Annex 6 to Appendix 3 to AMC 20-20A Background to the need for damage-tolerance-based inspection programmes for repairs

Repairs are a concern on older aircraft because of the possibility that they may develop, cause, or obscure metal fatigue, corrosion, or other damage during service. This damage might occur within the repair itself or in the adjacent structure, and might ultimately lead to structural failure.

In general, repairs present a more challenging problem to solve than the original structure because they are unique and tailored in design to correct particular damage to the original structure. While the performance of the original structure may be predicted from tests and from experience on other aircraft in service, the behaviour of a repair and its effect on the fatigue characteristics of the original structure are generally known to a lesser extent than for the basic unrepaired structure.

Repairs may be of concern as time in service increases for the following reasons:

As aircraft age, both the number and age of the existing repairs increase. Along with this increase is the possibility of unforeseen repair interaction, failure, or other damage occurring in the repaired area. The continued operational safety of these aircraft depends primarily on a satisfactory maintenance programme (with inspections conducted at the right time, in the right place, using the most appropriate technique, or in some cases, replacement of the repair). To develop this programme, a DTE of repairs to aircraft structure is essential. The longer an aircraft is in service, the more important this evaluation and a subsequent inspection programme become.

The practice of repair justification has evolved gradually over the last 20 years. Some repairs described in the aircraft manufacturers’ SRMs were not designed in accordance with fatigue and damage tolerance principles (Ref. AAWG Report: Recommendations concerning ARAC taskings FR Doc 04‑10816; Aging Aircraft Safety Final Rule. 14 CFR 121.370a and 129.16.). Repairs accomplished in accordance with the information contained in the early versions of the SRMs may require additional inspections if evaluated using the fatigue and damage tolerance methodology.

Damage tolerance is a structural design and inspection methodology used to maintain safety by considering the possibility of metal fatigue or other structural damage (i.e. safety is maintained by adequate structural inspection until the damage is repaired). One prerequisite for the successful application of the damage tolerance approach for managing fatigue is that crack growth and residual strength can be anticipated with sufficient precision to allow inspections to be established that will detect cracking before it reaches a size that will degrade the strength to less than a specified level. A DTE entails the prediction of sites where fatigue cracks are most likely to initiate in the aircraft’s structure, the prediction of the crack path and rates of growth under repeated aircraft structural loading, the prediction of the size of the damage at which strength limits are exceeded, and an analysis of the potential opportunities for inspection of the damage as it progresses. This information is used to establish an inspection programme for the structure that will be able to detect cracking that may develop before it could contribute to a catastrophic failure.

The evidence to date is that when all the critical structure is included, damage-tolerance-based inspections and procedures, including modification and replacement, provide the best assurance of continued structural integrity that is currently available. In order to apply this concept to existing transport aeroplanes, the competent authorities have issued a series of ADs requiring compliance with the first supplemental inspection programmes resulting from the application of this concept to existing aeroplanes. Generally, these ADs require that operators incorporate SSIDs into their maintenance programmes for the affected aeroplanes. These documents were derived from damage tolerance assessments of the originally certified type designs for these aeroplanes. For this reason, the majority of ADs written for the SSIP did not attempt to address the issues related to the damage tolerance of repairs that had been made to the aeroplanes. The objective of repair assessment and repair evaluation guidelines is to provide the same level of assurance for areas of the structure that have been repaired as that achieved by the SSIP for the baseline structure as originally certified.

[AMC 20/20]