ALL THE STRAINS WHICH ORIGINALLY CAUSED
THE CRACK ARE CONCENTRATED AT POINT A - TENDING TO EXTEND THE CRACK. THEREFORE DRILL, WITH A #30 OR 1/8" DRILL BIT, A SMALL HOLE AT END OF THE CRACK POINT A1 TO DISTRIBUTE THE STRAIN OVER A WIDER AREA.
EACH CRACK OCCURRING AT ANY HOLE OR TEAR IS DRILLED IN SAME MANNER.
FIGURE 3-21. Stop-drilling cracks.
Surface Patch. If a surface patch is to be installed, trim away the damaged area and round all corners. Cut a RFACE PATCH piece of plastic of sufficient size to cover the damaged area and extend at least 3/4 inch on each side of the STOI crack or hole. Bevel the edges as shown in figure 3-22. CRÁCK DRILLEC 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 °F, or it may be heated on a hot-plate until soft. Boiling water should not be used SURFACE PATCH FOR CRACKS BEVELED EDGE 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 psi on the patch TRAM AL CORNERND SURFACE PATCH for a minimum of 3 hours. Allow the patch to dry CORNE SHARP 24 to 36 hours before sanding or polishing → BEVELED Plug Patch. When using inserted patches to repair SURFACE PATCH FOR IRREGULAR SHAPED DAMAGE / "EDGE 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 ACE PATCH ROUND HOLE SÉVELED EDGE
SURFACE PATCH FOR ROUND HOLES FIGURE 3-22. Surface patches.
Par 3-23 Page 3-96
Par 3-24 Page 3-97 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.
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.
Clean the plastic by washing it with plenty of water and mild soap, using a cl ean, 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.
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.
Do not attempt hand polishing or buffing until the surface is clean. A soft, ope n-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.
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
Par 3-40 Page 3-98 temperatures, resistance to discolora tion when exposed to sunlight, surface checking, etc. Information on these properties is in MIL-HDBK-17A, Plastics for Flight Vehicles, Part II Transparent Glazing Materials, available from the Government Printing 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.
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.
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.
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 pl astic 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 3-40 Page 3-99 In bolted installations, use spacers, collars, shoulders, or stop-nuts to prevent tightening the bolt excessively.
Whenever such devices are used by th e aircraft manufacturer, retain them in the replacement installation. It is important that the original number of bo lts, 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.
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.
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.
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 3-100 SECTION 4. WINDSHIELDS, ENCLOSURES, AND WINDOWS
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.
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 stre sses 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.
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 a nd prevent further breakage until the windshield can be properly repaired or replaced. (See figure 3-24.)
Permanent 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 us e 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.
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.
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.
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.
After using the finest abrasive paper, use rubbing compound and buff in a circular motion to remove all traces of the sanding.
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
Par 3-40 Page 3-101
FIGURE 3-24. Temporary repairs to cracked windshields or windows.
windshield cleaner especially approved for use on aircraft transparent plastic s. These cleaners may be purchased through aircraft supply houses.
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.
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.
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.
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 pl astic 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.
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.
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.
Par 3-42 Page 3-102 (and 3-26) 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.
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 4-103
CHAPTER 4. METAL STRUCTURE, WELDING, AND BRAZING
SECTION 1. IDENTIFICATION OF METALS
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.
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%
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.
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.
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 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 chromiummolybdenum 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.
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.
“1025” welded tubing as per Specification MIL-T-5066 and “1025” seamless tubing conforming to Specification MIL-T-5066A are interchangeable.
Par 4-1 Page 4-104
INTERCHANGEABILITY OF STEEL TUBING .
“4130” welded tubing conforming to Specification MIL-T-6731, and “4130” seam-less tubing conforming to Specification MIL-T-6736 are interchangeable.
NE-8630 welded tubing conforming to Specification MIL-T-6734, and NE-8630 seamless tubing conforming to Specification MIL-T-6732 are interchangeable.
IDENTIFICATION OF ALUMINUM. 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. 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.
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
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
Par 4-4 Page 4-105 (and 4-4) TABLE 4-4. Basic temper designations and subdivisions from aluminum alloys.
