(4) Deodorizing or Disinfecting Agents.
number of proprietary agents useful for aircraft cabin deodorizing or disinfecting are nonflammable.
Most of these are designed for spray application (aerosol type) and have a nonflammable pressurizing agent but it is well to check this carefully as some may contain a flammable compressed gas for pressurization.
(5) Abrasives. Some proprietary nonflammable mild abrasive materials are available for rejuvenating painted or polished surfaces. They present no
(6) Dry Cleaning Agents. Perchlorethylene and trichlorethylene as used at ambient temperatures are examples of nonflammable dry cleaning agents.
These materials do have a toxicity hazard requiring care in their use. Fire retardant treated materials may be adversely affected by the application of these agents as is true of the water-soluble agents.
Flammable and Combustible Agents
(1) High Flash Point Solvents. Specially refined petroleum products, first developed as ¿Stoddard Solvent" but now sold under a variety of trade names by different companies, have solvent properties approximating gasoline but have fire hazard properties similar to those of kerosene as commonly used (not heated). Most of these are stable products having a flash point from 100° F.
to 140° F. with a comparatively low degree of toxicity.
(2) Low Flash Point Solvents. Class I (flash point at below 100° F.) flammable liquids should not be used for aircraft cleaning or refurbishing.
Common materials falling into this "Class" are acetone, aviation gasoline, methyl ethyl ketone, naphtha and toluol.
In cases where it is absolutely necessary to use a flammable liquid, high flash point liquids (those having a flash point of 100° F. or more) should be used.
(3) Mixed Liquids. Some commercial solvents are mixtures of liquids with differing rates of evaporation such as a mixture of one of the various naphthas and a chlorinated material. The different rates of evaporation may present problems from both the toxicity and fire hazard viewpoints and such mixtures should not be used unless they are stored and handled with full knowledge of these hazards and appropriate precautions taken.
Container Controls Flammable liquids should be handled only in approved containers or safety cans appropriately labeled.
Fire Prevention Precautions During aircraft cleaning or refurbishing operations where flammable or combustible liquids are used, the following general safeguards are recom-
(1) Aircraft cabins should be provided with ventilation sufficient at all times to prevent the accumulation of flammable vapors. To accomplish this, doors to cabins shall be open to secure maximum advantage of natural ventilation. Where such natural ventilation is insufficient under all conditions to prevent the accumulation of flammable vapors, approved mechanical ventilation equipment shall be provided and used. The accumulation of flammable vapors above 25 percent of the lower flammability limit of the particular vapor being used, measured at a point five feet from the location of use, shall result in emergency revisions of operations in progress.
(2) All open fame and spark producing equipment or devices that might be brought within the vapor hazard area should be shut down and not operated during the period when flammable vapors may exist.
Electrical equipment of a hand portable nature used within an aircraft cabin shall be of the type approved for use in Class I, Group D, Hazardous Locations as defined by the National Electrical Code.
Switches to aircraft cabin lighting and to the aircraft electrical system components within the cabin area should not be worked on or switched on or off during cleaning
(5) Suitable warning signs should be placed in conspicuous locations at aircraft doors to indicate that flammable liquids are being or have been used in the cleaning or refurbishing operation in progress.
Fire Protection Recommendations During aircraft cleaning or refurbishing operations where flammable liquids are used the following general fire protection safeguards are recommended:
(1) Aircraft undergoing such cleaning or refurbishing should preferably be located outside of the hangar buildings when weather conditions permit. This provides for added natural ventilation and normally assures easier access to the aircraft in the event of fire.
(2) It is recommended that during such cleaning or refurbishing operations in an aircraft outside of the hangar that portable fire extinguishers be provided at cabin entrances having a minimum rating of 20-B and, at minimum, a booster hoseline with an adjustable water spray nozzle be available capable of reaching the cabin area for use pending the arrival of airport fire equipment. As an alternate to the previous recommendations a Class A fire extinguisher having a minimum rating of 4-A plus or a Class B fire extinguisher having a minimum rating of 20-B should be placed at aircraft cabin doors for immediate use if required.
NOTE 1: All-purpose (dry chemical) type extinguishers should not be used in situations where aluminum corrosion is a problem.
NOTE 2: Portable and semi-portable fire detection and extinguishing equipment has been developed, tested and installed to provide protection to aircraft during construction and maintenance operations. Operators are urged to investigate the feasibility of utilizing such equipment during aircraft cabin cleaning and refurbishing operations.
(3) Aircraft undergoing such cleaning or refurbishing where the work must be done under cover should be in hangars equipped with automatic fire protection equipment.
POWERPLANT CLEANING Cleaning the powerplant is an important job and should be done thoroughly. Grease and dirt accumulations on an air-cooled engine provide an effective insulation against the cooling effect of air flowing over it. Such an accumulation can also cover up cracks or other defects.
When cleaning an engine, open or remove the cowling. Beginning with the top, the engine and accessories are washed down with a fine spray of kerosene or solvent. A bristle brush may be used to help clean some of the surfaces.
Fresh water and soap, and approved cleaning solvents may be used for cleaning propeller and rotor blades. Except in the process of etching.
caustic material should not be used on a propeller.
Scrapers, power buffers, steel brushes, or any tool or substances that will mar or scratch the surface should not be used on propeller blades, except as recommended for etching and repair.
