DISTRIBUTION A. Approved for public release; distribution unlimited.
EN-SB-08-003, Rev A, Page 1 of 5
Number: EN-SB-08-003, Revision A
Date: 18 March 2011
Subject: Fail-Safe Assessments of Current Aircraft
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
size before they are detected and repaired. Initial crack sizes were specified in MIL-A-
83444 (and later in the Joint Services Specification Guide, JSSG-2006) for use in
design and in establishing inspection intervals.
Since the inception of damage tolerance, this slow crack growth inspection approach
has greatly diminished the incidence of catastrophic structural failure. However,
inspection reliability has become a significant issue both due to imperfections of the
nondestructive inspection (NDI) systems and concern over inspectors becoming
complacent as a result of performing numerous inspections looking for rogue flaws
without any finds. The inspection issue becomes more critical as aging weapon systems
approach the onset of widespread fatigue damage (WFD). The inspection burden and
aircraft down times will tend to overwhelm the depots and jeopardize both safety and
operational readiness. To prevent this from occurring, the recent Structures Bulletins
EN-SB-08-001 and EN-SB-08-002 provide guidelines for establishing life limits for fail-
safe and slow crack growth design concepts respectively. When these life limits
(updated with fleet tracking usage data) are reached, the structure should be modified
or replaced, or the aircraft retired.
Prior to reaching these life limits, there is still a need to minimize the possibility of
inspection misses and potential catastrophic failure by improving NDI equipment and
Structures Bulletin
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