Structural Damage

Secondary damage may have occurred at some distance from the initial point, due to abnormal loads being transmitted along the structure. Such damage may not be immediately noticeable; damage to one end of a member may result in the other end being strained, distorted, cracked or otherwise rendered unserviceable. A typical example of this type of damage, would be that of a wooden strut having fractured under compression. Any attachment bolt holes in the wooden structure would probably become elongated and cracks may well have developed in the attaching components.

Where wooden components are covered by fabric bindings or by heavy protective treatments, the indications of damage may not be obvious. Close examination for any damage which may be concealed beneath such bindings or protective treatments must therefore be carried out.

The lifting, wrinkling or the formation of a sharp raised line on the fabric bindings or coverings and in some cases, isolated damage to any paint finish on the fabric may be an indication of a fracture. Such signs of altered contour must never be ignored and the coverings in a suspect area must be removed to enable the wooden structure underneath to be examined closely. Where necessary, the damaged portion of the component must be removed and an approved repair carried out.

Compression failures

This is the most serious defect and its presence in any part of the aircraft structure is not permitted under any circumstances. Compression failure is indicative of a definite rupture across the fibres of a component caused by excessive compressive loads and/or shock loads along the grain of the timber.

Compression failures are invariably accompanied by a marked reduction in toughness and bending strength.

As the indications of the defect are very inconspicuous, great care is required to ensure that it is detected. Severe compression failure may appear on the surface of a component as a series of fine irregular lines, either light or dark coloured, running across the grain.

Whenever a fault is suspected, the affected component should be renewed.

Mechanical Damage

Damage caused by crushing of components by the over tightening of bolts and/or swelling of the timber due to moisture content increase after assembly may be detected during the examination.

Providing that the wood fibres have not been broken and the depth of the depression does not exceed 0.06 in (1.5mm), an approved patch repair may be embodied. Damage exceeding the depth quoted or breakage of the wood fibres will necessitate complete replacement of the affected component.

If bolt holes in wooden components are found to have become elongated due to wear or crushing, the components affected may be repairable by metal bushing of the hole where permissible in accordance with the relevant Aircraft Maintenance and Repair Manual (or equivalent) or, on application to the local airworthiness authority. Where repair is not permissible, the component must be renewed.

Distortion

Damage which may have been caused by shrinkage or distortion of wooden components can only be determined by a detailed examination of the stripped component.

Deterioration

When it is necessary for wooden or wood and metal composite aircraft to be kept outside for extended periods of time in extreme weather conditions and temperatures or, the age of the aircraft becomes significant, the detection of possible deterioration of timber and glued joints is essential.

A thorough examination of the airframe structure must be carried out to ensure that no damage has occurred through water soakage, wet or dry rot and that there are no signs of the structure being attacked by Woodworm, insects or bacteria peculiar to the geographical location which may be attributed to the unsatisfactory maintenance of protective treatments. Where any such damage has occurred, all affected components or sections of components must be renewed immediately, especially where joints in an old airframe may have been glued with Casein cement, which is not impervious to moisture or bacterial degradation.

Oil soakage does not appreciably affect the strength of a component as it does not penetrate the grain of the timber as badly as does water. Where however, oil soakage is detected and for instance, layers of plywood are soggy and tending to bulge or lift, the component should be renewed.

Wooden components affected by wet or dry rot, other fungal defects or damage caused by insects must be examined fully before the full extent of any damage can be fully assessed and the causes eliminated. A full visual examination of significant areas of structure will necessitate the complete removal of fabric coverings from the components in the affected area where any form of deterioration has been detected. Initial detection of contaminated areas under fabric coverings maybe identified by signs of fabric lifting or bubbling with an build up of a white powdery substance in the vicinity.

Shakes.

A shake is a partial or complete separation between adjoining layers of a components timber fibres.Any appreciable shakes will require the affected component to be renewed. A shake will mainly be found in end grain material.

Checks

A check is a longitudinal separation of the timber fibres resulting from shrinkage of the wood not necessarily along the grain. The defect may result in merely cosmetic surface cracks, but if the checking is severe or extends for a significant distance it will seriously lower the strength of the component. Any component which exhibits evidence of significant check defects must be renewed.

Splits

Splits are separations of the timber fibres following the grain. They usually are caused by rough handling of the timber prior to installation but in existing structure may be the result of damage to the aircraft and may be accompanied by other defects which will require further investigation.

Defects caused by Fungi

Wood is so seriously weakened by fungal decay that any components in which there is evidence of such must be immediately rejected and completely renewed.

Advanced decay or dry rot is usually recognised without difficulty, but much care and experience is necessary to enable an inspector to detect slight infection or incipient decay (Dote) with certainty.

Dote is generally accompanied by some discoloration of the wood and flecks or streaks of stain will therefore call for close examination. Even though discoloration may prove to be not directly caused by the attack of harmful growths, it is useful to remember that the conditions which favour the development of natural staining also may encourage Dote, and timber that may contain staining in one area, may also contain Dote in another. Similarly, although insect holes are not evidence of decay, they suggest that the timber for some time was in a situation favourable to decay.
In Sitka Spruce, Dote finally takes the form of pockets of brown crumbly rot, but in the early days, it will appear as oval patches or streams of brownish discoloration. They may be distinguished from streaks of excessive resin, 'Strawberry marks', by the fact that areas of Dote end more or less abruptly, whereas, the resin streaks usually taper off and fade out at the ends. The penetration of the fungus is more pronounced in the direction of the grain at the end of the component than on the surface perpendicular to the grain.

Any component containing traces of Dote should be renewed immediately and other adjacent components in the vicinity should be closely examined for further evidence of fungal attack.

Chemical Conversion/Oxidation

Chemical conversion or Oxidation of timber (metal catalysed auto-oxidation - Marian and Wissing 1960) may be recognised by dark red or black discoloration, rust build-up and local hardening of the timber and is caused by the absorption of iron oxide into the fibres and cells of the timber. This form of deterioration is likely to become significant in climates where the residual moisture content of the timber approaches 20% (Imamura et al. 1987) and, significantly reduces the tensile strength of the component affected (Dinwoodie 1971).

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