JAR 25.571 Damage-tolerance and fatigue
evaluation of structure
(a) General. An evaluation of the strength,
detail design, and fabrication must show that
catastrophic failure due to fatigue, corrosion, or
accidental damage, will be avoided throughout the
operational life of the aeroplane. This evaluation
must be conducted in accordance with the
provisions of sub-paragraphs (b) and (e) of this
paragraph, except as specified in sub-paragraph
(c) of this paragraph, for each part of the structure
which 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). (See ACJ 25.571 (a).) For turbine
engine powered aeroplanes, those parts which
could contribute to a catastrophic failure must
also be evaluated under sub-paragraph (d) of this
paragraph. In addition, the following apply:
(1) Each evaluation required by this
paragraph 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 aeroplane; and
(iii) An analysis, supported by test
evidence, of the principal structural
elements and detail design points
identified in sub-paragraph (a) (1) (ii) of
this paragraph.
(2) The service history of aeroplanes of
similar structural design, taking due account of
differences in operating conditions and
procedures, may be used in the evaluations
required by this paragraph.
(3) Based on the evaluations required
by this paragraph, 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 JAR 25.1529.
(b) Damage-tolerance (fail-safe) evaluation.
The evaluation must include a determination of
the probable locations and modes of damage due
to fatigue, corrosion, or accidental damage. The
determination must be by analysis supported by
test evidence and (if available) service
experience. Damage at multiple sites due to prior
fatigue exposure must be included where the
design is such that this type of damage can be
expected to occur. The evaluation must
incorporate repeated load and static analyses
supported by test evidence. The extent of damage
for residual strength evaluation at any time within
the operational life must be consistent with the
initial detectability and subsequent growth under
repeated loads. The residual 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 manoeuvring
conditions specified in JAR 25.337 up to VC
and in JAR 25.345.
(2) The limit gust conditions specified
in JAR 25.341 at the specified speeds up to]
VC and in JAR 25.345.
(3) The limit rolling conditions
specified in JAR 25.349 and the limit
unsymmetrical conditions specified in JAR
25.367 and JAR 25.427(a) through (c), at]
speeds up to VC.
(4) The limit yaw manoeuvring
conditions specified in JAR 25.351 at the
specified speeds up to VC.
(5) For pressurised cabins, the
following conditions:
(i) The normal operating
differential pressure combined with the
expected external aerodynamic pressures
applied simultaneously with the flight
loading conditions specified in sub-paragraphs
(b)(1) to (b)(4) of this
paragraph if they have a significant
effect.
(ii) The maximum value of
normal operating differential pressure
(including the expected external
aerodynamic pressures during 1 g level
flight) multiplied by a factor of 1·15
omitting other loads.
(6) For landing gear and directly-affected
airframe structure, the limit ground
loading conditions specified in JAR 25.473,
JAR 25.491 and JAR 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. (See ACJ
25.571 (b).) The residual strength requirements of
this sub-paragraph (b) apply, where the critical
damage is not readily detectable. On the other
hand, in the case of damage which is readily
detectable within a short period, smaller loads
than those of sub-paragraphs (b)(1) to (b)(6)
inclusive may be used by agreement with the
Authority. A probability approach may be used in
these latter assessments, substantiating that
catastrophic failure is extremely improbable. (See
ACJ 25.571 (a), paragraph 2.1.2.)
(c) Fatigue (safe-life) evaluation.
Compliance with the damage-tolerance
requirements of sub-paragraph (b) of this
paragraph is not required if the applicant
establishes that their application for particular
structure is impractical. This structure must be
shown by analysis, supported by test evidence, to
be able to withstand the repeated loads of variable
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 aeroplanes of similar structural
design and sonic excitation environment, that –
(1) Sonic fatigue cracks are not
probable 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 sub-paragraph (b) of this
paragraph are applied to all areas affected by
those cracks.
(e) Damage-tolerance (discrete source)
evaluation. The aeroplane must be capable of
successfully completing a flight during which
likely structural damage occurs as a result of –
(1) Bird impact as specified in JAR
25.631;
(2) Reserved
(3) Reserved
(4) Sudden decompression of
compartments as specified in JAR 25.365 (e)
and (f).
The damaged structure must be able to
withstand the static loads (considered as ultimate
loads) which are reasonably expected to occur at
the time of the occurrence and during the
completion of the flight. Dynamic effects on these
static loads need not be considered. Corrective
action to be taken by the pilot following the
incident, such as limiting manoeuvres, avoiding
turbulence, and reducing speed, may be
considered. 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.
(See ACJ 25.571(a), paragraph 2.7.2 and ACJ
25.571 (b).)
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