Repairing Damaged Aircraft

Both metallic and composite structures present their own particular problems, not the least of which is actually finding cracks and defects before catastrophe occurs. Non Destructive Testing (NDT) is still the basic tool for detection, but there are limitations. Even improved methods still need some idea of position to be fully effective in discovering faults and the complexity of modern faults makes life very difficult. Considerable research is in hand in the US on new techniques.

Delamination is also a major problem in composite materials, where large loss of compressive strength can occur with little or no surface indication. An interesting new technique is emerging, in which fibre optic cables are embedded in new composites, and, when damage occurs, the resulting signals are read off and analysed externally.

Another new method, under development at the Australian Defence Force Academy in Canberra, is the use of holography to find very small cracks about 3 microns long. Perhaps operators would still like to have the old time inspectors who could smell invisible cracks from a hundred metres away on a wet and windy night!

Once having located the defects, there is the problem of devising repairs. Badly designed repairs are often worse than no repairs, since they give false security and make inspection more difficult. Things are even more difficult where an old aircraft, designed on safe life principles but with an unknown life remaining, needs repairs. Both the Australian and UK Civil Aviation Authorities have been involved in various international talks on the matter, and the UK CAA has produced its requirements for the design and approval of repairs, particularly where the manufacturer gives little guidance. The essence of all such requirements is the prescription of analysis, materials, components, incorporation and documentation. Some elegant theoretical methods of stress analysis and life prediction are being developed, and the bad old days of rivetting on a thicker patch over the crack are long since gone (although this writer well remembers doing so in a previous incarnation forty-odd years ago).

For composite materials, an International Air Transport Association Task Force has published its “Guidance Material for Design, Maintenance and Repair of Thermo-setting Epoxy Matrix Composite Aircraft Structures”. Composite repairs have the advantage over metallic ones in that, if they are properly done, they do not cause stress concentration, they give stiffer load paths and they generally do not inhibit future inspections. They can be used on thick structures as well as skins.

Air forces have a special interest in repair techniques for Aircraft Battle Damage Repair work. This is the practice of making the best use of combat aircraft by doing just enough repairs to make them fit for the next missions when they have received damage, or, where damage is severe, making them flyable to move them from the operational areas to bases where deep repairs can be done. The UK Royal Air Force views ABDR as a “force multiplier”, based on its Gulf experiences. Effective ABDR depends on meticulous pre-planning, experienced staff and the right repair kits and practices already in place.

Civil Aviation Authorities and airlines, as major players in the airworthiness business, need to work closely together to maintain standards, in the face of reduced funding by Governments and sharply rising maintenance costs. The Australian Government has said that, while standards are to be maintained, it will not continue funding aviation safety regulation after 1992. The burden of proof has shifted: those who wish to maintain existing standards now have to argue their case for retention, not those seeking relaxation of standards.

Airlines find it difficult to obtain and keep the qualified and experienced staffs needed to uphold the maintenance and safety requirements, and the maintenance load is increasing faster than the number of staff. There is no doubt that some costs can be cut, but maintenance of safety is inherently a costly business, and there is the old saying that “if you think safety is expensive, try having an accident!”.