Miami’s “Corrosion Corner” is getting busier. In January, the FAA’s new rules addressing widespread fatigue damage (WFD) in aging aircraft became effective. Drafted as amendments to Parts 25, 26, 121 and 129 of the U.S. Federal Aviation Regulations (FARs), the FAA has effectively set “soft” life limits for aircraft. While these regulations only effect U.S.-registered aircraft, it is expected that EASA and other regulators will follow suit.
The issue of WFD gained widespread attention in April 1988 when the upper fuselage of an Aloha Airlines 737 separated in flight. While not caused exclusively by fatigue, the era of increased scrutiny of the maintenance of aging aircraft formally began.
In fact the FAA began efforts in the 1970’s to address fatigue damage in aircraft when it transitioned from a “fail safe” to “damage tolerance” approach, according to Ray Valeika, former airline executive and most recently the former Senior Vice President of Technical Operations at Delta Air Lines. According to Valeika, the “fail safe” approach sought to prevent any fatigue damage to aircraft structures. As a result, Valeika said, inspections were time-consuming and aircraft were over-engineered to provide a cushion of protection. The “damage tolerance” approach, Valeika explained, presumes that some fatigue damage will exist, but aims to manage it through targeted inspections based on engineering analysis to detect and repair it before a structural failure occurs. Design service goals were expectancies that could be exceeded with a “detect and repair” aging aircraft inspection program.
Under “damage tolerance”, the concern is not fatigue cracking itself. Small fatigue cracks occur over the course of an aircraft’s life. However, unless discovered during inspection, small cracks in close proximity can rapidly and unexpectedly join to make large cracks. Engineered tear stoppers were the last defense against a catastrophic failure if cracks remained undetected. More recently and following a number of news making in-flight decompressions, the FAA concluded that aircraft inspections that sometimes uncovered fatigue cracks by happenstance and the patchwork of ADs that addressed them didn’t go far enough. “More cracks resulted in more ADs which then became a very complicated way to manage aircraft structure,” Valeika said. “This new rule is a way to amalgamate how you handle this issue.” The “damage tolerance” approach is not dead, it is now just part of a broader approach to WFD.
The new rules apply to turbine powered aircraft certificated after January 1, 1958, which have a maximum takeoff gross weight greater than 75,000 pounds and which are operated under FAR Parts 121 or 129. Boeing 707s and Caravelles, among others, are exempted. The FAA clearly wanted to move away from detect and repair for WFD to a more managed and predictable process, thus resulting in fewer ADs necessitated by unexpected failures. In fact, the FAA requires design approval holders to demonstrate that their aircraft will not suffer from WFD during their design service lives. “Design approval holders” include holders of and applicants for type certificates, supplemental type certificates and amended type certificates. Design approval holders must establish a limit of validity of the engineering data that supports the structure maintenance program (LOV) as their aircraft are phased under the rules by type over the next few years. For future aircraft, design approval holders and applicants for type certificates must establish an LOV. Operators may not fly an airplane beyond its LOV unless an extended LOV is approved.
The LOV approach was taken to address industry resistance to hard time operational limits. For existing aircraft types, design approval holders are not required to identify and develop maintenance actions if they can show that such actions are not necessary to prevent WFD before their LOV. If they are necessary, the maintenance actions must be identified unless service information already exists, in which case they would become mandatory by future ADs.
The fleets of first tier operators (original purchasers of aircraft) should not be impacted by these new rules. This is not the case as aircraft age and transition to smaller second tier operators. Although it may be extended, “it is not practical that an airline will have the capabilities to extend the LOV; the operators that have these aircraft won’t have the technical resources to do this,” Valeika said.
These new rules should not affect aircraft values. Using current manufacturer design service goals as the guide, the FAA concluded that most aircraft are currently retired before the LOVs that design approval holders anticipate establishing under the rules. Thus, for example, a 737NG has an expected LOV of 75,000 cycles and an A319, 48,000 cycles or 60,000 flight hours. In reality, by ultimately reducing available aircraft by forced retirement, the values of remaining aircraft could be increased. As the new rules became effective, the FAA determined that just 21 aircraft with eight small operators were over 75% of LOV, with just one aircraft over 100%.
Whether this enhanced approach is an improvement over one that was arguably working remains to be seen. Nevertheless, by forcing a substantive reevaluation of an aircraft as it approaches its initial design service goal, the FAA wants to make certain breakups remain matters of the heart and not the sky.