Water spray, rain, or other airborne abrasive material strikes a whirling propeller blade with such force that small pits are formed in the blade's leading edge. If preventive measures are not taken, corrosion causes these pits to rapidly grow larger.
The pits may become so large that it is necessary to file the blade's leading edge until it is smooth.
Steel propeller blades have more resistance to abrasion and corrosion than aluminum alloy blades. Steel blades, if rubbed down with oil after each flight, retain a smooth surface for a long time.
Propellers should be examined regularly because cracks in steel or aluminum alloy blades can become filled with oil which tends to oxidize. This can readily be seen when the blade is inspected Keeping the surface wiped with oil serves as a safety feature by helping to make cracks more Propeller hubs must be inspected regularly for cracks and other defects. Unless the hubs are kept clean, defects may not be found. Steel hubs should be cleaned with soap and fresh water, or with an approved cleaning solvent. These cleaning solvents may be applied by cloths or brushes. Tools and abrasives that scratch or otherwise damage the plating should be avoided.
In special cases in which a high polish is desired, the use of a good grade of metal polish is recommended. Upon completion of the polishing, all traces of polish must be removed immediately, the blades cleaned, and then coated with clean engine oil.
All cleaning substances must be removed immediately after completion of the cleaning of any propeller part. Soap in any form can be removed by rinsing repeatedly with fresh water.
After rinsing, all surfaces should be dried and coated with clean engine oil.
After cleaning the powerplant, all control arms, belleranks, and moving parts should be lubricated according to instructions in the applicable maintenance manual.
SOLVENT CLEANERS In general, solvent cleaners used in aircraft cleaning should have a flashpoint of not less than 105° F., if explosion proofing of equipment and other special precautions are to be avoided.
Chlorinated solvents of all types meet the nonflammable requirements but are toxic, and safety precautions must be observed in their use. Use of carbon tetrachloride should be avoided.
Dry-cleaning Solvent Stoddard solvent is the most common petroleum-base solvent used in aircraft cleaning. Its 188
flashpoint is slightly above 105° F. and can be used to remove grease, oils, or light soils. Drycleaning solvent is preferable to kerosene for all cleaning purposes, but like kerosene it leaves a slight residue upon evaporation which may interfere with the application of some final paint films.
Aliphatic and Aromatic Naphtha Aliphatic naphtha is recommended for wipedown of cleaned surfaces just before painting. This material can also be used for cleaning acrylics and rubber. It flashes at approximately 80° F. and must be used with care.
Aromatic naphtha should not be confused with the aliphatic material. It is toxic and attacks acrylics and rubber products, and must be used with adequate controls.
Safety Solvent Safety solvent, trichloroethane (methyl chloroform), is used for general cleaning and grease removal. It is nonflammable under ordinary circumstances, and is used as a replacement for carbon tetrachloride. The use and safety precautions necessary when using chlorinated solvents must be observed. Prolonged use can cause dermatitis on some persons.
Methyl Ethyl Ketone (MEK) MEK is also available as a solvent cleaner for metal surfaces and paint stripper for small areas.
This is a very active solvent and metal cleaner, with a flashpoint of about 24° F. It is toxic when inhaled, and safety precautions must be observed during its use.
Kerosene is mixed with solvent-emulsion type cleaners for softening heavy preservative coatings.
It is also used for general solvent cleaning, but its use should be followed by a coating or rinse with some othes type of protective agent. Kerosene does not evaporate as rapidly as dry cleaning solvent and generally leaves an appreciable film on cleaned surfaces, which may actually be corrosive. Kerosene films may be removed with safety solvent, water emulsion cleaners, or detergent mixtures.
Cleaning Compound For Oxygen Systems Cleaning compounds for use in the oxygen system are anhydrous (waterless) ethyl alcohol, isopropyl (anti-icing fluid) alcohol, or a mixture of may ho wed freon and isopropyl alcohol. These may be used to clean accessible components of the oxygen system such as crew masks and lines. Fluids should not be put into tanks or regulators.
Do not use any cleaning compounds which may leave an oily film when cleaning oxygen equipment.
Avoid prolonged skin contact with the freon-alcohol mixture or breathing of the vapors. Instructions of the manufacturer of the oxygen equipment and cleaning compounds must be followed at all times.
EMULSION CLEANERS Solvent- and water-emulsion compounds are used in general aircraft cleaning. Solvent emulsions are particularly useful in the removal of heavy deposits, such as carbon, grease, oil, or tar. When used in accordance with instructions, these solvent emulsions do not affect good paint coatings or organic finishes.
Water-Emulsion Cleaner Material available under Specification MIL-C- 22543A is a water-emulsion cleaning compound intended for use on both painted and unpainted aircraft surfaces. This material is also acceptable for cleaning fluorescent painted surfaces and is safe for use on acrylics. However, these properties will vary with the material available, and a sample application should be checked carefully before general uncontrolled use.
Solvent-Emulsion Cleaners One type of solvent-emulsion cleaner is nonphenolic and can be safely used on painted surfaces without softening the base paint. Repeated use may soften acrylic nitrocellulose lacquers. It is effective, however, in softening and lifting heavy preservative coatings. Persistent materials should be given a second or third treatment as necessary Another type of solvent-emulsion cleaner has a phenolic base which is more effective for heavyduty application, but it also tends to soften paint coatings. It must be used with care around rubber, plastics, or other nonmetallic materials. Rubber gloves and goggles should be worn for protection when working with phenolic-base cleaners.