NON HEAT-TREATABLE ALLOYS HEAT-TREATABLE ALLOYS Temper Designation
Definition Temper Designation
Definition -0 Annealed recrystallized (wrought products only) applies to softest temper of wrought products.
-0 Annealed recrystallized (wrought products only) applies to softest temper of wrought products.
-H1 Strain-hardened only. Applies to products which are strain-hardened to obtain the desired strength without supplementary thermal treatment.
-T1 Cooled from an elevated temperature shaping process (such as extrusion or casting) and naturally aged to a substantially stable condition.
-H12 Strain-hardened one-quarter-hard tem per. -T2 Annealed (castings only).
-H14 Strain-hardened half-hard temper. -T3 Solution heat-treated and cold-worked by the flattening or straightening operation.
-H16 Strain-hardened three-quarters-hard temper. -T36 Solution heat-treated and cold-worked by reduction of 6 percent -H18 Strain-hardened full-hard temper. -T4 Solution heat-treated.
-H2 Strain-hardened and then partially annealed.
Applies to products which are strain-hardened more than the desired final amount and then reduced in strength to the desired level by partial annealing.
-T42 Solution heat-treated by the user regardless of prior temper (applicable only to 2014 and 2024 alloys).
-H22 Strain-hardened and partially annealed to one-quarter-hard temper.
-T5 Artificially aged only (castings only).
-H24 Strain-hardened and partially annealed to half-hard temper.
-T6 Solution heat-treated and artificially aged.
-H26 Strain-hardened and partially annealed to three-quarters-hard temper.
-T62 Solution heat-treated and aged by user regardless of prior temper (applicable only to 2014 and 2024 alloys).
-H28 Strain-hardened and partially annealed to full-hard temper.
-T351, -T451, -T3510, -T3511, -T4510, -T4511.
Solution heat-treated and stress relieved by stretching to produce a permanent set of 1 to 3 percent, depending on the product.
-H3 Strain-hardened and then stabilized. Applies to products which are strain-hardened and then stabilized by a low temperature heating to slightly lower their strength and increase ductility.
-T651, -T851, -T6510, -T8510, -T6511, -T8511.
Solution heat-treated, stress relieved by stretching to produce a permanent set of 1 to 3 percent, and artificially aged.
-H32 Strain-hardened and then stabilized. Final temper is one-quarter hard.
-T652 Solution heat-treated, compressed to produce a permanent set and then artificially aged.
-H34 Strain-hardened and then stabilized. Final temper is one-half hard.
-T8 Solution heat-treated, cold-worked and then artificially aged.
-H36 Strain-hardened and then stabilized. Final temper is three-quarters hard.
-T/4 Solution heat-treated, cold-worked by the flattening or straightening operation, and then artificially aged.
-H38 Strain-hardened and then stabilized. Final temper is full-hard.
-T86 Solution heat-treated, cold-worked by reduction of 6 percent, and then artificially aged.
-H112 As fabricated; with specified mechanical property limits.
-T9 Solution heat-treated, artificially aged and then cold-worked.
-F For wrought alloys; as fabricated. No mechanical properties limits. For cast alloys; as cast.
-T10 Cooled from an elevated temperature shaping process artificially aged and then cold-worked.
-F For wrought alloys; as fabricated. No mechanical properties limits. For cast alloys; as cast.
4-5.4-15. [RESERVED.]
Par 4-16 Page 4-106
SECTION 2. TESTING OF METALS
HARDNESS TESTING. If the material type is known, a hardness test is a simple way to verify that the part has been properly heattreated. Hardness testers such as Rockwell, Brinell, and Vickers can be useful to check metals for loss of strength due to exposure to fire or abusive heating. Also, under-strength bolts can be found and removed from the replacement part inventory by checking the hardness of the bolt across the hex flats.
Although hardness tests are generally considered nondestructive, hardness testing does leave a small pit in the surface; therefore, hardness tests should not be used on sealing surfaces, fatigue critical parts, load bearing areas, etc., that will be returned to service.
