Federal Aviation Administration, DOT

Section 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 structure, protection must be provided to
prevent axially compressive loads exceeding 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]

Section 25.563 Structural
sions.

ditching

provi-

Structural strength considerations of
ditching provisions must be in accordance with Section 25.801(e).
FATIGUE EVALUATION

spaschal on DSK3GDR082PROD with CFR

Section 25.571 Damage - tolerance and
tigue evaluation of structure.

fa-

(a) General. An evaluation of the
strength, detail design, and fabrication
must show that catastrophic failure
due to fatigue, corrosion, manufacturing defects, or accidental damage,
will be avoided throughout the operational life of the airplane. This evaluation must be conducted in accordance with the provisions of paragraphs
(b) and (e) of this section, except as
specified in paragraph (c) of this section, for each part of the structure that
could contribute to a catastrophic failure (such as wing, empennage, control
surfaces and their systems, the fuselage, engine mounting, landing gear,
and their related primary attachments). For turbojet powered airplanes, those parts that could contribute to a catastrophic failure must
also be evaluated under paragraph (d)
of this section. In addition, the following apply:
(1) Each evaluation required by this
section must include - 
(i) The typical loading spectra, temperatures, 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 identified in paragraph (a)(1)(ii) of this section.
(2) The service history of airplanes of
similar structural design, taking due
account of differences in operating conditions and procedures, may be used in
the evaluations required by this section.
(3) Based on the evaluations required
by this section, inspections or other
procedures must be established, as necessary, to prevent catastrophic failure,
and must be included in the Airworthiness Limitations section of the Instructions for Continued Airworthiness
required by Section 25.1529. The limit of validity of the engineering data that supports the structural maintenance program (hereafter referred to as LOV),
stated as a number of total accumulated 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 Section 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 demonstrated that load path failure, partial failure, or crack arrest will be detected and repaired during normal
maintenance, inspection, or operation
of an airplane prior to failure of the remaining structure.
(b) Damage-tolerance evaluation. The
evaluation must include a determination of the probable locations and
modes of damage due to fatigue, corrosion, 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 consideration for widespread fatigue damage
must be included where the design is

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