TERMS AND DEFINITIONS
Primary structure: Primary structure is structure that significantly
contributes to the carrying of flight, ground, or pressure loads. It is also
known as a structurally significant item (SSI).
Principal structural elements (PSE): These are those elements of
primary structure which contribute significantly to carrying flight, ground,
and pressurization loads, and whose failure could result in catastrophic
failure of the airplane.
Hence, integrity of PSE is essential in maintaining the
overall structural integrity of the aeroplane.
Single load path is where the applied loads are eventually distributed
through a single member, the failure of which would result in the loss of the
structural capability to carry the applied loads.
Multiple load path is identified with redundant structures in which, (with the
failure of individual elements) the applied loads would be safely distributed to
other load-carrying members.
Fail safe: Fail safe means the structure has been evaluated to assure that
catastrophic failure is not probable after fatigue failure or obvious partial
failure of a single, principal structural element.
Safe life: Safe life means that the structure has been evaluated to be able to
withstand the repeated loads of variable magnitude expected during its service
life without detectable cracks.
Damage tolerance means that the
structure has been evaluated to ensure that should serious fatigue, corrosion,
or accidental damage occur within the operational life of the airplane, the
remaining structure can withstand reasonable loads without failure or excessive
structural deformation until the damage is detected.
In other
words, DT is the attribute of the
structure that permits it to retain its required residual strength without
detrimental structural deformation for a period of use after the structure has
sustained a given level of fatigue, corrosion, and accidental or discrete
source damage.
Design Service Goal (DSG) is the period of time (in flight
cycles/hours) established at design and/or certification during which the
principal structure will be reasonably free from significant cracking including
widespread fatigue damage.
Extended Service Goal (ESG) is an adjustment to the design
service goal established by service experience, analysis, and/or test during
which the principal structure will be reasonably free from significant cracking
including widespread fatigue damage.
Widespread Fatigue Damage
(WFD) in a
structure is characterized by the simultaneous presence of cracks at multiple
structural details that are of sufficient size and density whereby the
structure will no longer meet its damage-tolerance requirement (i.e., to
maintain its required residual strength after partial structural failure).
Multiple Site Damage (MSD) is a source of widespread fatigue
damage characterized by the simultaneous presence of fatigue cracks in the same
structural element (i.e., fatigue cracks that may coalesce with or without
other damage leading to a loss of required residual strength).
Multiple Element Damage
(MED) is a source
of widespread fatigue damage characterized by the simultaneous presence of
fatigue cracks in similar adjacent structural elements.
DAMAGE TOLERANT STRUCTURES
a. The
damage tolerance assessment of the airplane structure should be based on the
best information available. The
assessment should include a review of analysis, test data, operational
experience, and any special inspections related to the type design. A determination should then be made of the
site or sites within each structural part or component considered likely to
crack, and the time or number of flights at which this might occur.
b. The
growth characteristics of damage and interactive effects on adjacent parts in
promoting more rapid or extensive damage should be determined. This determination should be based on study
of those sites that may be subject to the possibility of crack initiation due
to fatigue, corrosion, stress corrosion, disbonding, accidental damage, or
manufacturing defects in those areas shown to be vulnerable by service
experience or design judgment.
c. The
minimum size of damage that is practical to detect and the proposed method of
inspection should be determined. This determination should take into account
the number of flights required for the crack to grow from detectable to the
allowable limit, such that the structure has a residual strength corresponding
to the conditions stated under § 25.571.
NOTE: In determining the proposed method
of inspection, consideration should be given to visual inspection,
nondestructive testing, and analysis of data from built-in load and defect
monitoring devices.
d. The
continuing assessment of structural integrity may involve more extensive damage
than might have been considered in the original fail-safe evaluation of the
airplane, such as:
(1) a number of small adjacent cracks, each of
which may be less than the typically detectable length, developing suddenly
into a long crack;
(2) failures or partial failures in other
locations following an initial failure due to redistribution of loading causing
a more rapid spread of fatigue; and
(3) concurrent failure or partial failure of
multiple load path elements (e.g., lugs, planks, or crack arrest features)
working at similar stress levels.