These hardness tests provide a convenient means for determining, within reasonable limits, the tensile strength of steel. It has several limitations in that it is not suitable for very soft or very hard steels. Hardness testing of aluminum alloys should be limited to distinguishing between annealed and heattreated material of the same aluminum alloy. In hardness testing, the thickness and the edge distance of the specimen being tested are two factors that must be considered to avoid distortion of the metal. Several readings should be taken and the results averaged. In general, the higher the tensile strength, the greater its hardness. Common methods of hardness testing are outlined in the following paragraphs. These tests are suitable for determining the tensile properties resulting from the heat treatment of steel. Care should be taken to have case-hardened, corroded, pitted, decarburized, or otherwise nonuniform surfaces removed to a sufficient depth.
Exercise caution not to cold-work, and consequently harden, the steel during removal of the surface.
ROCKWELL HARDNESS TEST. The Rockwell hardness test is the most common method for determining hardness of ferrous and many nonferrous metals. (See table 4-5.) It differs from Brinell hardness testing in that the hardness is determined by the depth of indentation made by a constant load impressing on an indenter. In this test, a standard minor load is applied to set a hardened steel ball or a diamond cone in the surface of the metal, followed by the application of a standard major load. The hardness is measured by depth of penetration.
Rockwell superficial hardness tests are made using light minor and major loads and a more sensitive system for measuring depth of indentation. It is useful for thin sections, very small parts, etc. Calibration of Rockwell hardness testers is done in accordance with American Society of Testing Materials (ASTM E-18) specifications.
BRINELL HARDNESS TEST. In this test a standard constant load, usually 500 to 3,000 kg, is applied to a smooth flat metal surface by a hardened steel-ball type indenter, 10 mm in diameter. The 500-kg load is usually used for testing nonferrous metals such as copper and aluminum alloys, whereas the 3,000-kg load is most often used for testing harder metals such as steels and cast irons. The numerical value of Brinell Hardness (HB), is equal to the load, divided by the surface area of the resulting spherical impression.
Where P is the load, in kg; D is the diameter of the ball, in mm; and d is the diameter of the indentation, in mm.
Par 4-20 Page 4-107 General Precautions. To avoid misapplication of Brinell hardness testing, the fundamentals and limitations of the test procedure must be clearly understood. To avoid inaccuracies, the following rules should be followed.
Do not make indentations on a curved surface having a radius of less than 1 inch.
Do make the indentations with the correct spacing. Indentations should not be made too close to the edge of the work piece being tested.
Apply the load steadily to avoid overloading caused by inertia of the weights.
Apply the load so the direction of loading and the test surface are perpendicular to each other within 2 degrees.
The thickness of the work piece being tested should be such that no bulge or mark showing the effect of the load appears on the side of the work piece opposite the indentation.
The indentation diameter should be clearly outlined.
Limitations. The Brinell hardness test has three principal limitations.
The work piece must be capable of accommodating the relatively large indentations.
Due to the relatively large indentations, the work piece should not be used after testing.
The limit of hardness, 15 HB with the 500-kg load to 627 HB with the 3,000-kg load, is generally considered the practical range.
Calibration. A Brinell Hardness Tester should be calibrated to meet ASTM standard E10 specifications.
VICKERS HARDNESS TEST. In this test, a small pyramidal diamond is pressed into the metal being tested. The Vickers Hardness number (HV) is the ratio of the load applied to the surface area of the indention. This is done with the following formula.
The indenter is made of diamond, and is in the form of a square-based pyramid having an angle of 136 degrees between faces. The facets are highly-polished, free from surface imperfections, and the point is sharp. The loads applied vary from 1 to 120 kg; the standard loads are 5, 10, 20, 30, 50, 100, and 120 kg. For most hardness testing, 50 kg is maximum.
A Vickers hardness tester should be calibrated to meet ASTM standard E10 specifi cations, acceptable for use over a loading range.
MICROHARDNESS TESTING. This is an indentation hardness test made with loads not exceeding 1 kg (1,000 g). Such hardness tests have been made with a load as light as 1 g, although the majority of microhardness tests are made with loads of 100 to 500 g. In general, the term is related to the size of the indentation rather than to the load applied.