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CHAPTER 14 AIRCRAFT NONMETALLIC REPAIR Chapter Objective: Upon completion of this chapter, you will have a working structures. You will also have a working knowledge of advanced composite materials, their unique characteristics, and special techniques and safety procedures associated with composite repair materials. This chapter deals with the materials and procedures to be used in the repair of nonmetallic and advanced composite materials used in aircraft construction. The procedures discussed are general in nature. When actually repairing nonmetallic or advanced composite materials, you should refer to the applicable maintenance instruction manual (MIM) and structural repair manual (SRM). MAINTAINING AND REPAIRING AIRCRAFT NONMETALLIC MATERIALSLearning Objective: Recognize the proce-dures for cleaning and repairing or replacement of aircraft nonmetallic structures and surfaces.In the following text, we will discuss some of the procedures used in the repair or replacement of aircraft nonmetallic structures. Because no one set of rules applies to all aircraft, you should refer to the maintenance instruction manual (MIM) and structural repair manual (SRM) for the materials and procedures for a particular aircraft. MAINTAINING TRANSPARENT PLASTIC MATERIALS Because of the many uses of plastic materials in aircraft, optical quality is of great importance. These plastic materials are similar to plate glass in many of their optical characteristics. Ability to locate and identify other aircraft in flight, to land safely at high speeds, to maintain position in formation, and in some cases, to sight guns accurately through plastic enclosures all depend upon the surface cleanliness, clarity, and freedom from distortion of the plastic material. These factors depend entirely upon the amount of care exercised in the handling, fabrication, maintenance, and repair of the material. Plastics have many advantages over glass for aircraft application, particularly the lightness in weight and ease of fabrication and repairs. They lack the surface hardness of glass and are very easily scratched, with resulting impairment of vision. You must exercise care while servicing all aircraft to avoid scratching or otherwise damaging the plastic surface. Specific procedures are described later in this section for minor maintenance; however, the following general rules are emphasized: 1. Transparent plastic materials should be handled only with clean cotton gloves. 2. The use of harmful liquids, such as cleaning agents, should be avoided. 3. Fabrication, repair, installation, and mainte-nance instructions must be closely followed. 4. Operations that might tend to scratch or distort the plastic surface must be avoided. You must take care to avoid scratching plastic surfaces with finger rings or other sharp objects. Just as woods split and metals crack in areas of high, localized stress, plastic materials develop, under similar conditions, small surface fissures called crazing. These tiny cracks are approximately perpendicular to the surface, very narrow in width, and usually not over 0.01 inch in depth. These tiny fissures are not only an optical defect, but also a mechanical defect, as there is a separation or parting of material. If the crazing is in a random pattern, it is usually caused by the action of solvent or solvent vapors. If the crazing is approximately parallel, it is usually caused by directional stress, set up by cold forming, excessive loading, improper installation, improper machining, or a combination of these with the action of solvents or solvent vapors. Crazing can be caused by improper cleaning, improper installation, improper machining, or cold forming. Once a part has been crazed, neither the optical nor the mechanical defect can be removed permanently; therefore, prevention of crazing is very important. |
 
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