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4.3.4.1.5  The continuing structural integrity programme should be checked from time to time against current service experience. Any unexpected defect that occurs should be assessed as part of the continuing assessment of structural integrity to determine the need for revision of the programme. Future structural service bulletins should state their effect on the programme.
4.3.4.2 Damage-tolerance assessment
4.3.4.2.1 Damage tolerance characteristics should be based on the best information available, including analysis, test and operational experience and special inspections which can be related to the type. From this information, the site or sites of likely cracking within each structural part or component and the time or number of flights (cycles or hours) at which this might occur may be judged.
4.3.4.2.2 The growth characteristics of damage and the interactive effects on adjacent parts in promoting more rapid or extensive damage should be determined. This study should include those sites which may be subject to the possibility of crack initiation owing to fatigue, corrosion, stress corrosion, wear, disbonding, accidental damage, manufacturing defects or other discrepancies in those areas which service experience or design judgement has shown to be vulnerable.
4.3.4.2.3  The minimum size of damage that it is practical to detect and the proposed method of inspection should be determined together with the number of flights required for the crack to grow from detectable to the allowable final size of damage in such a way that the structure has a residual strength corresponding to the conditions stated for fail-safe qualification. It is recognized that the residual strength requirements include the provision that they apply only where the critical damage would not be readily detectable, whereas in the case of damage which is readily detectable within a relatively short period, a lower residual strength may be agreed with the certificating authority. A probability approach may be acceptable for these latter assessments.
Note.— In determining the proposed method of inspection, consideration should be given to:
a) visual inspection;
b)  non-destructive testing; and
c)  analysis of data from built-in load and defect monitoring devices.
4.3.4.2.4  The continuing assessment of structural integrity may involve more extensive damage than might have been considered in the original evaluation of the aeroplane, such as:
a)  a number of small adjacent cracks, each of which may be less than the minimum detectable length, developing suddenly into a long crack;
b)  failures or partial failures in other locations, due to a redistribution of loading and a more rapid spread of fatigue, following an initial failure in a particular location;
c)  concurrent failure or partial failure of multiple load path elements (e.g. lugs, planks or crack arrest features) working at similar stress levels;
d)  the influence of corrosion; and
e)  the influence of wear.
4.3.4.3  Safe-life structures
The basis for the determination of the safe-life of parts and components should be re-analysed using knowledge gained from service experience, including operational usage, loading assumptions and loading spectra and from any further tests that may have been conducted.
4.3.4.4  Information to be included in the assessment
4.3.4.4.1  The continuing assessment of structural integrity for the particular aeroplane type should be based on the principles outlined in 4.3.4.1 to 4.3.4.3 above. The following information should be included in the assessment and kept by the type design organization in a form available for reference:
a)  the current operational statistics of the fleet in terms of hours or flights;
b)  the typical operational mission or missions assumed in the assessment;
c)  the structural loading conditions from the chosen missions; and
　
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