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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