Par 3-23 Page 3-25
(2) PS-30 and Weld-On 40 should be used at temperatures no lower than 65 ºF. If cementing is done in a room cooler than 65 ºF, it will require a longer time to harden and the joint strength will be reduced.
(a) The cement should be prepared with the correct proportions of components as given in the manufacturer’s instructions and thoroughly mixed, making sure neither the mixing container nor mixing paddle adds color or affects the hardening of the cement. Clean glass or polyethylene mixing containers are preferred.
(b) Because of their short pot life (approximately 45 minutes) Cement PS-30 and Weld-On 40 must be used quickly once the components are mixed. Time consumed in preparation shortens the effective working time, making it necessary to have everything ready to be cemented before the cements are mixed. For better handling pour cement within 20 minutes of mixing.
(c) For maximum joint strength, the final cement joint should be free of bubbles. It will usually be sufficient to allow the mixed cement to stand for 10 minutes before cementing to allow bubbles to rise to the surface. (See Figure 3-20.)
FIGURE 3-20. APPLYING PRESSURE TO ACRYLIC PLASTICS
Par 3-23 Page 3-26
(d) The gap joint technique can only be used with colorless Plexiglas acrylic or in cases in which joints will be hidden. If inconspicuous joints in colored Plexiglas acrylic are needed, the parts must be fitted closely, using closed V groove, butt, or arc joints.
(3) Cement forms or dams may be made with masking tape as long as the adhesive surface does not contact the cement. This is easily done with a strip of cellophane tape placed over the masking tape adhesive. The tape must be chosen carefully. The adhesive on ordinary cellophane tape prevents the cure of PS-30 and Weld-On 40. Before actual fabrication of parts, sample joints should be tried to ensure that the tape system used will not harm the cement. Since it is important for all of the cement to remain in the gap, only contact pressure should be used.
(4) Bubbles will tend to float to the top of the cement bead in a gap joint after the cement is poured. These cause no problem if the bead is machined off. A small wire (not copper), or similar objects may be used to lift some bubbles out of the joint; however, the cement joint should be disturbed as little as possible.
(5) Polymerizable cements shrink as the cement hardens. Therefore, the freshly poured cement bead should be left above the surfaces being cemented to compensate for the shrinkage.
If it is necessary for appearances, the bead may be machined off after the cement has set.
3-24. REPAIR OF PLASTICS. Replace, rather than repair extensively damaged transparent plastic, whenever possible, since even a carefully patched part is not the equal of a new section, either optically or structurally. At the first sign of crack development, drill a small hole with a # 30 or a 1/8-inch drill at the extreme ends of the cracks as shown in Figure 3-21. This serves to localize the cracks and to prevent further splitting by distributing the strain over a large area. If the cracks are small, stopping them with drilled holes will usually suffice until replacement or more permanent repairs can be made. The following repairs are permissible; however, they are not to be located in the pilot’s line of vision during landing or normal flight.
FIGURE 3-21. STOP-DRILLING CRACKS
a.
Surface Patch. If a surface patch is to be installed, trim away the damaged area and round all corners. Cut a piece of plastic of sufficient size to cover the damaged area and extend at
Par 3-24 Page 3-27 least 3/4 inch on each side of the crack or hole. Bevel the edges as shown in Figure 3-22. If the section to be repaired is curved, shape the patch to the same contour by heating it in an oil bath at a temperature of 248 to 302 ºF, or it may be heated on a hot-plate until soft. Boiling water should not be used for heating. Coat the patch evenly with plastic solvent adhesive and immediately place it over the hole. Maintain a uniform pressure of 5 to 10 pounds per square inch (psi) on the patch for a minimum of 3 hours. Allow the patch to dry 24 to 36 hours before sanding or polishing.
FIGURE 3-22. SURFACE PATCHES
b. Plug Patch. When using inserted patches to repair holes in plastic structures, trim the holes to a perfect circle or oval and bevel the edges slightly. Make the patch slightly thicker than the material being repaired and similarly bevel its edges. Install patches in accordance with Figure 3-23. Heat the plug until soft and press it into the hole without cement and allow to cool to make a perfect fit. Remove the plug, coat the edges with adhesive, and then reinsert in the hole. Maintain a firm light pressure until the cement has set. Sand or file the edges level with the surface, then buff and polish.