SOAPS AND DETERGENT CLEANERS A number of materials are available for mild cleaning use. In this section, some of the more common materials are discussed.
Cleaning Compound, Aircraft Surfaces Specification MIL-C-5410 Type I and II materials are used in general cleaning of painted and unpainted aircraft surfaces for the removal of light-to-medium soils, operational films, oils, or greases. They are safe to use on all surfaces, 189
including fabrics, leather, and transparent plastics.
Nonglare finishes should not be cleaned more than necessary and should never be scrubbed with stiff Nonionic Detergent Cleaners These materials may be either water-soluble or oil-soluble. The oil-soluble detergent cleaner is effective in a 3- to 5-percent solution in dry.
cleaning solvent for softening and removing heavy preservative coatings. This mixture's performance is similar to the emulsion cleaners mentioned
MECHANICAL CLEANING MATERIALS Mechanical cleaning materials must be used with care and in accordance with directions given, if damage to finishes and surfaces is to be avoided.
Mila Abrasive Materials No attempt is made in this section to furnish detailed instructions for using various materials listed. Some do's and don'ts are included as an aid in selecting materials for specific cleaning jobs.
Powdered pumice is used for cleaning corroded aluminum surfaces. Similar mild abrasives may also be used.
Impregnated cotton wadding material is used for removal of exhaust-gas stains and polishing corroded aluminum surfaces. It may also be used on other metal surfaces to produce a high Aluminum metal polish is used to produce a a sure foul ot be wed on.
anodized surfaces because it will remove the oxide Three grades of aluminum wool, coarse, medium, and fine, are used for general cleaning of aluminum surfaces. Impregnated nylon webbing material is preferred over aluminum wool for the removal of corrosion products and stubborn paint films and for the scuffing of existing paint finishes prior to Lacquer rubbing compound material can be used to remove engine exhaust residues and minor oxidation. Heavy rubbing over rivet heads or edges where protective coatings may be worn thin should be avoided.
Abrasive Papers Abrasive papers used on aircraft surfaces should not contain sharp or needlelike abrasives which can imbed themselves in the base metal being cleaned or in the protective coating being maintained. The abrasives used should not corrode the material being cleaned. Aluminum oxide paper, 300 grit or finer, is available in several forms and is safe to use on most surfaces. Type I, Class 2 material under Federal Specification P-C-451 is available in 1½- and 2-inch widths. The use of carborundum (silicon carbide) papers should be avoided, particularly on aluminum or magnesium.
The grain structure of carborundum is sharp, and the material is so hard that individual grains will penetrate and bury themselves even in steel surfaces. The use of emery paper or crocus cloth on aluminum or magnesium can cause serious corrosion of the metal by imbedded iron oxide.
CHEMICAL CLEANERS Chemical cleaners must be used with great care in cleaning assembled aircraft. The danger of entrapping corrosive materials in faying surfaces and crevices counteracts any advantages in their speed and effectiveness. Any materials used must be relatively neutral and easy to remove. It is emphasized that all residue must be removed Soluble salts from chemical surface treatments such as chromic acid or dichromate treatment will liquefy and promote blistering in the paint Phosphoric-Citric Acid A phosphoric-citric acid mixture (Type I) for cleaning aluminum surfaces is available and is ready to use as packaged. Type II is a concentrate which must be diluted with mineral spirits and water. Skin contact should be avoided by wearing rubber gloves and goggles. Any acid burns may be neutralized by copious water washing, followed by treatment with a diluted solution of baking soda (sodium bicarbonate).
Baking Soda Baking soda may be used to neutralize acid deposits in lead-acid battery compartments and to treat acid burns from chemical cleaners and inhibitors.
STRUCTURAL METALS Knowledge and understanding of the uses, strengths, limitations, and other characteristics of structural metals is vital to properly construct and maintain any equipment, especially airframes. In aircraft maintenance and repair, even a slight deviation from design specification, or the substitution of inferior materials, may result in the loss of both lives and equipment. The use of unsuitable materials can readily erase the finest craftsmanship. The selection of the correct material for a specific repair job demands familiarity with the most common physical properties of various Properties of Metais Of primary concern in aircraft maintenance are such general properties of metals and their alloys as hardness malleability, ductility, elasticity, toughness, density, brittleness, fusibility, conductivity contraction and expansion, and so forth.
These terms are explained to establish a basis for further discussion of structural metals.
permits it to be permanently drawn, bent, or twisted into various shapes without breaking. This property is essential for metals used in making wire and tubing. Ductile metals are greatly preferred for aircraft use because of their ease of forming and resistance to failure under shock loads.
For this reason, aluminum alloys are used for cowl rings, fuselage and wing skin, and formed or extruded parts, such as ribs, spars, and bulkheads.
Chrome molybdenum steel is also easily formed into desired shapes. Ductility is similar to malleability.
Elasticity is that property which enables a metal to return to its original shape when the force which causes the change of shape is removed. This property is extremely valuable because it would be highly undesirable to have a part permanently distorted after an applied load was removed. Each metal has a point known as the elastic limit beyond which it cannot be loaded without causing permanent distortion. In aircraft construction, members and parts are so designed that the maximum loads to which they are subjected will not stress them beyond their elastic limits. This Explanation of Terms Hardness refers to the ability of a metal to resist abrasion, penetration, cutting action, or permanent distortion. Hardness may be increased by coldworking the metal and, in the case of steel and certain aluminum alloys, by heat treatment.
