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Federal Aviation Administration, DOT 

§ 25.1125 

(b) No screen may be in any part of 

the induction system that is the only 
passage through which air can reach 
the engine, unless it can be deiced by 
heated air; 

(c) No screen may be deiced by alco-

hol alone; and 

(d) It must be impossible for fuel to 

strike any screen. 

§ 25.1107

Inter-coolers and after-cool-


Each inter-cooler and after-cooler 

must be able to withstand any vibra-
tion, inertia, and air pressure load to 
which it would be subjected in oper-






§ 25.1121


For powerplant and auxiliary power 

unit installations the following apply: 

(a) Each exhaust system must ensure 

safe disposal of exhaust gases without 
fire hazard or carbon monoxide con-
tamination in any personnel compart-
ment. For test purposes, any accept-
able carbon monoxide detection meth-
od may be used to show the absence of 
carbon monoxide. 

(b) Each exhaust system part with a 

surface hot enough to ignite flammable 
fluids or vapors must be located or 
shielded so that leakage from any sys-
tem carrying flammable fluids or va-
pors will not result in a fire caused by 
impingement of the fluids or vapors on 
any part of the exhaust system includ-
ing shields for the exhaust system. 

(c) Each component that hot exhaust 

gases could strike, or that could be 
subjected to high temperatures from 
exhaust system parts, must be fire-
proof. All exhaust system components 
must be separated by fireproof shields 
from adjacent parts of the airplane 
that are outside the engine and auxil-
iary power unit compartments. 

(d) No exhaust gases may discharge 

so as to cause a fire hazard with re-
spect to any flammable fluid vent or 

(e) No exhaust gases may discharge 

where they will cause a glare seriously 
affecting pilot vision at night. 

(f) Each exhaust system component 

must be ventilated to prevent points of 
excessively high temperature. 

(g) Each exhaust shroud must be ven-

tilated or insulated to avoid, during 
normal operation, a temperature high 
enough to ignite any flammable fluids 
or vapors external to the shroud. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–40, 42 FR 15043, Mar. 17, 

§ 25.1123

Exhaust piping. 

For powerplant and auxiliary power 

unit installations, the following apply: 

(a) Exhaust piping must be heat and 

corrosion resistant, and must have pro-
visions to prevent failure due to expan-
sion by operating temperatures. 

(b) Piping must be supported to with-

stand any vibration and inertia loads 
to which it would be subjected in oper-
ation; and 

(c) Piping connected to components 

between which relative motion could 
exist must have means for flexibility. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–40, 42 FR 15044, Mar. 17, 

§ 25.1125

Exhaust heat exchangers. 

For reciprocating engine powered 

airplanes, the following apply: 

(a) Each exhaust heat exchanger 

must be constructed and installed to 
withstand each vibration, inertia, and 
other load to which it would be sub-
jected in operation. In addition— 

(1) Each exchanger must be suitable 

for continued operation at high tem-
peratures and resistant to corrosion 
from exhaust gases; 

(2) There must be means for the in-

spection of the critical parts of each 

(3) Each exchanger must have cooling 

provisions wherever it is subject to 
contact with exhaust gases; and 

(4) No exhaust heat exchanger or 

muff may have any stagnant areas or 
liquid traps that would increase the 
probability of ignition of flammable 
fluids or vapors that might be present 
in case of the failure or malfunction of 
components carrying flammable fluids. 

(b) If an exhaust heat exchanger is 

used for heating ventilating air— 

(1) There must be a secondary heat 

exchanger between the primary ex-
haust gas heat exchanger and the ven-
tilating air system; or 

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