight 
crew within one minute after the start 
of a fire. 

(b) The system must be capable of de-

tecting a fire at a temperature signifi-
cantly below that at which the struc-
tural integrity of the airplane is sub-
stantially decreased. 

(c) There must be means to allow the 

crew to check in flight, the functioning 
of each fire detector circuit. 

(d) The effectiveness of the detection 

system must be shown for all approved 
operating configurations and condi-
tions. 

[Amdt. 25–54, 45 FR 60173, Sept. 11, 1980, as 
amended by Amdt. 25–93, 63 FR 8048, Feb. 17, 
1998] 

§ 25.859

Combustion heater fire pro-

tection. 

(a) 

Combustion heater fire zones. The 

following combustion heater fire zones 
must be protected from fire in accord-
ance with the applicable provisions of 
§§ 25.1181 through 25.1191 and §§ 25.1195 
through 25.1203; 

(1) The region surrounding the heat-

er, if this region contains any flam-
mable fluid system components (ex-
cluding the heater fuel system), that 
could— 

(i) Be damaged by heater malfunc-

tioning; or 

(ii) Allow flammable fluids or vapors 

to reach the heater in case of leakage. 

(2) The region surrounding the heat-

er, if the heater fuel system has fit-
tings that, if they leaked, would allow 
fuel or vapors to enter this region. 

(3) The part of the ventilating air 

passage that surrounds the combustion 
chamber. However, no fire extinguish-
ment is required in cabin ventilating 
air passages. 

(b) 

Ventilating air ducts. Each ven-

tilating air duct passing through any 
fire zone 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. For each 

combustion heater there must be the 
following safety control means: 

(1) Means independent of the compo-

nents provided for the normal contin-
uous control of air temperature, air-
flow, and fuel flow must be provided, 
for each heater, to automatically shut 
off the ignition and fuel supply to that 
heater at a point remote from that 
heater when any of the following oc-
curs: 

(i) The heat exchanger temperature 

exceeds safe limits. 

(ii) The ventilating air temperature 

exceeds safe limits. 

(iii) The combustion airflow becomes 

inadequate for safe operation. 

(iv) The ventilating airflow becomes 

inadequate for safe operation. 

(2) The means of complying with 

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

(i) Be independent of components 

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

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