FIGURE 3-23. PLUG PATCH REPAIR
PATCHES PATCH SHOULD BE
THICKER
PATCH AND HOLE PATCH TAPERED
SHOULD BE TRIMMED ON SHARPER WITH TAPERED EDGES. ANGLE THAN MATERIAL.
HEAT EDGES OF DURING CEMENTING-
PATCH UNTIL PRESSURE NEED
SOFT, AND BE APPLIED
FORCE IT INTO ONLY ON TOP HOLE. HOLD SURFACE. TAPER
IT IN PLACE ASSURES EQUAL
UNTIL COOL PRESSURE ON ALL AND HARD TO SIDES.
ASSURE PERFECT FIT.
THEN REMOVE
PATCH FOR AFTER CEMENT HAS
CEMENTING HARDENED - SAND OR BATH. FILE EDGES LEVEL WITH SURFACE.
Par 3-24 Page 3-28
Par 3-25 Page 3-29
3-25. CLEANING AND POLISHING TRANSPARENT PLASTIC. Plastics have many advantages over glass for aircraft use, but they lack the surface hardness of glass and care must be exercised while servicing the aircraft to avoid scratching or otherwise damaging the surface.
a. Clean the plastic by washing it with plenty of water and mild soap, using a clean, soft, grit-free cloth, sponge, or bare hands. Do not use gasoline, alcohol, benzene, acetone, carbon tetrachloride, fire extinguisher or deicing fluids, lacquer thinners, or window cleaning sprays.
These will soften the plastic and cause crazing.
b. Plastics should not be rubbed with a dry cloth since this is likely to cause scratches, and also to build up an electrostatic charge that attracts dust particles to the surface. If after removing dirt and grease, no great amount of scratching is visible, finish the plastic with a good grade of commercial wax. Apply the wax in a thin even coat and bring to a high polish by rubbing lightly with a soft cloth.
c. Do not attempt hand polishing or buffing until the surface is clean. A soft, open-type cotton or flannel buffing wheel is suggested. Minor scratches may be removed by vigorously rubbing the affected area by hand, using a soft clean cloth dampened with a mixture of turpentine and chalk, or by applying automobile cleanser with a damp cloth. Remove the cleaner and polish with a soft, dry cloth. Acrylic and cellulose acetate plastics are thermoplastic. Friction created by buffing or polishing too long in one spot can generate sufficient heat to soften the surface. This condition will produce visual distortion and should be avoided.
3-26. REPLACEMENT PANELS. Use material equivalent to that originally used by the manufacturer of the aircraft for replacement panels. There are many types of transparent plastics on the market. Their properties vary greatly, particularly in regard to expansion characteristics, brittleness under low temperatures, resistance to discoloration when exposed to sunlight, surface checking, etc. Information on these properties is in MIL-HDBK-17 (current revision), Plastics for Flight Vehicles, Part II—Transparent Glazing Materials, available from the Government Publishing Office (GPO). These properties are considered by aircraft manufacturers in selecting materials to be used in their designs and the use of substitutes having different characteristics may result in subsequent difficulties.
3-27. INSTALLATION PROCEDURES. When installing a replacement panel, use the same mounting method employed by the manufacturer of the aircraft. While the actual installation will vary from one type of aircraft to another, consider the following major principles when installing any replacement panel.
a. Never force a plastic panel out of shape to make it fit a frame. If a replacement panel does not fit easily into the mounting, obtain a new replacement or heat the whole panel and reform. When possible, cut and fit a new panel at ordinary room temperature.
b. In clamping or bolting plastic panels into their mountings, do not place the plastic under excessive compressive stress. It is easy to develop more than 1,000 psi on the plastic by over-torquing a nut and bolt. Tighten each nut to a firm fit, then back the nut off one full turn (until they are snug and can still be rotated with the fingers).
