471 

Federal Aviation Administration, DOT 

§ 25.933 

the most adverse center of gravity, and 
with the propeller in the most adverse 
pitch position, may not be less than 
the following: 

(a) 

Ground clearance. There must be a 

clearance of at least seven inches (for 
each airplane with nose wheel landing 
gear) or nine inches (for each airplane 
with tail wheel landing gear) between 
each propeller and the ground with the 
landing gear statically deflected and in 
the level takeoff, or taxiing attitude, 
whichever is most critical. In addition, 
there must be positive clearance be-
tween the propeller and the ground 
when in the level takeoff attitude with 
the critical tire(s) completely deflated 
and the corresponding landing gear 
strut bottomed. 

(b) 

Water clearance. There must be a 

clearance of at least 18 inches between 
each propeller and the water, unless 
compliance with § 25.239(a) can be 
shown with a lesser clearance. 

(c) 

Structural clearance. There must 

be— 

(1) At least one inch radial clearance 

between the blade tips and the airplane 
structure, plus any additional radial 
clearance necessary to prevent harmful 
vibration; 

(2) At least one-half inch longitudinal 

clearance between the propeller blades 
or cuffs and stationary parts of the air-
plane; and 

(3) Positive clearance between other 

rotating parts of the propeller or spin-
ner and stationary parts of the air-
plane. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–72, 55 FR 29784, July 20, 
1990] 

§ 25.929

Propeller deicing. 

(a) For airplanes intended for use 

where icing may be expected, there 
must be a means to prevent or remove 
hazardous ice accumulation on propel-
lers or on accessories where ice accu-
mulation would jeopardize engine per-
formance. 

(b) If combustible fluid is used for 

propeller deicing, §§ 25.1181 through 
25.1185 and 25.1189 apply. 

§ 25.933

Reversing systems. 

(a) For turbojet reversing systems— 
(1) Each system intended for ground 

operation only must be designed so 

that during any reversal in flight the 
engine will produce no more than flight 
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] 

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