312 

14 CFR Ch. I (1–1–24 Edition) 

§ 25.925 

peaks and resonant conditions, 
throughout the operational envelope of 
the airplane by either: 

(1) Measurement of stresses or loads 

through direct testing or analysis 
based on direct testing of the propeller 
on the airplane and engine installation 
for which approval is sought; or 

(2) Comparison of the propeller to 

similar propellers installed on similar 
airplane installations for which these 
measurements have been made. 

(b) The applicant must demonstrate 

by tests, analysis based on tests, or 
previous experience on similar designs 
that the propeller does not experience 
harmful effects of flutter throughout 
the operational envelope of the air-
plane. 

(c) The applicant must perform an 

evaluation of the propeller to show 
that failure due to fatigue will be 
avoided throughout the operational life 
of the propeller using the fatigue and 
structural data obtained in accordance 
with part 35 of this chapter and the vi-
bration data obtained from compliance 
with paragraph (a) of this section. For 
the purpose of this paragraph, the pro-
peller includes the hub, blades, blade 
retention component and any other 
propeller component whose failure due 
to fatigue could be catastrophic to the 
airplane. This evaluation must include: 

(1) The intended loading spectra in-

cluding all reasonably foreseeable pro-
peller vibration and cyclic load pat-
terns, identified emergency conditions, 
allowable overspeeds and overtorques, 
and the effects of temperatures and hu-
midity expected in service. 

(2) The effects of airplane and pro-

peller operating and airworthiness lim-
itations. 

[Amdt. 25–126, 73 FR 63345, Oct. 24, 2008] 

§ 25.925

Propeller clearance. 

Unless smaller clearances are sub-

stantiated, propeller clearances with 
the airplane at maximum weight, with 
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) If certification for flight in icing 

is sought there must be a means to pre-
vent or remove hazardous ice accumu-
lations that could form in the icing 
conditions defined in Appendix C of 
this part and in the portions of Appen-
dix O of this part for which the air-
plane is approved for flight 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. 

[ Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–140, 79 FR 65525, Nov. 4, 
2014] 

§ 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 

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