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.