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313 

Federal Aviation Administration, DOT 

§ 25.941 

idle thrust. In addition, it must be 
shown by analysis or test, or both, 
that— 

(i) Each operable reverser can be re-

stored to the forward thrust position; 
and 

(ii) The airplane is capable of contin-

ued safe flight and landing under any 
possible position of the thrust reverser. 

(2) Each system intended for inflight 

use must be designed so that no unsafe 
condition will result during normal op-
eration of the system, or from any fail-
ure (or reasonably likely combination 
of failures) of the reversing system, 
under any anticipated condition of op-
eration of the airplane including 
ground operation. Failure of structural 
elements need not be considered if the 
probability of this kind of failure is ex-
tremely remote. 

(3) Each system must have means to 

prevent the engine from producing 
more than idle thrust when the revers-
ing system malfunctions, except that it 
may produce any greater forward 
thrust that is shown to allow direc-
tional control to be maintained, with 
aerodynamic means alone, under the 
most critical reversing condition ex-
pected in operation. 

(b) For propeller reversing systems— 
(1) Each system intended for ground 

operation only must be designed so 
that no single failure (or reasonably 
likely combination of failures) or mal-
function of the system will result in 
unwanted reverse thrust under any ex-
pected operating condition. Failure of 
structural elements need not be consid-
ered if this kind of failure is extremely 
remote. 

(2) Compliance with this section may 

be shown by failure analysis or testing, 
or both, for propeller systems that 
allow propeller blades to move from 
the flight low-pitch position to a posi-
tion that is substantially less than 
that at the normal flight low-pitch po-
sition. The analysis may include or be 
supported by the analysis made to 
show compliance with the require-
ments of § 35.21 of this chapter for the 
propeller and associated installation 
components. 

[Amdt. 25–72, 55 FR 29784, July 20, 1990] 

§ 25.934

Turbojet engine thrust re-

verser system tests. 

Thrust reversers installed on tur-

bojet engines must meet the require-
ments of § 33.97 of this chapter. 

[Amdt. 25–23, 35 FR 5677, Apr. 8, 1970] 

§ 25.937

Turbopropeller-drag limiting 

systems. 

Turbopropeller power airplane pro-

peller-drag limiting systems must be 
designed so that no single failure or 
malfunction of any of the systems dur-
ing normal or emergency operation re-
sults in propeller drag in excess of that 
for which the airplane was designed 
under § 25.367. Failure of structural ele-
ments of the drag limiting systems 
need not be considered if the prob-
ability of this kind of failure is ex-
tremely remote. 

§ 25.939

Turbine engine operating 

characteristics. 

(a) Turbine engine operating charac-

teristics must be investigated in flight 
to determine that no adverse charac-
teristics (such as stall, surge, or flame-
out) are present, to a hazardous degree, 
during normal and emergency oper-
ation within the range of operating 
limitations of the airplane and of the 
engine. 

(b) [Reserved] 
(c) The turbine engine air inlet sys-

tem may not, as a result of air flow dis-
tortion during normal operation, cause 
vibration harmful to the engine. 

[Amdt. 25–11, 32 FR 6912, May 5, 1967, as 
amended by Amdt. 25–40, 42 FR 15043, Mar. 17, 
1977] 

§ 25.941

Inlet, engine, and exhaust 

compatibility. 

For airplanes using variable inlet or 

exhaust system geometry, or both— 

(a) The system comprised of the 

inlet, engine (including thrust aug-
mentation systems, if incorporated), 
and exhaust must be shown to function 
properly under all operating conditions 
for which approval is sought, including 
all engine rotating speeds and power 
settings, and engine inlet and exhaust 
configurations; 

(b) The dynamic effects of the oper-

ation of these (including consideration 

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314 

14 CFR Ch. I (1–1–24 Edition) 

§ 25.943 

of probable malfunctions) upon the aer-
odynamic control of the airplane may 
not result in any condition that would 
require exceptional skill, alertness, or 
strength on the part of the pilot to 
avoid exceeding an operational or 
structural limitation of the airplane; 
and 

(c) In showing compliance with para-

graph (b) of this section, the pilot 
strength required may not exceed the 
limits set forth in § 25.143(d), subject to 
the conditions set forth in paragraphs 
(e) and (f) of § 25.143. 

[Amdt. 25–38, 41 FR 55467, Dec. 20, 1976, as 
amended by Amdt. 25–121, 72 FR 44669, Aug. 8, 
2007] 

§ 25.943

Negative acceleration. 

No hazardous malfunction of an en-

gine, an auxiliary power unit approved 
for use in flight, or any component or 
system associated with the powerplant 
or auxiliary power unit may occur 
when the airplane is operated at the 
negative accelerations within the 
flight envelopes prescribed in § 25.333. 
This must be shown for the greatest 
duration expected for the acceleration. 

[Amdt. 25–40, 42 FR 15043, Mar. 17, 1977] 

§ 25.945

Thrust or power augmentation 

system. 

(a) 

General.  Each fluid injection sys-

tem must provide a flow of fluid at the 
rate and pressure established for proper 
engine functioning under each intended 
operating condition. If the fluid can 
freeze, fluid freezing may not damage 
the airplane or adversely affect air-
plane performance. 

(b) 

Fluid tanks. Each augmentation 

system fluid tank must meet the fol-
lowing requirements: 

(1) Each tank must be able to with-

stand without failure the vibration, in-
ertia, fluid, and structural loads that it 
may be subject to in operation. 

(2) The tanks as mounted in the air-

plane must be able to withstand with-
out failure or leakage an internal pres-
sure 1.5 times the maximum operating 
pressure. 

(3) If a vent is provided, the venting 

must be effective under all normal 
flight conditions. 

(4) [Reserved] 

(5) Each tank must have an expan-

sion space of not less than 2 percent of 
the tank capacity. It must be impos-
sible to fill the expansion space inad-
vertently with the airplane in the nor-
mal ground attitude. 

(c) Augmentation system drains 

must be designed and located in ac-
cordance with § 25.1455 if— 

(1) The augmentation system fluid is 

subject to freezing; and 

(2) The fluid may be drained in flight 

or during ground operation. 

(d) The augmentation liquid tank ca-

pacity available for the use of each en-
gine must be large enough to allow op-
eration of the airplane under the ap-
proved procedures for the use of liquid- 
augmented power. The computation of 
liquid consumption must be based on 
the maximum approved rate appro-
priate for the desired engine output 
and must include the effect of tempera-
ture on engine performance as well as 
any other factors that might vary the 
amount of liquid required. 

(e) This section does not apply to fuel 

injection systems. 

[Amdt. 25–40, 42 FR 15043, Mar. 17, 1977, as 
amended by Amdt. 25–72, 55 FR 29785, July 20, 
1990; Amdt. 25–115, 69 FR 40527, July 2, 2004] 

F

UEL

S

YSTEM

 

§ 25.951

General. 

(a) Each fuel system must be con-

structed and arranged to ensure a flow 
of fuel at a rate and pressure estab-
lished for proper engine and auxiliary 
power unit functioning under each 
likely operating condition, including 
any maneuver for which certification is 
requested and during which the engine 
or auxiliary power unit is permitted to 
be in operation. 

(b) Each fuel system must be ar-

ranged so that any air which is intro-
duced into the system will not result 
in— 

(1) Power interruption for more than 

20 seconds for reciprocating engines; or 

(2) Flameout for turbine engines. 
(c) Each fuel system for a turbine en-

gine must be capable of sustained oper-
ation throughout its flow and pressure 
range with fuel initially saturated with 
water at 80 

°

F and having 0.75cc of free 

water per gallon added and cooled to 

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