Par 3-27 Page 3-30
c. In bolted installations, use spacers, collars, shoulders, or stop-nuts to prevent tightening the bolt excessively. Whenever such devices are used by the aircraft manufacturer, retain them in the replacement installation. It is important that the original number of bolts, complete with washers, spacers, etc., be used. When rivets are used, provide adequate spacers or other satisfactory means to prevent excessive tightening of the frame to the plastic.
d. Mount plastic panels between rubber, cork, or other gasket material to make the installation waterproof, to reduce vibration, and to help to distribute compressive stresses on the plastic.
e. Plastics expand and contract considerably more than the metal channels in which they are mounted. Mount windshield panels to a sufficient depth in the channel to prevent it from falling out when the panel contracts at low temperatures or deforms under load. When the manufacturer’s original design permits, mount panels to a minimum depth of 1-1/8 inch, and with a clearance of 1/8 inch between the plastic and bottom of the channel.
f. In installations involving bolts or rivets, make the holes through the plastic oversize by 1/8-inch diameter and center so that the plastic will not bind or crack at the edge of the holes.
The use of slotted holes is also recommended.
3-28. –3-39. [RESERVED.]
Par 3-40 Page 3-31 SECTION 4. WINDSHIELDS, ENCLOSURES, AND WINDOWS
3-40. GENERAL. These repairs are applicable to plastic windshields, enclosures, and windows in nonpressurized airplanes. For pressurized airplanes, replace or repair plastic elements in accordance with the manufacturer’s recommendation. When windshields and side windows made of acrylic plastics are damaged, they are usually replaced unless the damage is minor and a repair would not be in the line of vision. Repairs usually require a great deal of labor.
Replacement parts are readily available, so replacement is normally more economical than repair.
a. Minor Repairs. There are times, however, when a windshield may be cracked and safety is not impaired. In that case, repairs can be made by stop-drilling the ends of the crack with a # 30 drill (1/8 inch) to prevent the concentration of stresses causing the crack to continue.
Drill a series of number 40 holes a half-inch from the edge of the crack about a half-inch apart, and lace through these holes with brass safety wire (see Figure 3-24) and seal with clear silicone to waterproof.
b. Temporary Repairs. One way to make a temporary repair is to stop-drill the ends of the crack, and then drill number 27 holes every inch or so in the crack. Use AN515-6 screws and AN365-632 nuts with AN960-6 washers on both sides of the plastic. This will hold the crack together and prevent further breakage until the windshield can be properly repaired or replaced.
(See Figure 3-24.)
FIGURE 3-24. TEMPORARY REPAIRS TO CRACKED WINDSHIELDS OR
WINDOWS
c. P ermanent Repairs. Windshields or side windows with small cracks that affect only the appearance rather than the airworthiness of a sheet may be repaired by first stop-drilling the ends of the crack with a # 30 or a 1/8-inch drill. Then use a hypodermic syringe and needle to fill the crack with polymerizable cement such as PS-30 or Weld-On 40, and allow capillary action to fill the crack completely. Soak the end of a 1/8-inch acrylic rod in cement to form a cushion and insert it in the stop-drilled hole. Allow the repair to dry for about 30 minutes, and then trim the rod off flush with the sheet.
Par 3-40 Page 3-32
d. Polishing and Finishing. Scratches and repair marks, within certain limitations, can be removed from acrylic plastic. No sanding that could adversely affect the plastic’s optical properties and distort the pilot’s vision should be done on any portion of a windshield.
(1) If there are scratches or repair marks in an area that can be sanded, they may be removed by first sanding the area. Use 320- or 400-grit abrasive paper that is wrapped around a felt or rubber pad.
(2) Use circular rubbing motions, light pressure, and a mild liquid soap solution as a lubricant. After the sanding is complete, rinse the surface thoroughly with running water. Then, using a 500-grit paper, continue to sand lightly. Keep moving to higher grit paper and sand and rinse until all of the sanding or repair marks have been removed.
(3) After using the finest abrasive paper, use rubbing compound and buff in a circular motion to remove all traces of the sanding.
e. Cleaning. Acrylic windshields and windows may be cleaned by washing them with mild soap and running water. Rub the surface with your bare hands in a stream of water. Follow with the same procedure but with soap and water. After the soap and dirt have been flushed away, dry the surface with a soft, clean cloth or tissue and polish it with a windshield cleaner especially approved for use on aircraft transparent plastics. These cleaners may be purchased through aircraft supply houses.
f. Waxing. A thin coating of wax will fill any minute scratches that may be present and will cause rain to form droplets that are easily blown away by the wind.