Structural parts are often formed from metals in their soft state and are then heat treated to harden them so that the finished shape will be retained. pereined, Hardness and strength are closely associated properties of metals.
Brittleness is the property of a metal which allows little bending or deformation without shattering. A brittle metal is apt to break or crack without change of shape. Because structural metals are often subjected to shock loads, brittleness is not a very desirable property. Cast iron, cast aluminum, and very hard steel are examples of brittle metals.
A metal which can be hammered, rolled, or pressed into various shapes without cracking, breaking, or having some other detrimental effect, is said to be malleable. This property is necessary in sheet metal that is worked into curved shapessuch as cowlings, fairings, or wingtips. Copper is an example of a malleable metal.
Ductility is the property of a metal which A material which possesses toughness will withstand tearing or shearing and may be stretched or otherwise deformed without breaking. Toughness is a desirable property in aircraft metals.
Density is the weight of a unit volume of a material. In aircraft work, the specified weight of a material per cubic inch is preferred since this figure can be used in determining the weight of a part before actual manufacture. Density is an important consideration when choosing a material to be used in the design of a part in order to maintain the proper weight and balance of the Fusibility is the ability of a metal to become liquid by the application of heat. Metals are fused in welding. Steels fuse around 2,600° F. and aluminum alloys at approximately 1,100° F.
Conductivity is the property which enables a metal to carry heat or electricity. The heat conductivity of a metal is especially important in welding because it governs the amount of heat that will be required for proper fusion. Conductivity of the metal, to a certain extent, determines the type of jig to be used to control expansion and contraction. In aircraft, electrical conductivity must also be considered in conjunction with bonding, to eliminate radio interference.
Contraction and expansion are reactions pro- 191
duced in metals as the result of heating or cooling.
Heat applied to a metal will cause it to expand or become larger. Cooling and heating affect the design of welding jigs, castings, and tolerances necessary for hot-rolled material.
Selection Factors Strength, weight, and reliability are three factors which determine the requirements to be met by any material used in airframe construction and repair. Airframes must be strong and yet as light in weight as possible. There are very definite limits to which increases in strength can be accompanied by increases in weight. An airframe so heavy that it could not support a few hundred pounds of additional weight would be of little use.
All metals, in addition to having a good strength/weight ratio, must be thoroughly reliable, thus minimizing the possibility of dangerous and unexpected failures. In addition to these general properties, the material selected for a definite application must possess specific qualities suitable for the purpose.
The material must possess the strength required by the dimensions, weight, and use. There are five basic stresses which metals may be required to withstand. These are tension, compression, shear, bending, and torsion.
The tensile strength of a material is its resistance to a force which tends to pull it apart. Tensile strength is measured in p.s.i. (pounds per square inch) and is calculated by dividing the load, in pounds, required to pull the material apart by its cross-sectional area, in square inches.
The compression strength of a material is its resistance to a crushing force which is the opposite of tensile strength. Compression strength is also measured in p.s.i.
When a piece of metal is cut, the material is subjected, as it comes in contact with the cutting edge, to a force known as shear. Shear is the tendency on the part of parallel members to slide in opposite directions. It is like placing a cord or thread between the blades of a pair of scissors. The shear strength is the shear force in p.s.i. at which a material fails. It is the load divided by the shear area.
Bending can be described as the deflection or curving of a member due to forces acting upon it.
The bending strength of material is the resistance it offers to deflecting forces.
Torsion is a twisting force. Such action would occur in a member fixed at one end and twisted at the other. The torsional strength of material is its resistance to twisting.
The relationship between the strength of a material and its weight per cubie inch, expressed as a ratio, is known as the strength/weight ratio.
This ratio forms the basis for comparing the desirability of various materials for use in airframe construction and repair. Neither strength nor weight alone can be used as a means of true comparison. In some applications, such as the skin of monocoque structures, thickness is more important than strength, and, in this instance, the thickness or gage is best. Thickness or bulk is necessary to prevent buckling or damage caused by careless handling.
Corrosion is the eating away or pitting of the surface or the internal structure of metals. Because of the thin sections and the safety factors used in aircraft design and construction, it would be dangerous to select a material possessing poor corrosion-resistant characteristics.
Another significant factor to consider in maintenance and repair is the ability of a material to be formed, bent, or machined to required shapes. The hardening of metals by cold-working or forming is termed work-hardening. If a piece of metal is forned (shaped or bent) while cold, it is said to be cold-worked. Practically all the work an aviation mechanic does on metal is cold-work. While this is convenient, it causes the metal to become harder and more brittle.
If the metal is cold-worked too much, that is, if it is bent back and forth or hammered at the same place too often, it will crack or break.
Usually, the more malleable and ductile a metal is, the more cold-working it can stand. Any process which involves controlled heating and cooling of metals to develop certain desirable characteristics (such as hardness, softness, ductility, tensile strength, or refined grain structure) is called heat treatment or heat treating. With steels the term "heat treating" has a broad meaning and includes such processes as annealing, normalizing, hardening, and tempering.
In the heat treatment of aluminum alloys, only two processes are included: (1) The hardening and toughening process, and (2) the softening process The hardening and toughening process is called heat treating, and the softening process is called annealing.
