title-14-part-25-sec25-907

311

Federal Aviation Administration, DOT

§ 25.907

(c)

Control of engine rotation. There

must be means for stopping the rota-
tion of any engine individually in
flight, except that, for turbine engine
installations, the means for stopping
the rotation of any engine need be pro-
vided only where continued rotation
could jeopardize the safety of the air-
plane. Each component of the stopping
system on the engine side of the fire-
wall that might be exposed to fire must
be at least fire-resistant. If hydraulic
propeller feathering systems are used
for this purpose, the feathering lines
must be at least fire resistant under
the operating conditions that may be
expected to exist during feathering.

(d)

Turbine engine installations. For

turbine engine installations—

(1) Design precautions must be taken

to minimize the hazards to the airplane
in the event of an engine rotor failure
or of a fire originating within the en-
gine which burns through the engine
case.

(2) The powerplant systems associ-

ated with engine control devices, sys-
tems, and instrumentation, must be de-
signed to give reasonable assurance
that those engine operating limitations
that adversely affect turbine rotor
structural integrity will not be exceed-
ed in service.

(e)

Restart capability. (1) Means to re-

start any engine in flight must be pro-
vided.

(2) An altitude and airspeed envelope

must be established for in-flight engine
restarting, and each engine must have
a restart capability within that enve-
lope.

(3) For turbine engine powered air-

planes, if the minimum windmilling
speed of the engines, following the
inflight shutdown of all engines, is in-
sufficient to provide the necessary
electrical power for engine ignition, a
power source independent of the en-
gine-driven electrical power generating
system must be provided to permit in-
flight engine ignition for restarting.

(f)

Auxiliary Power Unit. Each auxil-

iary power unit must be approved or

meet the requirements of the category
for its intended use.

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25–23, 35 FR 5676, Apr. 8,
1970; Amdt. 25–40, 42 FR 15042, Mar. 17, 1977;
Amdt. 25–57, 49 FR 6848, Feb. 23, 1984; Amdt.
25–72, 55 FR 29784, July 20, 1990; Amdt. 25–73,
55 FR 32861, Aug. 10, 1990; Amdt. 25–94, 63 FR
8848, Feb. 23, 1998; Amdt. 25–95, 63 FR 14798,
Mar. 26, 1998; Amdt. 25–100, 65 FR 55854, Sept.
14, 2000; Amdt. 25–140, 79 FR 65525, Nov. 4,
2014; Amdt. No. 25–148, 87 FR 75710, Dec. 9,
2022; 88 FR 2813, Jan. 18, 2023]

§ 25.904

Automatic takeoff thrust con-

trol system (ATTCS).

Each applicant seeking approval for

installation of an engine power control
system that automatically resets the
power or thrust on the operating en-
gine(s) when any engine fails during
the takeoff must comply with the re-
quirements of appendix I of this part.

[Amdt. 25–62, 52 FR 43156, Nov. 9, 1987]

§ 25.905

Propellers.

(a) Each propeller must have a type

certificate.

(b) Engine power and propeller shaft

rotational speed may not exceed the
limits for which the propeller is certifi-
cated.

system must meet the requirements of
§§ 35.21, 35.23, 35.42 and 35.43 of this
chapter.

(d) Design precautions must be taken

to minimize the hazards to the airplane
in the event a propeller blade fails or is
released by a hub failure. The hazards
which must be considered include dam-
age to structure and vital systems due
to impact of a failed or released blade
and the unbalance created by such fail-
ure or release.

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25–54, 45 FR 60173, Sept.
11, 1980; Amdt. 25–57, 49 FR 6848, Feb. 23, 1984;
Amdt. 25–72, 55 FR 29784, July 20, 1990; Amdt.
25–126, 73 FR 63345, Oct. 24, 2008]

§ 25.907

Propeller vibration and fa-

tigue.

This section does not apply to fixed-

pitch wood propellers of conventional
design.

(a) The applicant must determine the

magnitude of the propeller vibration
stresses or loads, including any stress

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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