3-41. PROTECTION. Acrylic windshields are often called “lifetime” windshields, to distinguish them from those made of the much shorter-lived acetate material. However, even acrylic must be protected from the ravages of the elements.
a. When an aircraft is parked in direct sunlight, the windshield will absorb heat and will actually become hotter than either the inside of the aircraft or the outside air. The sun will cause the inside of a closed aircraft to become extremely hot, and this heat is also absorbed by the plastic windshield.
b. To protect against this damage, it is wise to keep the aircraft in a hangar. If this is not possible, some type of shade should be provided to keep the sun from coming in direct contact with the windshield. Some aircraft owners use a close-fitting, opaque, reflective cover over the windshield. In many cases, this has done more harm than good. This cover may absorb moisture from the air and give off harmful vapors, and if it touches the surface of the plastic, it can cause crazing or minute cracks to form in the windshield. Another hazard in using such a cover is that sand can blow up under the cover and scratch the plastic.
3-42. WINDSHIELD INSTALLATION. Aircraft windshields may be purchased either from the original aircraft manufacturer or from any of several FAA-PMA sources. These windshields are formed to the exact shape required, but are slightly larger than necessary so they may be trimmed to the exact size.
Par 3-42 Page 3-33
a. After removing the damaged windshield, clean all of the sealer from the grooves and cut the new windshield to fit. New windshields are covered with either protective paper or film to prevent damage during handling or installation. Carefully peel back just enough of this covering to make the installation. The windshield must fit in its channels with about 1/8- to 1/4-inch clearance to allow for expansion and contraction. If any holes are drilled in the plastic for screws, they should be about 1/8 inch oversize.
b. Place the sealing tape around the edges of the windshield and install the windshield in its frame. Screws that go through the windshield should be tightened down snug and then backed out a full turn, so the plastic can shift as it expands and contracts.
c. Do not remove the protective paper or film until the windshield is installed and all of the securing screws are in place.
3-43. –3-47. [RESERVED.]
Par 4-1 Page 4-1 CHAPTER 4. METAL STRUCTURE, WELDING, AND BRAZING SECTION 1. IDENTIFICATION OF METALS
4-1. GENERAL. Proper identification of the aircraft structural material is the first step in ensuring that the continuing airworthiness of the aircraft will not be degraded by making an improper repair using the wrong materials.
a. Ferrous (iron) alloy materials are generally classified according to carbon content.
(See Table 4-1.)
TABLE 4-1. FERROUS (IRON) ALLOY MATERIALS
MATERIALS CARBON CONTENT Wrought iron Trace to 0.08% Low carbon steel 0.08% to 0.30% Medium carbon steel 0.30% to 0.60% High carbon steel 0.60% to 2.2% Cast iron 2.3% to 4.5%
b. The strength and ductility, or toughness of steel, is controlled by the kind and quantity of alloys used and also by cold-working or heat-treating processes used in manufacturing. In general, any process that increases the strength of a material will also decrease its ductility.
c. Normalizing is heating steel to approximately 150 °F to 225 °F above the steel’s critical temperature range, followed by cooling to below that range in still air at ordinary temperature. Normalizing may be classified as a form of annealing. This process also removes stresses due to machining, forging, bending, and welding. After the metal has been held at this temperature for a sufficient time to be heated uniformly throughout, remove the metal from the furnace and cool in still air. Avoid prolonging the soaking of the metal at high temperatures, as this practice will cause the grain structure to enlarge. The length of time required for the soaking temperature depends on the mass of the metal being treated. The soaking time is roughly 1/4 hour per inch of the diameter of thickness (refer to Military Technical Order (TO) 1-1A-9).