Aircraft metals are subjected to both shock and 192
fatigue (vibrational) stresses. Fatigue occurs in materials which are exposed to frequent reversals of loading or repeatedly applied loads, if the fatigue limit is reached or exceeded. Repeated vibration or bending will ultimately cause a minute crack to occur at the weakest point. As vibration or bending continues, the crack lengthens until the part completely fails. This is termed shock and fatigue failure. Resistance to this condition is known as shock and fatigue resistance.
It is essential that materials used for critical parts be resistant to these stresses.
METALWORKING PROCESSES There are three methods of metalworking:
(1) Hot-working, (2) cold-working, and (3) extruding. The method used will depend on the metal involved and the part required, although in some instances both hot- and cold-working methods may be used to make a single part.
Hot-working Almost all steel is hot-worked from the ingot into some form from which it is either hot- or cold-worked to the finished shape. When an ingot is stripped from its mold, its surface is solid, but the interior is still molten. The ingot is then placed in a soaking pit which retards loss of heat, and the molten interior gradually solidifies. After soaking, the temperature is equalized throughout the ingot, then it is reduced to intermediate size by rolling, The rolled shape is called a bloom when its section dimensions are 6 x 6 inches or larger and approximately square. The section is called a billet when it is approximately square and less than 6 x 6 inches. Rectangular sections which have a width greater than twice their thickness are called slabs. The slab is the intermediate shape from Blooms, billets, or slabs are heated above the critical range and rolled into a variety of shapes of uniform cross section. The more common of these rolled shapes are sheet, bar, channels, angles, I-beams, and the like. As will be discussed later in this chapter, hot-rolled material is frequently finished by cold-rolling or drawing to obtain accurate finish dimensions and a bright, smooth surface.
Complicated sections which cannot be rolled, or sections of which only a small quantity is required, are usually forged. Forging of steel is a mechanical working at temperatures above the critical range to shape the metal as desired. Forging is done either by pressing or hammering the heated steel until the desired shape is obtained, Pressing is used when the parts to be forged are large and heavy; this process also replaces hammering where high-grade steel is required.
Since a press is slow acting, its force is uniformly transmitted to the center of the section, thus affecting the interior grain structure as well as the exterior to give the best possible structure throughout.
Hammering can be used only on relatively small pieces. Since hammering transmits its force almost instantly, its effect is limited to a small depth.
Thus, it is necessary to use a very heavy hammer or to subject the part to repeated blows to ensure complete working of the section. If the force applied is too weak to reach the center, the finished forged surface will be concave. If the center was properly worked, the surface will be convex or bulged. The advantage of hammering is that the operator has control over both the amount of pressure applied and the finishing temperature, and is able to produce small parts of the highest grade. This type of forging is usually referred to as smith forging. It is used extensively where only a small number of parts are needed. Considerable machining time and material are saved when a part is smith forged to approximately the finished shape.
Steel is often harder than necessary and too brittle for most practical uses when put under severe internal strain. To relieve such strain and reduce brittleness, it is tempered after being hardened. This consists of heating the steel in a furnace to a specified temperature and then cooling it in air, oil, water, or a special solution. Temper condition refers to the condition of metal or metal alloys with respect to hardness or toughness.
Rolling, hammering, or bending these alloys, or heat treating and aging them, causes them to become tougher and harder. At times these alloys become too hard for forming and have to be re-heat treated or annealed.
Metals are annealed to relieve internal stresses, soften the metal, make it more ductile, and refine the grain structure. Annealing consists of heating the metal to a prescribed temperature, holding it there for a specified length of time, and then cooling the metal back to room temperature. To produce maximum softness, the metal must be cooled very slowly. Some metals must be furnace cooled;
others may be cooled in air.
Normalizing applies to iron-base metals only.
Normalizing consists of heating the part to the proper temperature, holding it at that temperature until it is uniformaly heated, and then cooling it in still air. Normalizing is used to relieve stresses in metals.
Cold-Working Cold-working applies to mechanical working performed at temperatures below the critical range. It results in a strain hardening of the metal.
In fact, the metal often becomes so hard that it is difficult to continue the forming process without softening the metal by annealing.
Since the errors attending shrinkage are eliminated in cold-working, a much more compact and better metal is obtained. The strength and hardness, as well as the elastic limit, are increased;
but the ductility decreases. Since this makes the metal more brittle, it must be heated from time to time during certain operations to remove the undesirable effects of the working.
While there are several cold-working processes, the two with which the aviation mechanic will be principally concerned are cold-rolling and colddrawing. These processes give the metals desirable qualities which cannot be obtained by hot-working.
Cold-rolling usually refers to the working of metal at room temperature. In this operation, the materials that have been rolled to approximate sizes are pickled to remove the scale, after which they are passed through chilled finishing rolls. This gives a smooth surface and also brings the pieces to accurate dimensions. The principal forms of cold-rolled stocks are sheets, bars, and rods.
Cold-drawing is used in making seamless tubing, wire, streamlined tie rods, and other forms of stock.
Wire is made from hot-rolled rods of various diameters. These rods are pickled in acid to remove scale, dipped in lime water, and then dried in a steam room where they remain until ready for drawing. The lime coating adhering to the metal serves as a lubricant during the drawing operation.
