title-14-part-25-sec25-601

262

14 CFR Ch. I (1–1–24 Edition)

§ 25.581

external aerodynamic pressures applied
simultaneously with the flight loading
conditions specified in paragraphs
(b)(1) through (4) of this section, if they
have a significant effect.

(ii) The maximum value of normal

operating differential pressure (includ-
ing the expected external aerodynamic
pressures during 1 g level flight) multi-
plied by a factor of 1.15, omitting other
loads.

(6) For landing gear and directly-af-

fected airframe structure, the limit
ground loading conditions specified in
§§ 25.473, 25.491, and 25.493.

If significant changes in structural
stiffness or geometry, or both, follow
from a structural failure, or partial
failure, the effect on damage tolerance
must be further investigated.

(c)

Fatigue (safe-life) evaluation. Com-

pliance with the damage-tolerance re-
quirements of paragraph (b) of this sec-
tion is not required if the applicant es-
tablishes that their application for par-
ticular structure is impractical. This
structure must be shown by analysis,
supported by test evidence, to be able
to withstand the repeated loads of vari-
able magnitude expected during its
service life without detectable cracks.
Appropriate safe-life scatter factors
must be applied.

(d)

Sonic fatigue strength. It must be

shown by analysis, supported by test
evidence, or by the service history of
airplanes of similar structural design
and sonic excitation environment,
that—

(1) Sonic fatigue cracks are not prob-

able in any part of the flight structure
subject to sonic excitation; or

(2) Catastrophic failure caused by

sonic cracks is not probable assuming
that the loads prescribed in paragraph
(b) of this section are applied to all
areas affected by those cracks.

(e)

Damage-tolerance (discrete source)

evaluation. The airplane must be capa-
ble of successfully completing a flight
during which likely structural damage
occurs as a result of—

(1) Impact with a 4-pound bird when

the velocity of the airplane relative to
the bird along the airplane’s flight
path is equal to V

at sea level or 0.85V

at 8,000 feet, whichever is more critical;

(2) Uncontained fan blade impact;
(3) Uncontained engine failure; or

(4) Uncontained high energy rotating

machinery failure.

The damaged structure must be able to
withstand the static loads (considered
as ultimate loads) which are reason-
ably expected to occur on the flight.
Dynamic effects on these static loads
need not be considered. Corrective ac-
tion to be taken by the pilot following
the incident, such as limiting maneu-
vers, avoiding turbulence, and reducing
speed, must be considered. If signifi-
cant changes in structural stiffness or
geometry, or both, follow from a struc-
tural failure or partial failure, the ef-
fect on damage tolerance must be fur-
ther investigated.

[Amdt. 25–45, 43 FR 46242, Oct. 5, 1978, as
amended by Amdt. 25–54, 45 FR 60173, Sept.
11, 1980; Amdt. 25–72, 55 FR 29776, July 20,
1990; Amdt. 25–86, 61 FR 5222, Feb. 9, 1996;
Amdt. 25–96, 63 FR 15714, Mar. 31, 1998; 63 FR
23338, Apr. 28, 1998; Amdt. 25–132, 75 FR 69781,
Nov. 15, 2010; Amdt. No. 25–148, 87 FR 75710,
Dec. 9, 2022; 88 FR 2813, Jan. 18, 2023]

IGHTNING

ROTECTION

§ 25.581

Lightning protection.

(a) The airplane must be protected

against catastrophic effects from light-
ning.

(b) For metallic components, compli-

ance with paragraph (a) of this section
may be shown by—

(1) Bonding the components properly

to the airframe; or

(2) Designing the components so that

a strike will not endanger the airplane.

compliance with paragraph (a) of this
section may be shown by—

(1) Designing the components to min-

imize the effect of a strike; or

(2) Incorporating acceptable means of

diverting the resulting electrical cur-
rent so as not to endanger the airplane.

[Amdt. 25–23, 35 FR 5674, Apr. 8, 1970]

Subpart D—Design and

Construction

ENERAL

§ 25.601

General.

The airplane may not have design

features or details that experience has
shown to be hazardous or unreliable.
The suitability of each questionable

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§ 25.613

design detail and part must be estab-
lished by tests.

§ 25.603

Materials.

The suitability and durability of ma-

terials used for parts, the failure of
which could adversely affect safety,
must—

(a) Be established on the basis of ex-

perience or tests;

(b) Conform to approved specifica-

tions (such as industry or military
specifications, or Technical Standard
Orders) that ensure their having the
strength and other properties assumed
in the design data; and

environmental conditions, such as tem-
perature and humidity, expected in
service.

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25–38, 41 FR 55466, Dec. 20,
1976; Amdt. 25–46, 43 FR 50595, Oct. 30, 1978]

§ 25.605

Fabrication methods.

(a) The methods of fabrication used

must produce a consistently sound
structure. If a fabrication process (such
as gluing, spot welding, or heat treat-
ing) requires close control to reach this
objective, the process must be per-
formed under an approved process spec-
ification.

(b) Each new aircraft fabrication

method must be substantiated by a
test program.

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25–46, 43 FR 50595, Oct. 30,
1978]

§ 25.607

Fasteners.

(a) Each removable bolt, screw, nut,

pin, or other removable fastener must
incorporate two separate locking de-
vices if—

(1) Its loss could preclude continued

flight and landing within the design
limitations of the airplane using nor-
mal pilot skill and strength; or

(2) Its loss could result in reduction

in pitch, yaw, or roll control capability
or response below that required by
Subpart B of this chapter.

(b) The fasteners specified in para-

graph (a) of this section and their lock-
ing devices may not be adversely af-
fected by the environmental conditions
associated with the particular installa-
tion.

on any bolt subject to rotation in oper-
ation unless a nonfriction locking de-
vice is used in addition to the self-lock-
ing device.

[Amdt. 25–23, 35 FR 5674, Apr. 8, 1970]

§ 25.609

Protection of structure.

Each part of the structure must—
(a) Be suitably protected against de-

terioration or loss of strength in serv-
ice due to any cause, including—

(1) Weathering;
(2) Corrosion; and
(3) Abrasion; and
(b) Have provisions for ventilation

and drainage where necessary for pro-
tection.

§ 25.611

Accessibility provisions.

(a)Means must be provided to allow

inspection (including inspection of
principal structural elements and con-
trol systems), replacement of parts
normally requiring replacement, ad-
justment, and lubrication as necessary
for continued airworthiness. The in-
spection means for each item must be
practicable for the inspection interval
for the item. Nondestructive inspection
aids may be used to inspect structural
elements where it is impracticable to
provide means for direct visual inspec-
tion if it is shown that the inspection
is effective and the inspection proce-
dures are specified in the maintenance
manual required by § 25.1529.

(b) EWIS must meet the accessibility

requirements of § 25.1719.

[Amdt. 25–23, 35 FR 5674, Apr. 8, 1970, as
amended by Amdt. 25–123, 72 FR 63404, Nov. 8,
2007]

§ 25.613

Material strength properties

and material design values.

(a) Material strength properties must

be based on enough tests of material
meeting approved specifications to es-
tablish design values on a statistical
basis.

(b) Material design values must be

chosen to minimize the probability of