FATIGUE EVALUATION
JAR 29.571 Fatigue evaluation of structure
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(a) General. An evaluation of the strength of
principal elements, detail design points, and
fabrication techniques must show that catastrophic
failure due to fatigue, considering the effects of
environment, intrinsic/discrete flaws, or
accidental damage will be avoided. Parts to be
evaluated include, but are not limited to, rotors,
rotor drive systems between the engines and rotor
hubs, controls, fuselage, fixed and movable
control surfaces, engine and transmission
mountings, landing gear, and their related primary
attachments. In addition, the following apply:
(1) Each evaluation required by this
[paragraph must include –]
(i) The identification of principal
structural elements, the failure of which
could result in catastrophic failure of the
rotorcraft;
(ii) In-flight measurement in
determining the loads or stresses for
items in sub-paragraph (a) (1) (i) of this
paragraph in all critical conditions
throughout the range of limitations in
JAR 29.309 (including altitude effects),
except that manoeuvring load factors
need not exceed the maximum values
expected in operations; and
(iii) Loading spectra as severe as
those expected in operation based on
loads or stresses determined under subparagraph
(a) (1) (ii) of this paragraph,
including external load operations, if
applicable, and other high frequency
power cycle operations.
(2) Based on the evaluations required
[by this paragaph, inspections, replacement]
times, combinations thereof, or other
procedures must be established as necessary to
avoid catastrophic failure. These inspections,
replacement times, combinations thereof, or
other procedures must be included in the
airworthiness limitations section of the
Instructions for Continued Airworthiness
required by JAR 29.1529 and Paragraph A29.4
of Appendix A.
(b) Fatigue tolerance evaluation (including
tolerance to flaws). The structure must be shown
by analysis supported by test evidence and, if
available, service experience to be of fatigue
tolerant design. The fatigue tolerance evaluation
must include the requirements of either subparagraph
(b) (l), (2), or (3) of this paragraph, or
a combination thereof, and also must include a
determination of the probable locations and
modes of damage caused by fatigue, considering
environmental effects, intrinsic/discrete flaws, or
accidental damage. Compliance with the flaw
tolerance requirements of sub-paragraph (b) (1) or
(2) of this paragraph is required unless the
applicant establishes that these fatigue flaw
tolerant methods for a particular structure cannot
be achieved within the limitations of geometry,
inspectability, or good design practice. Under
these circumstances, the safe-life evaluation of
sub-paragraph (b)(3) of this paragraph is required.
(1) Flaw tolerant safe-life evaluation.
It must be shown that the structure, with flaws
present, is able to withstand repeated loads of
variable magnitude without detectable flaw
growth for the following time intervals:
(i) Life of the rotorcraft; or
(ii) Within a replacement time
furnished under Paragraph A29.4 of
Appendix A.
(2) Fail-safe (residual strength after
flaw growth) evaluation. It must be shown that
the structure remaining after a partial failure is
able to withstand design limit loads without
failure within an inspection period furnished
under Paragraph A29.4 of Appendix A. Limit
loads are defined in JAR 29.301 (a).
(i) The residual strength
evaluation must show that the remaining
structure after flaw growth is able to
withstand design limit loads without
failure within its operational life.
(ii) Inspection intervals and
methods must be established as necessary
to ensure that failures are detected prior
to residual strength conditions being
reached.
(iii) If significant changes in
structural stiffness or geometry, or both,
follow from a structural failure or partial
failure, the effect on flaw tolerance must
be further investigated.
(3) Safe-life evaluation. It must be
shown that the structure is able to withstand
repeated loads of variable magnitude without
detectable cracks for the following time
intervals:
(i) Life of the rotorcraft; or
(ii) Within a replacement time
furnished under Paragraph A29.4 of
Appendix A.
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