Aircraft Material and Processes

Aircraft Construction, Repair, and Modification

Aircraft Construction, Repair, and Modification

AIRCRAFT MATERIALS AND AIRCRAFT MATERIALS AND PROCESSES PROCESSES prepared by : Engr. Eric John Velasco prepared by : Engr. Eric John Velasco

RECIPROCATING ENGINE

Aircraft Construction, Repair, and Modification (15%) Aircraft Materials and Processes; Methods and Techniques in Repair and Modification in Accordance With Civil Aviation Regulation; Manufacturing, Production Processes and Quality Assurance

IV. AIRCRAFT CONSTRUCTION, REPAIR, AND MODIFICATION A. Objective: To determine the basic knowledge of the Examinees on Aircraft Materials, Construction Repair,and Modification Subject Contents: 1. Aircraft Materials and Processes a. Physical and Chemical Properties of Ferrous Metals and Alloys, Non-Ferrous Metals and Alloys, Non-Metals (Wood, Fiberglass, others) b. Identification of Metals c. Heat Treatment processes d. Forming/Shaping and Forging e. Joining of Metals 2. Aircraft Hardware, Cables, and Tools,Equipment a. Bolts, Nuts, Screws, Rivets, others b. Control Cables and Cable Assemblies c. Tools and Fabrication/Repair Equipment

Subject Contents: 3. Construction, Repair, and Modification a. Aircraft Structural Components b. Metal Structures c. Non-Metal Structures d. Composite Materials 4. Testing and Inspection a. Testing of Metals - Hardness Tests b. Non-Destructive Test and Inspection

5. Corrosion Protection and Control a. Types of Corrosion b. Corrosion Protection and Removal

6. Aircraft Weight and Balance a. Weighing Procedure b. Weight and Balance Computations c. Weight and Balance Extreme Conditions Most Forward and Rearward CG Positions

C. References: 1. Aircraft Materials and Processes - Titterton 2. Aircraft Inspection and Repair – US Printing Office 3. Maintenance and Repair of Aerospace Vehicle -McKinkey and Bent

Physical and Chemical Properties of

Ferrous Metals and Alloys,  Non-Ferrous Metals and  Alloys,  Non-Metals (Wood, Fiberglass,others) 

5 Major Stresses to which all Aircraft Subjected     

TENSION – is the stress that resist a force tends to null apart. COMPRESSION – is the stress that resist a crushing force. TORSION – is the stress that produce twisting. SHEAR – is the stress that resists the force tending to cause one material to slide over an adjacent layer. BENDING – is a combination of compression and tension. ○ STRESS – is an internal force of a substance which opposes or

resist deformation can cause strain. ○ STRAIN – is the deformation of a material or substance.

5 Major Stresses to which all Aircraft Subjected

5 Major Stresses to which all Aircraft Subjected

PROPERTIES OF MATERIALS       

HARDNESS - The property of a material that enables it to resist penetration, wear, or cutting action or permanent distortion. BRITTLENESS – is the property of a metal which allows little bending or deformation without shattering. MALLEABILITY – property of metals which allows them to be bent or permanently distorted without rupture. STRENGTH - The ability of a material to resist deformation. PLASTICITY - The capability of an object or material to be stretched and to recover its size and shape after its deformation. DUCTILITY - The property which allows metal to be drawn, bent or twisted into various shapes without breaking. ELASTICITY – property which enables a metal to return to its original shapes when the forces which causes the change of shape is removed.

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TOUGHNESS – a material which possesses toughness will withstand tearing or shearing and maybe stretched or otherwise deformed without breaking. DENSITY – the weight of a unit volume of the materials. FUSIBILITY – the ability of a metal to become liquid by the application of heat. CONDUCTIVITY – the ability of a metal which enables to carry heat or electricity THERMAL EXPANSION  Contraction – ability of metals to shrink when subjected to cooling.  Expansion – expand upon the application of heat.

Aircraft Metals 

Two Main Group of Aircraft Metals: NON-FERROUS METALS – the term that

describes metals which are have elements other than Iron as their base. Aluminum, Copper, Titanium, and Magnesium are some of the common non-ferrous metals used in Aircraft Construction and Repair. FERROUS METALS – any alloy containing iron as its chief constituent, most common ferrous metal in aircraft structure is steel, an alloy of iron with a controlled amount of carbon added.

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NON-FERROUS METALS: 1. ALUMINUM AND ITS ALLOYS ○ - Pure aluminum lacks sufficient strength to be used in aircraft Quenching ○ construction. However, its strength increases considerably when it is

ALLOYED, or mixed with compatible metals. TYPES OF ALUMINUM ALLOYS: 1. Cast Alloys – those suitable for casting in sand, permanent mold or die casting. 2. Wrought Alloys – those which may be shaped by rolling, drawing or forging. These are the most widely used in aircraft construction, being used for stringers, bulkheads, skin, rivets, and extruded sections. GENERAL CLASSES OF WROUGHT ALUMINUM ALLOYS: 1. Non-Heat Treatable Alloy – the mechanical properties obtained by cold working are destroyed and any subsequent heating cannot restore it except by additional cold working. 2. Heat Treatable Alloy – alloy which responds readily to heat treatment which results in considerable improvement of the strength characteristics. Greater strength is obtained and used for structural purposes.

