262 

14 CFR Ch. I (1–1–19 Edition) 

§ 25.571 

such that this type of damage could 
occur. An LOV must be established 
that corresponds to the period of time, 
stated as a number of total accumu-
lated flight cycles or flight hours or 
both, during which it is demonstrated 
that widespread fatigue damage will 
not occur in the airplane structure. 
This demonstration must be by full- 
scale fatigue test evidence. The type 
certificate may be issued prior to com-
pletion of full-scale fatigue testing, 
provided the Administrator has ap-
proved a plan for completing the re-
quired tests. In that case, the Air-
worthiness Limitations section of the 
Instructions for Continued Airworthi-
ness required by § 25.1529 must specify 
that no airplane may be operated be-
yond a number of cycles equal to 

1

⁄

2

the 

number of cycles accumulated on the 
fatigue test article, until such testing 
is completed. The extent of damage for 
residual strength evaluation at any 
time within the operational life of the 
airplane must be consistent with the 
initial detectability and subsequent 
growth under repeated loads. The resid-
ual strength evaluation must show 
that the remaining structure is able to 
withstand loads (considered as static 
ultimate loads) corresponding to the 
following conditions: 

(1) The limit symmetrical maneu-

vering conditions specified in § 25.337 at 
all speeds up to V

c

and in § 25.345. 

(2) The limit gust conditions speci-

fied in § 25.341 at the specified speeds up 
to V

C

and in § 25.345. 

(3) The limit rolling conditions speci-

fied in § 25.349 and the limit unsymmet-
rical conditions specified in §§ 25.367 
and 25.427 (a) through (c), at speeds up 
to V

C

. 

(4) The limit yaw maneuvering condi-

tions specified in § 25.351(a) at the spec-
ified speeds up to V

C

. 

(5) For pressurized cabins, the fol-

lowing conditions: 

(i) The normal operating differential 

pressure combined with the expected 
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 

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