261 

Federal Aviation Administration, DOT 

§ 25.571 

(t) is in seconds, and (a) is in units of gravity 

(g). 

(6) Where leg injuries may result 

from contact with seats or other struc-
ture, protection must be provided to 
prevent axially compressive loads ex-
ceeding 2,250 pounds in each femur. 

(7) The seat must remain attached at 

all points of attachment, although the 
structure may have yielded. 

(8) Seats must not yield under the 

tests specified in paragraphs (b)(1) and 
(b)(2) of this section to the extent they 
would impede rapid evacuation of the 
airplane occupants. 

[Amdt. 25–64, 53 FR 17646, May 17, 1988] 

§ 25.563

Structural ditching provi-

sions. 

Structural strength considerations of 

ditching provisions must be in accord-
ance with § 25.801(e). 

F

ATIGUE

E

VALUATION

 

§ 25.571

Damage—tolerance and fa-

tigue evaluation of structure. 

(a) 

General. 

An evaluation of the 

strength, detail design, and fabrication 
must show that catastrophic failure 
due to fatigue, corrosion, manufac-
turing defects, or accidental damage, 
will be avoided throughout the oper-
ational life of the airplane. This eval-
uation must be conducted in accord-
ance with the provisions of paragraphs 
(b) and (e) of this section, except as 
specified in paragraph (c) of this sec-
tion, for each part of the structure that 
could contribute to a catastrophic fail-
ure (such as wing, empennage, control 
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 

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