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2. MAGNESIUM AND ITS ALLOYS  Magnesium alloy are used for cast and wrought form available in sheets, bars, tubing,

and extrusions. Magnesium is one of the lightest metals having sufficient strength and suitable working characteristics for use in aircraft hardware. However, it is susceptible to corrosion and tends to crack.

3. TITANIUM AND ITS ALLOYS  Titanium and its alloys are light metals with very high strength. It has an excellent

corrosion resistance characteristics, particularly to the effects of salt water.

4. NICKEL AND ITS ALLOYS  Nickel is the base element for most of the higher temperature heat-resistant alloys.

While it is much more expensive than iron, nickel provides an austenitic structure that has greater toughness and workability than ferrous alloys of the same strength.

MONEL – contains about 68 % nickel and 29% copper, along with iron and manganese. It works well in gears and parts that require high strength and corrosion resistance at elevated temperature. INCONEL – high strength, high temperature alloys containing approximately about 80% nickel, 14 % chromium, and small amounts of iron and other elements.

5. COPPER AND ITS ALLOYS  It is easily identified by its reddish color and by the green and blue colors of its oxides

and salt. Copper has excellent electrical and thermal conductivity and it is primary metal used for electrical wiring. BRASS – an alloy of copper and zinc. BRONZE – an alloy of copper and tin.

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FERROUS METALS:

1. IRON  Is like a chemical which is fairy soft, malleable and ductile in its pure form. It is silvery

white in color and is quite heavy, having a density of 7.9 grams per cubic centimeter.

2. STEEL  To make steel, pig iron is re-melted in a special furnace. Pure oxygen is the forced

through the molten where it combines with carbon and burns. A control amount of carbon is then put back into the molten. The molten steel is then poured into molds where it solidifies into ingots. The ingots are then placed in a soaking pit where they are heated to a uniform temperature of about 2200 degrees F. They are then taken from the soaking pit and passed through steel rollers to form late or sheet metal.

a. CARBON  Carbon is the most common alloying element found in steel. When mixed with iron

core compounds of iron carbides called CEMETITE form. It is the carbon in steel that allows the steel to be heat treated to obtain varying degrees of hardness, strength and toughness. The greater the carbon content, the more receptive steel is to heat treatment and therefore, the higher its tensile strength, and hardness. However, higher carbon content decreases the malleability and weldability of steel. LOW CARBON STEELS – contains between 0.10 and 0.30 percent carbon. Primarily used in safety wire, cable bushing, and threaded rod ends. MEDIUM CARBON STEELS – contains between 0.30 and 0.50 percent carbon. HIGH CARBON STEELS – contains between 0.50 to 1.05 percent carbon and are very hard. Primarily used in springs, files, and some cutting tools.

b. SILICON  When it is alloyed with steel it acts as a hardener. When used in small quantities, it

also improves ductility.

c. PHOSPHOROUS  Raises the yield strength of steel and improves low carbon steel’s resistance of

atmospheric condition. However, no more than 0.05 percent is normally used in steel, since higher amounts cause the alloy to become brittle when cold.

d. NICKEL  Adds strength and hardness to steel and increase yield strength. It also slows the

rate of hardening when steel is heat treated, which increases the steels contains 3% nickel and 0.30% carbon, and used in producing aircraft hardwired such as bolts, nuts, rod end and pins.

e. CHROMIUM  Alloyed with steel to increase strength and hardness as well as improve its wear

and corrosion resistance. It is used in balls and rollers of anti-friction bearings.

f. STAINLESS STEEL  Is a classification of CORROSION-RESISTANT STEEL (CRES) that contain large

amount of chromium and nickel. Their strength and resistant to corrosion make than well suited for high-temperature applications such as firewalls and exhaust system components. It contains 18% chromium and 8% nickel. It is referred as 18-8. AUSTENITIC STEELS – refers to 200 and 300 series stainless steel. Hardened only by cold-working. FERRITIC STEELS – contains no carbon. They do not respond to heat treatment. MARTENSITIC STEELS - the 400 series of stainless steel. These are magnetic and it becomes extremely hard if allowed to cool rapidly by cooling from an elevated temperature.

g. CHROME – MOLYBDENUM (chrome-moly) STEELS  Commonly used alloy in aircraft. Making it an ideal choice for landing gear

structures and engine mounts.

h. VANADIUM  When combined with chromium, vanadium produces a strong, tough, ductile steel

alloys. Most wrenches and ball bearings are made of chrome-vanadium steel.

i. TUNGSTEN  Has an extremely high melting point and adds this characteristics to steel when it

is alloyed. Typically used for breaker contacts in magnetos and for high speed cutting tools.