The size of the rod used for drawing depends upon the diameter wanted in the finished wire. To reduce the rod to the desired size, it is drawn cold through a die. One end of the rod is filed or hammered to a point and slipped through the die opening. Here it is gripped by the jaws of the drawing block and pulled through the die. This series of operations is done by a mechanism known as a drawbench.
In order to reduce the rod gradually to the desired size, it is necessary to draw the wire through successively smaller dies. Because each of these drawings reduces the ductility of the wire, it must be annealed from time to time before further drawings can be accomplished. Although cold-working reduces the ductility, it increases the tensile strength of the wire.
In making seamless steel aircraft tubing, the tubing is cold-drawn through a ring-shaped die with a mandrel or metal bar inside the tubing to support it while the drawing operations are being performed. This forces the metal to flow between the die and the mandrel and affords a means of controlling the wall thickness and the inside and outside diameters.
Extruding The extrusion process involves the forcing of metal through an opening in a die, thus causing the metal to take the shape of the die opening.
Some metals such as lead, tin, and aluminum may be extruded cold; but generally metals are heated before the operation is begun.
The principal advantage of the extrusion process is its flexibility. Aluminum, because of its workability and other favorable properties, can be economically extruded to more intricate shapes and larger sizes than is practicable with many other metals.
Extruded shapes are produced in very simple as well as extremely complex sections. In this process a cylinder of aluminum, for instance, is heated to 750° F. to 850° F. and is then forced through the opening of a die by a hydraulic ram. The opening is the shape desired for the cross section of the finished extrusion.
Many structural parts, such as channels, angles, T-sections, and Z-sections are formed by the extrusion process.
FERROUS AIRCRAFT METALS Many different metals are required in the repair of aircraft. This is a result of the varying needs with respect to strength, weight, durability, and resistance to deterioration of specific structures of parts. In addition, the particular shape or form of the material plays an important role. In selecting materials for aircraft repair, these factors plus many others are considered in relation to the mechanical and physical properties. Among the common materials used are ferrous metals. The term "ferrous" applies to the group of metals having iron as their principal constituent.
Identification If carbon is added to iron, in percentages 194
ranging up to approximately 1 percent, the product is vastly superior to iron alone and is classified as carbon steel. Carbon steel forms the base of those alloy steels produced by combining carbon steel with other elements known to improve the properties of steel. A base metal (such as iron) to which small quantities of other metals have been added is called an alloy. The addition of other metals changes or improves the chemical or physical properties of the base metal for a particular use.
Nomenclature and Chemical Compositions of Steels In order to facilitate the discussion of steels, some familiarity with their nomenclature is desirable. A numerical index, sponsored by the Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI), is used to identify the chemical compositions of the structural steels.
In this system, a four-numeral series is used to designate the plain carbon and alloy steels; five numerals are used to designate certain types of alloy steels. The first two digits indicate the type of steel, the second digit also generally (but not always) gives the approximate amount of the major alloying element and the last two (or three) digits are intended to indicate the approximate middle of the carbon range. However, a deviation from the rule of indicating the carbon range is sometimes necessary.
Small quantities of certain elements are present in alloy steels that are not specified as required.
These elements are considered as incidental and may be present to the maximum amounts as follows: copper, 0.35 percent; nickel, 0.25 percent;
chromium, 0.20 percent; molybdenum, 0.06 per- The list of standard steels is altered from time to time to accommodate steels of proven merit and to provide for changes in the metallurgical and engineering requirements of industry. Refer to the numerical index table in figure 6-62.
Metal stock is manufactured in several forms and shapes, including sheets, bars, rods, tubings, extrusions, forgings, and castings. Sheet metal is of an inch. Bars and rods are supplied in a variety of shapes, such as round, square, rectangular, hexagonal, and octagonal. Tubing can be obtained in round, oval, rectangular, or streamlined shapes.
The size of tubing is generally specified by outside diameter and wall thickness.
The sheet metal is usually formed cold in such machines as presses, bending brakes, drawbenches, Series designation Types 100xx - Nonsulphurized carbon stects Resulphurized carbon steels (free machining) 12xx Rephosphorized and resulphurized carbon steels (free machining) 13xx Manganese 1.75% #23xx Nickel 3.50% *25xx Nickel 5.00% 31xx Nickel 1,25%, chromium 0.65% 33xx Nickel 3.50%, chromium 1.55% 40xx Molybdenum 0.20 or 0.25% 41xx Chromium 0.50 or 0.95%, molybdenum 0.12 or 0.20% 43xx Nickel 1.80%, chromium 0.50 or 0.80%, molybdenum 0.25% 44xx - Molybdenum 0.40% 45xx - Molyhdenum 0.52% 46xx - Nickel 1.80%, molybdenum 0.25% 47xx - Nickel 1.05%, chromium 0.45%, molybdemim 0.20 or 0.35% 48xx - Nickel 3.50%, molybdenum 0.25% 50xx - Chromium 0.25, or 0.40 or 0.50% 50xxx- Carbon 1.00%, chromium 0.50% 51xx - Chromium 0.80, 0.90, 0.95, or 1.00% 51xxx - Carban 1.00%, chromium 1.05% 52xxx - Carbon 1.00%, chromium 1.45% 6lxx - Chromium 0.60, 0.80, or 0.95%, Vitalium 0.12';, 0.10% min., or 0.15% min.