4-2. IDENTIFICATION OF STEEL STOCK. The Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI) use a numerical index system to identify the composition of various steels. The numbers assigned in the combined listing of standard steels issued by these groups represent the type of steel and make it possible to readily identify the principal elements in the material.
a. The basic numbers for the four digit series of the carbon and alloy steel may be found in Table 4-2. The first digit of the four number designation indicates the type to which the steel belongs. Thus, “1” indicates a carbon steel, “2” a nickel steel, “3” a nickel chromium steel, etc. In the case of simple alloy steels, the second digit indicates the approximate percentage of the predominant alloying element. The last two digits usually indicate the mean of the range
Par 4-2 Page 4-2 of carbon content. Thus, the designation “1020” indicates a plain carbon steel lacking a principal alloying element and containing an average of 0.20 percent (0.18 to 0.23) carbon. The designation “2330” indicates a nickel steel of approximately 3 percent (3.25 to 3.75) nickel and an average of 0.30 percent (0.28 to 0.33) carbon content. The designation “4130” indicates a chromium molybdenum steel of approximately 1 percent (0.80 to 1.10) chromium, 0.20 percent
(0.15 to 0.25) molybdenum, and 0.30 percent (0.28 to 0.33) carbon.
TABLE 4-2. NUMERICAL SYSTEM FOR STEEL IDENTIFICATION
TYPES OF STEELS NUMERALS AND DIGITS Plain carbon steel 10XX Carbon steel with additional sulfur for easy machining 11XX Carbon steel with about 1.75% manganese 13XX .25% molybdenum 40XX 1% chromium, .25% molybdenum 41XX 2% nickel, 1% chromium, .25% molybdenum 43XX
1.7% nickel, .2% molybdenum 46XX
3.5% nickel, .25% molybdenum 48XX 1% chromium steels 51XX 1% chromium, 1.00% carbon 51XXX
1.5% chromium steels 52XX
1.5% chromium, 1.00% carbon 52XXX 1% chromium steel with .15% vanadium 61XX .5% chromium, .5% nickel, .20% molybdenum 86XX .5% chromium, .5% nickel, .25% molybdenum 87XX 2% silicon steels, .85% manganese 92XX
3.25% nickel, 1.20% chromium, .12% molybdenum 93XX
b. There are numerous steels with higher percentages of alloying elements that do not fit into this numbering system. These include a large group of stainless and heat resisting alloys in which chromium is an essential alloying element. Some of these alloys are identified by three digit AISI numbers and many others by designations assigned by the steel company that produces them. The few examples in Table 4-3 will serve to illustrate the kinds of designations used and the general alloy content of these steels.
Par 4-2 Page 4-3 TABLE 4-3. EXAMPLES OF STAINLESS AND HEAT-RESISTANT STEELS
NOMINAL COMPOSITION (PERCENT) ALLOY
DESIGNATION CARBON CHROMIUM NICKEL OTHER GENERAL CLASS OF STEEL 302 0.15 18 9 Austenitic 310 0.25 25 20 Austenitic 321 0.08 18 11 Titanium Austenitic 347 0.08 18 11 Columbium or Tantalum Austenitic 410 0.15 12.5 Martensitic, Magnetic 430 0.12 17 Ferritic, Magnetic 446 0.20 25 Nitrogen Ferritic, Magnetic PH15-7 Mo 0.09 15 7 Molybdenum, Aluminum Precipitation Hardening 17-4 PH 0.07 16.5 4 Copper, Columbium, or Tantalum Precipitation Hardening
c. “1025” welded tubing as per SAE Specification AMST5066B and “1025” seamless tubing are interchangeable.
4-3. INTERCHANGEABILITY OF STEEL TUBING.
a. “4130” welded tubing conforming to SAE Specification AMST6736B and “4130” seamless tubing are interchangeable.
b. NE-8630 welded tubing conforming to SAE Specification AMST6736B and NE-8630 seamless tubing are interchangeable.
4-4. IDENTIFICATION OF ALUMINUM.
a. To provide a visual means for identifying the various grades of aluminum and aluminum alloys, such metals are usually marked with symbols such as a government specification number, the temper or condition furnished, or the commercial code marking.
b. Plate and sheet are usually marked with specification numbers or code markings in rows approximately 5 inches apart. Tubes, bars, rods, and extruded shapes are marked with specification numbers or code markings at intervals of 3 to 5 feet along the length of each piece.
The commercial code marking consists of a number which identifies the particular composition of the alloy. In addition, letter suffixes (see Table 4-4) designate the basic temper designations and subdivisions of aluminum alloys.