ALLOYS

ALLOYS

WOOD STRUCTURES: 

WOOD – wood structures requires a great deal of handwork and therefore, are extremely expensive.  SOLID WOOD – used for some aircraft wing spars and is made of solid pie cut from a

log. Most solid cut by quarter sawing to prevent war page.  LAMINATED WOOD – made up of two or three pieces of thin wood glued together with the same direction.  PLY WOOD – consist of three or more layers of thin veneer glued together so the grain of each successive layer crosses the others at an angle of 45 degrees of 90 degrees.

2 BASIC SPECIES OF WOOD USED IN AIRCRAFT CONSTRUCTION: 1. HARDWOOD – come from deciduous trees having broad leaves. a. MAHOGANY – this hardwood is heavier and stronger than spruce. Primary use in aircraft construction is for face sheets of plywood used in aircraft skin. b. BIRCH – a heavy hardwood with very good shock resistant characteristics. It is recommended for the face plies of plywood used as reinforcement plates on wing spars and in the construction of wooden propellers.

2. SOFTWOOD – come from coniferous trees with needle like or scale like leaves. a. SITKA SPRUCE – most common wood used in aircraft structures. It is relatively free from defects, has a high strength to weight ration and available in large size. FAA chosen Sitka Spruce as the reference wood for aircraft construction. b. DOUGLAS FIR – the strength properties exceed those of spruce; however, it is much heavier. Further more, it is more difficult to work than spruce, and has a tendency to split. c. NOBLE FIR – slightly lighter than spruce and is equal or superior to spruce in all properties except hardness and shock resistance. It is often used for structural parts that are subject to heavy bending and compression loads such as spars, spar flange, and has tendency to split. d. BALSA – an extremely light wood. Balsa lacks of structural strength, it is often sliced across its grain for use as a core material for sandwich-type panels that requires lightweight and rigidity.

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QUALITY OF WOOD:  Some of the categories a woods quality is based on include how straight

the grain is, the number of knots, pitch pockets, splits and presence of decay. 1. GRAIN DEVIATION – regardless of the species of wood used aircraft construction, it must

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have a straight grain. This means all of the woods fiber must be oriented parallel to the materials longitudinal axis. A maximum of deviation of 1:15 is allowed. In other words, the grain must not slope more than 1 inch in 15 inches. KNOTS – it identifies where a branch grew from the tree trunk. PITCH POCKETS – small opening within the annual rings of a tree can fill resin and form pitch pocket. It slightly weaken the piece of wood. CHECKS, SHAKE AND SPLITS CHECKS – a crack that runs across the annual rings of a board and occurs during the seasoning process. SHAKE – a crack or separation that occurs when two annual rings separates along their boundary. SPLITS – a lengthwise separation of the wood caused by the wood fibers tearing apart. STRAINS AND DECAY STRAINS – It is caused by decay usually appears streaks in the grain. Strains that uniformly discolor the annual rings are evidence of decay. DECAY – is caused by fungi that grow in damp wood, and is prevented by proper seasoning and dry storage. A simple way of identfying decayed wood is to pick at a suspected area with the point of a knife. Sound wood will splinter, while a knife point will bring up a chunk of decayed wood.

PLASTICS OR RESINS 1. THERMOSETTING RESINS – it hardens or set when heat of the correct value is applied. It cannot softened and reshaped after having been solidified. 2. THERMOPLASTIC RESINS – can be soften by heat and reshaped or reformed many times without changing composition, provided that the heat applied is held with proper limits. 

Types of Thermoplastic Material used for Aircraft Windshield and Side Windows:  1. CELLULOSE ACETATE – transparent and lightweight. It has a tendency to

shrink and turn yellow. When applied with acetone it softens.  2. ACRYLIC – identified by trade names as Lucite or Plexiglas or in Britain Perspex. It is stiffer than cellulose acetate. More transparent and for all purpose is colorless. It burns with a clear flame and produces a fairly pleasant odor. If acetone is applied to acrylic it leaves a white residue but remains hard.

THERMOPLASTIC RESINS:  1. CELLULOSE ACETATE  2. POLYETHYLENE – is made in low and high-density qualities. Low-

density polyethylene is made in thin, flexible sheet or film and is used for plastic bags, protective sheeting and electrical insulation. High-density polyethylene is used for containers such as fuel tanks, large drums and bottles.  3. VINYLS – manufactured in a variety of types and has a wide range of application. Their used in aircraft includes seat covering, electrical insulation, moldings, and tubing. They are flexible and resistant to most chemical and moisture.  4. ACRYLIC RESIN – a water clear plastic that has a light transmission of 92%. This property, together with its weather and moisture resistance, makes it an excellent product for aircraft windows and windshields.  5. POLYTETRAFLOUROETHYLENE (Teflon) – is encountered in nonlubricated bearings, tubing, electrical devices and other applications.