81xx - Nickel 0.30%, chromium 0.40%, molybelenum 0.12% 86xx - Nickel 0.55%, chromium 0.50%, molybdenum 0.20% 87xx - Nickel 0.55%, chromium 0.05%, molybdenum 0,25% 88xx - Nickel 0.55%, chromium 0.05°%, molyilenum 0.35% 92xx - Manganese 0.85%, silicon 2.00%, chrominn 0 or 0.35% 93xx - Nickel 3.25%, chromium 1.20%, molybdenum 0.12% 94xx ~ Nickel 0.45%, chromium 0.40, molybdenum 0.12% 98xx - Nickel 1.00%, chromium 0.80%, molybdenum 0.25% *Not included in the current list of standard steels.
FIGURE 6-62. SAE numerical index.
or rolls. Forgings are shaped or formed by pressing or hammering heated metal in dies. Castings are produced by pouring molten metal into molds. The casting is finished by machining.
Spark testing is a common means of identifying various ferrous metals. In this test the piece of iron or steel is held against a revolving grinding stone and the metal is identified by the sparks thrown off. Each ferrous metal has its own peculiar spark characteristics. The spark streams vary from a few tiny shafts to a shower of sparks several feet in length. (Few nonferrous metals give off sparks when touched to a grinding stone. Therefore, these metals cannot be successfully identified by the spark test.)
Identification by spark testing is often inexact unless performed by an experienced person, or the 195
test pieces differ greatly in their carbon content and alloying constituents.
Wrought iron produces long shafts that are straw colored as they leave the stone and white at the end. Cast iron sparks are red as they leave the stone and turn to a straw color. Low-carbon steels give off long, straight shafts having a few white sprigs. As the carbon content of the steel increases, the number of sprigs along each shaft increases and the stream becomes whiter in color. Nickel steel causes the spark stream to contain small white blocks of light within the main burst.
Types, Characteristics, and Uses of Alloyed Steels Steel containing carbon in percentages ranging from 0.10 to 0.30 percent is classed as low-carbon steel. The equivalent SAE numbers range from 1010 to 1030. Steels of this grade are used for making such items as safety wire, certain nuts, cable bushings, or threaded rod ends. This steel in sheet form is used for secondary structural parts and clamps, and in tubular form for moderately Steel containing carbon in percentages ranging from 0.30 to 0.50 percent is classed as mediumcarbon steel. This steel is especially adaptable for machining or forging, and where surface hardness is desirable. Certain rod ends and light forgings are made from SAE 1035 steel.
Steel containing carbon in percentages ranging from 0.50 to 1.05 percent is classed as high-carbon steel. The addition of other elements in varying quantities adds to the hardness of this steel. In the fully heat-treated condition it is very hard, will withstand high shear and wear, and will have little 16f5 a shon deformation. It has limited use in aircraft. SAE 1095 in sheet form is used for making flat springs and in wire form for making coil springs.
The various nickel steels are produced by combining nickel with carbon steel. Steels containing from 3 to 3.75 percent nickel are commonly used. Nickel increases the hardness, tensile strength, and elastic limit of steel without appreciably decreasing the ductility. It also intensifies the hardening effect of heat treatment.
SAE 2330 steel is used extensively for aircraft parts, such as bolts, terminals, keys, clevises, and pins.
Chromium steel is high in hardness, strength, and corrosion-resistant properties, and is particularly adaptable for heat-treated forgings which require greater toughness and strength than may be obtained in plain carbon steel. It can be used for such articles as the balls and rollers of antifriction bearings.
Chrome-nickel or stainless steels are the corrosion-resistant metals. The anticorrosive degree of this steel is determined by the surface condition of the metal as well as by the composition, temperature, and concentration of the corrosive agent.
The principal alloy of stainless steel is chromium.
The corrosion-resistant steel most often used in aircraft construction is known as 18-8 steel because of its content of 18 percent chromium and 8 percent nickel. One of the distinctive features of 18-8 steel is that its strength may be increased by coldworking.
Stainless steel may be rolled, drawn, bent, or formed to any shape. Because these steels expand about 50 percent more than mild steel and conduct heat only about 40 percent as rapidly, they are more difficult to weld. Stainless steel can be used for almost any part of an aircraft. Some of its common applications are in the fabrication of exhaust collectors, stacks and manifolds, structural and machined parts, springs, castings, tie rods, and control cables.
The chrome-vanadium steels are made of approximately 18 percent vanadium and about 1 percent chromium. When heat treated, they have strength, toughness, and resistance to wear and fatigue. A special grade of this steel in sheet form can be cold-formed into intricate shapes. It can be folded and flattened without signs of breaking or failure. SAE 6150 is used for making springs; and chrome-vanadium with high-carbon content, SAE 6195, is used for ball and roller bearings.
Molybdenum in small percentages is used in combination with chromium to form chromemolybdenum steel, which has various uses in aircraft. Molybdenum is a strong alloying element.
It raises the ultimate strength of steel without affecting ductility or workability. Molybdenum steels are tough and wear resistant, and they harden throughout when heat treated. They are especially adaptable for welding and, for this reason, are used principally for welded structural parts and assemblies. This type steel has practically replaced carbon steel in the fabrication of fuselage tubing, engine mounts, landing gears, and other structural parts. For example, a heat-treated SAE X4130 tube is approximately four times as strong as an SAE 1025 tube of the same weight and size.
