The Technical Oversight Group for Aging Aircraft (TOGAA) of the
Federal Aviation Administration adopted the following definition of widespread
fatigue damage (WFD) for aging aircraft [Lincoln, 2000]:
“The simultaneous presence of cracks
at multiple structural details characterizes the onset of WFD. These cracks are of sufficient size and
density whereby the structure will no longer meet its damage tolerance
requirement (e.g. maintaining required residual strength after partial structural
Where damage tolerance is defined as
Damage tolerance is the attribute of a
structure that permits it to retain its required residual strength for a period
of unrepaired usage. It must be able to
do this after it has sustained specified levels of fatigue, corrosion,
accidental, or discrete source damage.
Examples of such damage are (a) unstable propagation of fatigue cracks,
(b) unstable propagation of initial or service induced damage, and /or (c)
impact damage from a discrete source.”
Current critical aircraft structures are designed to be damage
tolerant. The structure is designed to
withstand failures or discrete source damage for a defined period of operation
during which the damage will be detected.
For fail-safe designed structures, the analyses and tests for
demonstrating fail-safety are based on the redundant or crack-stopping
component to be essentially undamaged.
However, if an aging airframe is experiencing WFD, the remaining
structure in the load path may not be capable of stopping the propagation of
the damage. Thus, WFD considerations
shift the emphasis from the growth of a dominant, monolithic crack to the loss
of fail-safety due to many small cracks.
This shift in emphasis has major ramifications with respect to the
application of the ASIP damage tolerance process.
A damage tolerance criterion for scheduling inspections for WFD
would need to be based both on the size of the cracks to be reliably detected
and on the number and location of the cracks in the crack-stopping
structure. It has been shown that
cracks on the order of 0.040 in. in the crack stopper
can compromise fail-safety [Swift, 1987, 1992a, 1992b]. At present, the reliable detection of
such small cracks, while possible, is cost prohibitive for the many details
over the broad expanse of structure that would need inspection. Further, the damage tolerance analysis
process is essentially deterministic.
The loss of fail-safety can occur as a result of many combinations of crack sizes and locations in the crack stopper of
the propagating damage. The use of
conservative, fixed-crack sizes in all of the crack stopper details
would permit a deterministic analysis but would lead to unacceptably short
inspection intervals. Therefore,
maintenance planning for WFD cannot be done with the ASIP damage tolerance
Since the aircraft can perform normal flight operations with
WFD, its presence can easily be overlooked.
The problem for maintenance planning is to predict to onset of WFD so
that repair, replacement, or retirement decisions can be made. At present, there is no standard method for
predicting the onset of WFD but structural risk analysis has been used in the
decision making process. The risk
analysis objective is to determine the number of flight hours at which the
probability of structural failure given a discrete source damage event exceeds
a defined level. For example, in a risk
analysis of the C-5A, probability of failure given the discrete source damage
greater than 10-4 was judged to be an unacceptable level of fail
safety [Lincoln, 2000]. Risk analysis
is discussed in Section 8.2.3, but it might be noted that predicting the growth
of small cracks can play an integral part of risk analyses.
There are two general scenarios for WFD that affect
fail-safety. These are referred to as
multiple-site damage (MSD) and multiple-element damage (MED). MSD is usually considered to be fatigue
cracking in multiple details of the same structural element. A discrete source damage event (i.e. failure
of an integral detail of an element) would raise stress levels in the remainder
of the structural element. The discrete
source damage event could be caused by an external disturbance or by the sudden
linking of cracks in the element. An
example of MSD leading to the loss of fail safety is provided by the failure in
an Aloha Airlines Boeing 737 in April 1988.
The failure occurred after the airframe had experienced 89,960
flights. Subsequent analyses have shown
that the airframe had lost fail-safety at about 40,000 flights due to MSD (see
NTSB  and Lincoln ).
In the MED scenario, fatigue cracking occurs in two or more
multiple elements that support the same load path. Failure of selected combinations of the elements may not lead to
system failure, but the effects of the failures may well lead to load and
geometry effects that do influence the integrity of the remaining
structure. An example of MED is
provided by the fatigue cracking at WS-405 of the C-141 aircraft [Alford, et