DISTRIBUTION A. Approved for public release; distribution unlimited.
EN-SB-08-001, Rev A, Page 1 of 13
Number: EN-SB-08-001, Revision A
Date: 18 March 2011
Subject: Revised Damage Tolerance Requirements and Determination of Fail-
Safety Life Limits for Fail-Safe Metallic Structures
Background:
The Air Force formally introduced damage tolerance requirements with the release of
MIL-A-83444 in July of 1974. While this specification allowed the use of either fail-safe
or slow crack growth design concepts, the primary focus was on the slow crack growth
concept since most combat aircraft were designed with many single load path
structures. With the slow crack growth concept, it is mandatory that material,
manufacturing and/or service induced defects not be allowed to reach their critical crack
sizes before they are detected and repaired. Initial crack sizes were specified in MIL-A-
83444, and later in Joint Services Specification Guide JSSG-2006, for use in design and
in establishing initial inspection intervals. These assumed initial flaw sizes were selected
as surrogates for the myriad of manufacturing, material and in-service defects (i.e.,
rogue flaws) that can and occasionally do exist in aircraft.
At that time, commercial aircraft as well as some military transport aircraft were
designed to the commercial aviation regulation, CAR 4b.270, fail-safe requirements.
These requirements stated that the structure must be able to sustain 80% of limit load
multiplied by a 1.15 dynamic factor after failure or partial failure (and crack arrest) of a
load path. However, even then it was recognized that the shortcomings of this
requirement were that it didn’t address the issues of continuing damage in adjacent
structure, safe periods of unrepaired usage after a load path failure and the fact that the
fail-safety would be jeopardized later in life by the onset of widespread fatigue damage
(WFD). The FAA later addressed these shortcomings in FAR 25.571 Amendment 96.
The Air Force addressed some of these issues in MIL-A-83444 and later in JSSG-2006
by imposing slow crack growth requirements on multiple load path and crack arrest
structures that were essentially the same as for single load path structures.
Structures Bulletin
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