Repair criteria differ in the same way that initial design requirements for aircraft differ. Criteria for a repair can be less demanding if the repair is considered to be temporary. Temporary repairs are performed for such requirements as a onetime flight to a repair facility or one more mission under combat conditions. However, most repairs are intended to be permanent, and, except for special conditions, criteria are applied so that the repair will remain acceptable for the life of the aircraft.

One of the major factors that influences the repair quality is the environment where the repairs are to made. For example, the presence of moisture is critical to bonded repairs. Epoxy resins can absorb 1.5 to 2 times their weight in moisture, thereby reducing the ability of the resins to support the fibers. Dirt and dust can seriously affect bonded repairs. Oils, vapors, and solvents prevent good adhesion in bonded surfaces and can lead to voids or delamination. To perform quality repairs, personnel must have a knowledge of the composite system to be repaired, type of damage, damage limitations/ classifications, repair publications, materials, tools and equipment, and repair procedures. The repair facilities where the work is to be performed will be clean and climate controlled if possible. The relative humidity should be 25 percent to 60 percent and temperatures stable at 65 to 75F. If repairs are to be made in an uncontrolled environment (hangar/flight deck), patches and adhesives will be prepared in a controlled environment and sealed in an airtight bag before being brought to the repair site.

Full strength repairs are desirable and should be made unless the cost is prohibitive or the facilities are inadequate. Less than full strength repairs are sometimes allowed on secondary structures that are lightly loaded, stiffness-critical structures designed for limited deflections rather than for carrying large loads (doors), or structures designed to a minimum thickness requirement for general resistance to handling damage (fuselage skins). Repair manuals for specific aircraft frequently "zone" the structure to show the amount of strength restoration needed or the kinds of standard repairs that are acceptable. Repair zones help to identify and classfy damage by limiting repairs to the load-carrying requirements. Repair zone borders indicate changes in load-carrying requirements due to changes in the structure, skin thickness, ply drop-offs, location of supporting members (ribs and spars), ply orientation, core density, size and type of materials. Damage in one zone may be repairable, where as the same type of damage in an adjacent zone may not be repairable. See figure 14-27.

Aerodynamic Smoothness 

High-performance aircraft depend on smooth external surfaces to minimize drag. During initial fabrication, smoothness requirements are specified, usually by defining zones where different levels of aerodynamic smoothness are required. These most

Figure 14-27.Repair zones. 

critical zones include leading edges of wings and tails, forward nacelles and inlet areas, forward fuselages, and overwing areas of the fuselage. The least critical zones include trailing edges and aft fuselage areas.

Drill motors should be capable of speeds of 2,000 to 5,000 rpm. These drills should be equipped with feed rate limiting surge controls to prevent backside breakout caused by feeding the drill too fast and excessive heat buildup from feeding the drill too slow. Feed rates should not exceed 30 seconds per inch, with 10 to 15 seconds per inch producing the best results on graphite-epoxy composites. The drill should be turning full speed prior to surface contact and during withdrawal from completed holes. These holes should be drilled slightly undersize and reamed to the required size. The various types of drill bits used for drilling composites are either twist, flat fluted/spade/dagger, single flute, or piloted countersink, and they are made out of carbide or carbon steel.

A drill stop (fig. 14-28) is an adjustable spring damper that is attached to the drill bit shank. This mechanically stops the drill at a predetermined depth prior to exiting the material backside, thus reducing backside breakout caused by the follow through. Firm pressure is required to overcome this spring tension for the drill to penetrate the laminates backside.

Routers are high-speed, hand-held, portable cutters used for removing damaged skin or core materials. They are designed to operate on shop air at

Figure 14-28.-Drill stop.

speeds of 25,000 to 40,000 rpm. Routers are normally used with a template to define a smooth regular cut with the depth of the cut set and locked. Hole saws are good for removing small areas of damage on laminates, although they have a tendency to damage honeycomb rather than cut it. Hole saws also easily clean up damages, providing a good surface for repairs. Backup plates should be taped to the backside of the material being sawed to prevent backside breakout. Fine tooth metal or diamond saws work the best for sawing laminates.