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

c

at sea level or 0.85V

c

 

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] 

L

IGHTNING

P

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. 

(c) For nonmetallic components, 

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 

G

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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