‘WFD’ – What is it and what’s ‘LOV’ got to do with it?

Abstract

Fatigue cracking of any kind can significantly reduce the load carrying capability of a structure. Because of this it is considered to be a major threat to structural integrity. Chapter 14 of the Code of Federal Regulations (14 CFR) includes specific requirements intended to preclude catastrophic failures due to fatigue. Fatigue cracking may be symptomatic or incidental, localized or at multiple locations. The 14 CFR requirements make no distinction. They are intended to mitigate the strength reducing effects of fatigue regardless of why or how it manifests itself. The Aloha Airlines incident of 1988 was caused by fatigue cracking at multiple locations that could be considered symptomatic. This incident precipitated an avalanche of activities related to symptomatic fatigue cracking at multiple locations. This included research, development and even new rulemaking. In many respects, this cracking was treated like a new phenomena. For many it was seen as a new and singular threat that needed special attention. Consistent with this a new term, “widespread fatigue damage” (WFD), was coined. Is WFD really something new? Does it need special treatment and maybe even its own rule? These questions are considered and it is concluded that WFD is not something new. It is also argued that WFD is and always has been within the scope of existing rules. However, one thing that is missing in the rules is the concept of a “limit of validity” (LOV) of an airplane’s fatigue management program (FMP). The LOV is the cumulative amount of operation beyond which it has not been validated that the FMP will keep the threat of catastrophic failure due to fatigue sufficiently low. The LOV is very much dependent on the fatigue knowledge base that has been acquired at any given time. The lack of an LOV is a deficiency relative to fatigue management in general and not just for what has been labeled WFD. It is concluded that rulemaking is needed to require the establishment of an LOV for certain existing airplane models and future certified ones. Additionally, the existing requirement to provide full-scale fatigue test evidence should not be limited to addressing the threat of WFD but should be required to characterize the fatigue performance of all primary structure in support of establishing an LOV for the FMP.