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Federal Aviation Administration, DOT 

§ 25.1093 

(2) The stage of flight is completed; 


(3) An operating limitation is 


(d) For reciprocating engine powered 

airplanes, it may be assumed, for cool-
ing test purposes, that the takeoff 
stage of flight is complete when the 
airplane reaches an altitude of 1,500 
feet above the takeoff surface or 
reaches a point in the takeoff where 
the transition from the takeoff to the 
en route configuration is completed 
and a speed is reached at which compli-
ance with § 25.121(c) is shown, which-
ever point is at a higher altitude. The 
airplane must be in the following con-

(1) Landing gear retracted. 
(2) Wing flaps in the most favorable 


(3) Cowl flaps (or other means of con-

trolling the engine cooling supply) in 
the position that provides adequate 
cooling in the hot-day condition. 

(4) Critical engine inoperative and its 

propeller stopped. 

(5) Remaining engines at the max-

imum continuous power available for 
the altitude. 

(e) For hull seaplanes and amphib-

ians, cooling must be shown during 
taxiing downwind for 10 minutes, at 
five knots above step speed. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–57, 49 FR 6848, Feb. 23, 






§ 25.1091

Air induction. 

(a) The air induction system for each 

engine and auxiliary power unit must 

(1) The air required by that engine 

and auxiliary power unit under each 
operating condition for which certifi-
cation is requested; and 

(2) The air for proper fuel metering 

and mixture distribution with the in-
duction system valves in any position. 

(b) Each reciprocating engine must 

have an alternate air source that pre-
vents the entry of rain, ice, or any 
other foreign matter. 

(c) Air intakes may not open within 

the cowling, unless— 

(1) That part of the cowling is iso-

lated from the engine accessory section 
by means of a fireproof diaphragm; or 

(2) For reciprocating engines, there 

are means to prevent the emergence of 
backfire flames. 

(d) For turbine engine powered air-

planes and airplanes incorporating aux-
iliary power units— 

(1) There must be means to prevent 

hazardous quantities of fuel leakage or 
overflow from drains, vents, or other 
components of flammable fluid systems 
from entering the engine or auxiliary 
power unit intake system; and 

(2) The airplane must be designed to 

prevent water or slush on the runway, 
taxiway, or other airport operating 
surfaces from being directed into the 
engine or auxiliary power unit air inlet 
ducts in hazardous quantities, and the 
air inlet ducts must be located or pro-
tected so as to minimize the ingestion 
of foreign matter during takeoff, land-
ing, and taxiing. 

(e) If the engine induction system 

contains parts or components that 
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-


(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; 

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