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328 

14 CFR Ch. I (1–1–24 Edition) 

§ 25.1093 

could be damaged by foreign objects 
entering the air inlet, it must be shown 
by tests or, if appropriate, by analysis 
that the induction system design can 
withstand the foreign object ingestion 
test conditions of §§ 33.76, 33.77 and 
33.78(a)(1) of this chapter without fail-
ure of parts or components that could 
create a hazard. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–38, 41 FR 55467, Dec. 20, 
1976; Amdt. 25–40, 42 FR 15043, Mar. 17, 1977; 
Amdt. 25–57, 49 FR 6849, Feb. 23, 1984; Amdt. 
25–100, 65 FR 55854, Sept. 14, 2000] 

§ 25.1093

Induction system icing pro-

tection. 

(a) 

Reciprocating engines. Each recip-

rocating engine air induction system 
must have means to prevent and elimi-
nate icing. Unless this is done by other 
means, it must be shown that, in air 
free of visible moisture at a tempera-
ture of 30 F., each airplane with alti-
tude engines using— 

(1) Conventional venturi carburetors 

have a preheater that can provide a 
heat rise of 120 F. with the engine at 60 
percent of maximum continuous power; 
or 

(2) Carburetors tending to reduce the 

probability of ice formation has a pre-
heater that can provide a heat rise of 
100 

°

F. with the engine at 60 percent of 

maximum continuous power. 

(b) 

Turbine engines. Except as pro-

vided in paragraph (b)(3) of this sec-
tion, each engine, with all icing protec-
tion systems operating, must: 

(1) Operate throughout its flight 

power range, including the minimum 
descent idling speeds, in the icing con-
ditions defined in Appendices C and O 
of this part, and Appendix D of part 33 
of this chapter, and in falling and blow-
ing snow within the limitations estab-
lished for the airplane for such oper-
ation, without the accumulation of ice 
on the engine, inlet system compo-

nents, or airframe components that 
would do any of the following: 

(i) Adversely affect installed engine 

operation or cause a sustained loss of 
power or thrust; or an unacceptable in-
crease in gas path operating tempera-
ture; or an airframe/engine incompati-
bility; or 

(ii) Result in unacceptable temporary 

power loss or engine damage; or 

(iii) Cause a stall, surge, or flameout 

or loss of engine controllability (for ex-
ample, rollback). 

(2) Operate at ground idle speed for a 

minimum of 30 minutes on the ground 
in the following icing conditions shown 
in Table 1 of this section, unless re-
placed by similar test conditions that 
are more critical. These conditions 
must be demonstrated with the avail-
able air bleed for icing protection at its 
critical condition, without adverse ef-
fect, followed by an acceleration to 
takeoff power or thrust in accordance 
with the procedures defined in the air-
plane flight manual. During the idle 
operation, the engine may be run up 
periodically to a moderate power or 
thrust setting in a manner acceptable 
to the Administrator. Analysis may be 
used to show ambient temperatures 
below the tested temperature are less 
critical. The applicant must document 
the engine run-up procedure (including 
the maximum time interval between 
run-ups from idle, run-up power set-
ting, and duration at power), the asso-
ciated minimum ambient temperature, 
and the maximum time interval. These 
conditions must be used in the analysis 
that establishes the airplane operating 
limitations in accordance with § 25.1521. 

(3) For the purposes of this section, 

the icing conditions defined in appen-
dix O of this part, including the condi-
tions specified in Condition 3 of Table 1 
of this section, are not applicable to 
airplanes with a maximum takeoff 
weight equal to or greater than 60,000 
pounds. 

T

ABLE

1—I

CING

C

ONDITIONS FOR

G

ROUND

T

ESTS

 

Condition 

Total air temperature 

Water concentration 

(minimum) 

Mean effective par-

ticle diameter 

Demonstration 

1. Rime ice condition

0 to 15 

°

F (18 to 

¥

°

C) Liquid—0.3 

g/m

3

........

