423 

Federal Aviation Administration, DOT 

§ 25.571 

surfaces and their systems, the fuse-
lage, engine mounting, landing gear, 
and their related primary attach-
ments). For turbojet powered air-
planes, those parts that could con-
tribute to a catastrophic failure must 
also be evaluated under paragraph (d) 
of this section. In addition, the fol-
lowing apply: 

(1) Each evaluation required by this 

section must include— 

(i) The typical loading spectra, tem-

peratures, and humidities expected in 
service; 

(ii) The identification of principal 

structural elements and detail design 
points, the failure of which could cause 
catastrophic failure of the airplane; 
and 

(iii) An analysis, supported by test 

evidence, of the principal structural 
elements and detail design points iden-
tified in paragraph (a)(1)(ii) of this sec-
tion. 

(2) The service history of airplanes of 

similar structural design, taking due 
account of differences in operating con-
ditions and procedures, may be used in 
the evaluations required by this sec-
tion. 

(3) Based on the evaluations required 

by this section, inspections or other 
procedures must be established, as nec-
essary, to prevent catastrophic failure, 
and must be included in the Airworthi-
ness Limitations section of the In-
structions for Continued Airworthiness 
required by § 25.1529. The limit of valid-
ity of the engineering data that sup-
ports the structural maintenance pro-
gram (hereafter referred to as LOV), 
stated as a number of total accumu-
lated flight cycles or flight hours or 
both, established by this section must 
also be included in the Airworthiness 
Limitations section of the Instructions 
for Continued Airworthiness required 
by § 25.1529. Inspection thresholds for 
the following types of structure must 
be established based on crack growth 
analyses and/or tests, assuming the 
structure contains an initial flaw of 
the maximum probable size that could 
exist as a result of manufacturing or 
service-induced damage: 

(i) Single load path structure, and 
(ii) Multiple load path ‘‘fail-safe’’ 

structure and crack arrest ‘‘fail-safe’’ 
structure, where it cannot be dem-

onstrated that load path failure, par-
tial failure, or crack arrest will be de-
tected and repaired during normal 
maintenance, inspection, or operation 
of an airplane prior to failure of the re-
maining structure. 

(b) 

Damage-tolerance evaluation. The 

evaluation must include a determina-
tion of the probable locations and 
modes of damage due to fatigue, corro-
sion, or accidental damage. Repeated 
load and static analyses supported by 
test evidence and (if available) service 
experience must also be incorporated 
in the evaluation. Special consider-
ation for widespread fatigue damage 
must be included where the design is 
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 

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