title-14-part-25-sec25-933

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.

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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§ 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