15–25 microns ...........

By test, analysis or com-

bination of the two. 

2. Glaze ice condition 

20 to 30 

°

F (

¥

7 to 

¥

°

C).

Liquid—0.3 g/m

3

........

15–25 microns ...........

By test, analysis or com-

bination of the two. 

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329 

Federal Aviation Administration, DOT 

§ 25.1105 

T

ABLE

1—I

CING

C

ONDITIONS FOR

G

ROUND

T

ESTS

—Continued 

Condition 

Total air temperature 

Water concentration 

(minimum) 

Mean effective par-

ticle diameter 

Demonstration 

3. Large drop condi-

tion.

15 to 30 

°

F (

¥

9 to 

¥

°

C).

Liquid—0.3 g/m

3

........

100 microns (min-

imum).

By test, analysis or com-

bination of the two. 

(c) 

Supercharged reciprocating engines. 

For each engine having a supercharger 
to pressurize the air before it enters 
the carburetor, the heat rise in the air 
caused by that supercharging at any 
altitude may be utilized in determining 
compliance with paragraph (a) of this 
section if the heat rise utilized is that 
which will be available, automatically, 
for the applicable altitude and oper-
ating condition because of super-
charging. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–38, 41 FR 55467, Dec. 20, 
1976; Amdt. 25–40, 42 FR 15043, Mar. 17, 1977; 
Amdt. 25–57, 49 FR 6849, Feb. 23, 1984; Amdt. 
25–72, 55 FR 29785, July 20, 1990; Amdt. 25–140, 
79 FR 65526, Nov. 4, 2014] 

§ 25.1101

Carburetor air preheater de-

sign. 

Each carburetor air preheater must 

be designed and constructed to— 

(a) Ensure ventilation of the pre-

heater when the engine is operated in 
cold air; 

(b) Allow inspection of the exhaust 

manifold parts that it surrounds; and 

(c) Allow inspection of critical parts 

of the preheater itself. 

§ 25.1103

Induction system ducts and 

air duct systems. 

(a) Each induction system duct up-

stream of the first stage of the engine 
supercharger and of the auxiliary 
power unit compressor must have a 
drain to prevent the hazardous accu-
mulation of fuel and moisture in the 
ground attitude. No drain may dis-
charge where it might cause a fire haz-
ard. 

(b) Each induction system duct must 

be— 

(1) Strong enough to prevent induc-

tion system failures resulting from 
normal backfire conditions; and 

(2) Fire-resistant if it is in any fire 

zone for which a fire-extinguishing sys-
tem is required, except that ducts for 
auxiliary power units must be fireproof 

within the auxiliary power unit fire 
zone. 

(c) Each duct connected to compo-

nents between which relative motion 
could exist must have means for flexi-
bility. 

(d) For turbine engine and auxiliary 

power unit bleed air duct systems, no 
hazard may result if a duct failure oc-
curs at any point between the air duct 
source and the airplane unit served by 
the air. 

(e) Each auxiliary power unit induc-

tion system duct must be fireproof for 
a sufficient distance upstream of the 
auxiliary power unit compartment to 
prevent hot gas reverse flow from burn-
ing through auxiliary power unit ducts 
and entering any other compartment 
or area of the airplane in which a haz-
ard would be created resulting from the 
entry of hot gases. The materials used 
to form the remainder of the induction 
system duct and plenum chamber of 
the auxiliary power unit must be capa-
ble of resisting the maximum heat con-
ditions likely to occur. 

(f) Each auxiliary power unit induc-

tion system duct must be constructed 
of materials that will not absorb or 
trap hazardous quantities of flammable 
fluids that could be ignited in the 
event of a surge or reverse flow condi-
tion. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–46, 43 FR 50597, Oct. 30, 
1978] 

§ 25.1105

Induction system screens. 

If induction system screens are 

used— 

(a) Each screen must be upstream of 

the carburetor; 

(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 

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