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4.3.7.3  The type design organization, in conjunction with operators, and in some cases the operators themselves, is expected to initiate development of a maintenance programme with the intent of predicting the onset of WFD and establishing an appropriate limit of validity (LoV) of the maintenance programme for the operation without multiple site damage or multiple element damage. Such programmes should be implemented before analysis, tests, and/or service experience indicates that widespread fatigue damage may develop in the fleet and substantially before LoV is reached on any aeroplane in service.
Note.— This may be based on typical construction, and may require a different methodology for composite structure.
4.3.8  Limit of validity of maintenance programmes
Associated with these programmes is the need to identify a limit of validity (LoV) of the maintenance programme that contains them. Operators may not operate aeroplanes beyond this LoV unless the structural integrity programmes have been reviewed and been found valid for an extension of the maintenance programme. A new LoV will then be defined.
4.3.9  Water-displacing corrosion-inhibiting fluids
4.3.9.1. Water-displacing corrosion preventatives (WDCPs) are a class of products widely used as a temporary and repetitive application to prevent corrosion and inhibit the progression of existing corrosion of metallic structures. There are many products available meeting a number of specifications with various classes of film hardness, tackiness and colour.
4.3.9.2. WDCPs may consist of a mixture of a water-displacing compound, a water-repelling agent and a corrosion-inhibiting agent contained in a low surface tension carrier solvent. Generally, the mixture is sprayed or brushed onto the structure and penetrates into cracks, crevices and contact surfaces of joints by capillary action. Evaporation of the carrier solvent leaves a waterproof corrosion-resistant film on surfaces, and seals cracks and crevices.
4.3.9.3. Apart from discouraging metal dissolution by displacing water, greases and oil films also help to exclude oxygen and simultaneously introduce a high electrical resistance between possible anodes and cathodes.
4.3.9.4. The inclusion of an inhibiting agent encourages the formation of a passive film on the metal surface, a primary corrosion control measure.
4.3.9.5. The efficacy of WDCP compounds depends purely on their ability to prevent corrosion in structural assemblies. They can protect metal surfaces when the original protective systems are no longer fully operative. By their very nature, however, these products raise some concerns.
a)  There has been considerable investigation into the effect of these fluids on the fatigue life of structural joints. Many joints transfer load through a clamping friction mechanism as well as by bearing on the fasteners. If the successful operation of a joint requires complete dependence upon friction between the members, WDCP or other lubricants should not be used during assembly. In general, however, the prevention of corrosion is even more important to the fatigue life and, except in very special cases, the advantages of using WDCP on joints outweigh any concerns of a possible fatigue life reduction.
b)  WDCPs can effectively seal pre-existing cracks, making it difficult to detect cracks by some common non-destructive testing (NDT) methods such as dye penetrant and ultrasonic. These products may be extremely difficult to remove from deep crevices to perform NDT procedures and hence an operator should consider the implications of using WDCPs in areas which require crack-checking procedures.
c)  The efficacy is maximized if applied during original manufacture and as early in the construction sequence as possible. Full coverage is more readily assured and corrosion has not already started. If applied to older aircraft, there is less likelihood of the product penetrating completely into deep lap joints or that moisture and other corrosive agents are truly displaced from the full depth of the joint. But again, the benefits of repetitive use, especially in aggressive environments, are usually worthwhile.
　
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