U.S. Department
of Transportation
Advisory
Circular
Subject: DAMAGE TOLERANCE AND
Date:
4129198
AC No: 25.571-‘C
FATIGUE EVALUATION OF STRUCTURE
Initiated By: ANhd-112
Change:
1. PURPOSF,. This advisory circular (AC) provides guidance for compliance with the
provisions of 14 CFR Part 25 of the Federal Aviation Regulations (FAR) pertaining to the
damage-tolerance and fatigue evaluation requirements for transport category aircraft structure.
It also provides rational guidelines for the evaluation of scatter factors for the determination of
life for parts categorized as safe-life. Like all advisory circular material, this advisory circular is’
not, in itself, mandatory and does not constitute a regulation. It is issued to provide an
acceptable means, but not the only means, of compliance with the rules. Terms used in this AC,
such as “shall” and “must” are used only in the sense of ensuring applicability of this particular
method of compliance when the acceptable method of compliance described herein is used.
While these guidelines are not mandatory, they are derived from extensive FAA and industry
experience in determining compliance with the pertinent FAR. This advisory circular does not
change, create any additional, authorize changes in, or permit deviations from, regulatory
requirements.
2 QQKEIJ.&TION. Advisory Circular 25.571- lB, dated February 18, 1997, is canceled.
3.
a.
Damage tolm is the attribute of the structure that permits it to retain its
required residual strength for a period of use after the structure has sustained a given level of
fatigue, corrosion, accidental or discrete source damage.
b. Fail-safe is the attribute of the structure that permits it to retain its required
residual strength for a period of unrepaired use after the failure or partial failure of a principal
structural element.
C.
Safe-l& of a structure is that number of events such as flights, landings, or flight
hours, during which there is a low probability that the strength will degrade below its design
ultimate value due to fatigue cracking.
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