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Section 8.2.2. Corrosion
Although corrosion is a major contributor to the costs of
structural maintenance of aging aircraft, corrosion has not been a safety issue
to date. Accordingly, corrosion has not
been emphasized in ASIP. JSSG-2006
recognizes that corrosion can affect operational readiness through enhanced
initiation of flaws that degrade damage tolerance, durability and residual
strength. Corrosion prevention and
control is addressed in Paragraph A.3.11.2 of JSSG-2006, but the emphasis here
is on material selection and corrosion prevention systems. The guidance states that corrosion will not
occur during the planned service life and usage because the corrosion
prevention system will remain effective
during the planned service life and usage.
Planning for corrosion maintenance is a not formal part of the
FSMP of MIL-HDBK-1530. In fact, there
is no reference to corrosion in the Force Management Tasks IV and V of
MIL-HDBK-1530. In Appendix B, “Additional
Guidance for Aging Aircraft”, of MIL-HDBK-1530, corrosion is recognized as an
aging aircraft issue. The guidance in
Appendix B states that inspections for corrosion in aging aircraft should be
conducted. If corrosion is found, it
should be removed. If, on rare
occasion, the corrosion cannot be removed, the effect of the corrosion on
structural integrity should be determined and the safety inspection schedule
should be modified. This approach to
maintenance is often referred to as “find it and fix it”.
Corrosion is an economic burden in sustainment. Inspections for hidden corrosion are
currently being performed during routine, depot level maintenance cycles. When corrosion is detected, the damage is
repaired or the damaged component is replaced.
Cost savings could be realized if the timing of corrosion maintenance
actions could be optimized. However, at
present there are no accepted analytical methods for predicting the initiation
and growth of corrosion, so that a severity-of-damage type approach to
scheduling inspections is not currently feasible. Such an “anticipate and manage” approach to corrosion maintenance
is under development (see, for example, Peeler, et al. [2001], Brooks, et al. [2001], and Lang, et al. [2001]). This approach depends on knowing the
condition of the corrosion damage through NDI, understanding the corrosion
growth rates as affected by the environment, and predicting the future
corrosion condition using models of corrosion growth. The present and predicted future states of the corrosion condition
can then be used in structural integrity calculations to determine remaining
strength and life. Disposition may now
include flying the aircraft with known corrosion present, among other
alternatives. Economical disposition
can be made while maintaining aircraft safety.
For damage-based inspection scheduling, the capability of NDI
systems must be characterized in terms of
the damage metric being modeled. Refer
to Section 3.1.3 for a discussion of characterizing the corrosion
detection capability of NDI systems.