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4.3.6.4.3 Uncracked structures of non-damage tolerant designs have long had to be re-evaluated in accordance with the damage tolerance philosophy and the results promulgated by way of supplemental inspection documents. A similar retrospective review of existing structural repairs of aircraft in service is needed.
Note.— This is considered to have been completed by a one-time review by TC holders.
4.3.6.4.4 Even the major organizations responsible for the type designs do not have the capability to handle the volume of work that individual appraisals would require. Accordingly, the organizations responsible for the type designs, with assistance from operators and airworthiness authorities, are working to provide a practical methodology that will allow operators to evaluate existing repairs without complex analysis.
4.3.6.4.5 The repair assessment programme should provide guidelines for the identification and documentation of all repairs in a three-stage programme which, generally, is as follows:
Stage 1. To identify areas where assessment is not required, e.g. secondary structure or low stress areas.
Stage 2. To provide operators with guidelines for dividing repairs into the following three categories:
Category A. Meets the design certification requirements of the aircraft, and requires no special inspections other than normal maintenance.
Category B. Meets design certification requirements of the aircraft; however, must be periodically inspected beyond normal maintenance requirements to ensure structural integrity.
Category C. Meets design certification requirements of the aircraft; however, repair is obviously of a temporary nature and to ensure structural integrity requires periodic inspection other than normal maintenance and must be replaced or upgraded to a Category B or better at a certain time limit.
Stage 3. To provide guidelines for operators to apply in establishing inspection intervals and removal time limits.
4.3.6.4.6 Typical repair parameters to be established by inspection of records or aircraft by the operator are: a) location; b) proximity to other repairs; c) condition; d) corrosion protection; e) size of damage or cut out; f) patch material and thickness; g) embodiment date; h) ratio of original to repaired thickness; i) fastener details for original and repair type, diameter, pitch, number of rows, edge margin;
and
j) extent of wear.
4.3.7 Widespread fatigue damage
4.3.7.1 The likelihood of the occurrence of fatigue damage in an aeroplane’s structure increases with aeroplane usage. The design process generally establishes a design service goal (DSG) in terms of flight cycles/hours for the airframe. It is expected that any cracking that occurs on an aeroplane operated up to the DSG will occur in isolation (i.e. local cracking), originating from a single source, such as a random manufacturing flaw (e.g. a mis-drilled fastener hole) or a localised design detail. The supplementary structural inspection programme (SSIP) described above or the maintenance review board (MRB) derived inspections for damage, are intended to find this form of damage before it becomes critical. Therefore, if aircraft are not operated beyond the initial limit of validity of the maintenance programme, it may not be required to perform a widespread fatigue damage (WFD) assessment.
4.3.7.2 With extended usage, uniformly loaded structure may develop cracks in adjacent fastener holes, or in adjacent similar structural details. These cracks, while they may or may not interact, can have an adverse effect on the structural capability before the cracks become detectable. The development of cracks at multiple locations may also result in strong interactions that can affect subsequent crack growth, in which case the predictions for local cracking would no longer apply. An example of this situation may occur at any skin joint where load transfer occurs. Simultaneous cracking at many fasteners along a common rivet line may reduce the residual strength of the joint below required levels before the cracks are detectable under the routine maintenance programme established at time of certification.
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