A series of chrome-molybdenum steel most used in aircraft construction is that series containing
0.25 to 0.55 percent carbon, 0.15 to 0.25 percent 196
3/31/99 molybdenum, and 0.50 to 1.10 percent chromium. These steels, when suitably heat treated, are deep hardening, easily machined, readily welded by either gas or electric methods, and are especially adapted to high-temperature service.
Inconel is a nickel-chromium-iron alloy closely resembling stainless steel (corrosion resistant steel, CRES) in appearance. Aircraft exhaust systems use both alloys interchangeably. Because the two alloys look very much alike, a distinguishing test is often necessary. One method of identification is to use an electrochemical technique to identify the nickel (Ni) content of the alloy.
Inconel has a nickel content greater than 50 percent, and the electrochemical test detects Ni.
Prepare a wiring assembly as shown in Figure 6-2A, and prepare the two reagents (ammonium fluoride and dimethylglyoxime solutions) placing them in separate dedicated dropper solution bottles. Before testing, the metal must be thoroughly cleaned in order for the electrolytic deposition to take place. You may use nonmetallic hand scrubbing pads or 320 to 600 grit "crocus cloth" to remove deposits and corrosion products (thermal oxide).
Connect the alligator clip of the wiring assembly to the bare metal being tested. Place one drop of a
0.05 percent reagent grade ammonium fluoride solution in deionized water on the center of a (1 by 1 inch) sheet of filter paper. Lay the moistened filter paper over the bare metal alloy being tested. Firmly press the end of the aluminum rod over the center of the moist paper.
Maintain connection for 10 seconds while rocking the aluminum rod on the filter paper. Ensure that the light emitting diode (LED) remains lit (indicating good electrical contact and current flow) during this period.
AC 65-9A CHG 1 Disconnect the wiring assembly, and set it aside.
Remove the filter paper and examine it to determine that a light spot appears where the connection was made.
9v battery LED Aluminum rod Alligator Clip stock FIGURE 6-2A. Wiring assembly schematic.
Deposit one drop of 1.0 percent solution of reagent grade dimethylglyoxime in ethyl alcohol on the filter paper (same side that was in contact with the test metal). A bright distinctly pink spot will appear within seconds on the filter paper if the metal being tested is Inconel. A brown spot will appear if the test metal is stainless steel. Some stainless steel alloys may leave a very light pink color. However, the shade and depth of color will be far less than would appear for Inconel. For flat surfaces the test spot will be circular while for curved surfaces, such as the outside of a tube or pipe, the test spot may appear as a streak. (See Figure 6-2B for sample test results.) This procedure should not be used in the heat affected zone of weldments and on nickel coated surfaces.
In 600 In 718 17-7 SS 304 SS 316 SS FIGURE 6-2B. Electrochemical test results, Inconel and stainless steel alloys.
The tensile strength of Inconel is 100,000 p.s.i., annealed, and 125,000 p.s.i., when hard rolled. It is highly resistant to sait water and is able to withstand temperatures as high as 1,600° F. Inconel welds readily and has working qualities quite similar to those of corrosion-resistant steels.
NONFERROUS AIRCRAFT METALS The term "nonferrous" refers to all metals which have elements other than iron as their base or principal constituent. This group includes such metals as aluminum, titanium, copper, and magnesium, as well as such alloyed metals as Monel and babbit.
Aluminum and Aluminum Alloys Commercially pure aluminum is a white lustrous metal which stands second in the scale of malleability, sixth in ductility, and ranks high in its resistance to corrosion. Aluminum combined with various percentages of other metals forms alloys which are used in aircraft construction.
Aluminum alloys in which the principal alloying ingredients are either manganese, chromium, or magnesium and silicon show little attack in corrosive environments. Alloys in which substantial percentages of copper are used are more susceptible to corrosive action.
The total percentage of alloying elements is seldom more than 6 or 7 percent in the wrought alloys.
Aluminum is one of the most widely used metals in modern aircraft construction. It is vial to the aviation industry because of its high strength-to-weight ratio and its comparative ease of fabrication. The outstanding characteristic of aluminum is its light weight. Aluminum metals at the comparatively low temperature of 1,250° F.
It is nonmagnetic and is an excellent conductor.
Commercially pure aluminum has a tensile strength of about 13,000 p.s.i., but by rolling or other cold-working processes its strength may be approximately doubled. By alloying with other metals, or by using heat-treating processes, the tensile strength may be raised to as high as 65,000 p.s.i. or to within the strength range of structural steel.
Aluminum alloys, although strong, are easily worked because they are malleable and ductile. They may be rolled into sheets as thin as .0017 inch or drawn into wire .004 inch in diameter. Most aluminum alloy sheet stock used in aircraft construction ranges from .016 to .096 inch in thickness; however, some of the larger aircraft use sheet stock which may be as thick as .356 inch.
The various types of aluminum may be divided into two general classes: (1) The casting alloys (those suitable for casting in sand, permanent mold, or die castings), and (2) the wrought alloys (those which may be shaped by rolling, drawing, or forging). Of these two, the wrought alloys are the most widely used in aircraft construction, being used for stringers, bulkheads, skin, rivets, and extruded sections.
Aluminum casting alloys are divided into two basic groups. In one, the physical properties of the alloys are determined by the alloying elements and cannot be 198