Composite ABBREVIATIONS:  AFRP - Aramide Fibre Reinforced Plastic  CFRP - Carbon Fibre Reinforced Plastic  GFRP - Glass Fibre Reinforced Plastic  HOBE - Honeycomb before Expansion  MSDS - Material Safety Data Sheet  NDT - Non Destructive Testing  NTM - Non Destructive Testing Manual  Prepeg - Pre impregnated Fabric  SRM - Structural Repair Manual

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Advantage  Composite materials are mainly used to reduce weight, that means if

weight can be saved, more cargo, fuel or passengers can be carried. More advantages are:  high strength to weight ratio  reducing of parts and fasteners  reducing wear  corrosion resistance

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Disadvantage Disadvantages are:  general expensive  not easy to repair; that means you need well trained staff, tools, equipment and facilities to repair composite components

Elements of Composite Structure   

Reinforcing Materials Core Materials Matrix

Aircraft Fabric

Aircraft Fabric Covering 

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Aircraft fabric covering is a term used for both the material used and the process of covering aircraft open structures. It is also used for reinforcing closed plywood structures Early aircraft used organic materials such as cotton and cellulose dope, modern fabric covered designs usually use synthetic materials such as Nylon and butyrate dope for adhesive, this method is often used in the restoration of older types that were originally covered using traditional methods.

Aircraft Dope  Aircraft dope is a plasticised lacquer that is applied to fabric-coated aircraft. It tautens and stiffens fabric stretched over airframes and adheres and protects fabric applied to other skin material.  Typical doping agents include nitrocellulose, cellulose acetate and cellulose acetate butyrate. Liquid dopes are highly flammable; nitrocellulose, for instance, is also known as the explosive propellant "guncotton". Dopes will often include colouring pigments to facilitate even application.

Problem Areas a.) Deterioration Fabric deteriorates only by exposure to ultraviolet radiation as used in an aircraft covering environment b.) Tension Most Fabrics obtains maximum tension on an airframe at 350 degrees Fahrenheit and will not be excessive on aircraft originally covered and doped

Aircraft Fabric Synthetic a.) STC approved covering material Difference in fabric may be denier, tenacity, thread count, weight, shrink, tension and weave style *tenacity- customary measure of strength of a fiber or yarn. * denier is a measure of the linear density, the tenacity works out to be not a measure of force per unit area, but rather a quasi-dimensionless measure analogous to specific strength

b.) Polyester Filaments Manufactured by polymerization of various select acids and alcohols, then extruding the resulting molten polymers through spinnerets to form filaments

c.) Covering Procedures Coating types, covering accessories and covering procedures also may vary; therefore, the covering procedures given in the pertinent manuals must be followed to comply with the STC. d,.) Installation Initial installation of polyester fabric is similar to natural fabric. The fabric is installed with as little slack as possible, considering fittings and other protrusions. *slack-not using due diligence, care, or dispatch

Aircraft Fabric-Natural 

Physical Specifications and minimum strength requirements for natural fabric fiber, cotton and linen, used to recover or repair components of an aircraft.

Recovering Aircraft Recover or repair aircraft with a fabric of equal quality and strength to that used by the original aircraft manufacturer *note: recovering or repairing aircraft with any type fabric and/or coating other than the type used by the original aircraft manufacturer is considered a major alteration. Obtain approval form from then FAA on fabric and installation data. Cotton and linen rib lacing cord, machine and hand sewing thread, and finishing tapes should not be used with polyester and glass fabric covering

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Reinforcing tape 

Reinforcing tape should have a minimum 40 lbs. resistance without failure when static tested in shear against a single rib lace, or a pull through resistance when tested against a single wire clip, rivet screw, or any other type of fabric to rib attachment.

Finishing tape 

Sometimes referred as surface tape, should have the same properties as the fabric used to cover the aircraft

Using the 2" Dacron straight finishing tape, measure and cut strips of the tape to be long enough to overlap both the leading and trailing spars.

Lacing Cord 

Should have a minimum breaking strength of 40 lbs.. Rib lace cord should have a micro-crystalline fungicidal wax, paraffin free wax, or beeswax coating, or other approved treatment to prevent wearing and fraying when pulling through the structure

Machine Thread and Hand sewing Thread  

Machine Thread-Shall have a minimum breaking strength of 5 lbs Hand sewing Thread-Shall have a minimum breaking strength of 14 lbs Hand Sewing Thread (FAA approved) Polyester cord. Replaces cotton.

Flutter Precautions When recovering or repairing control surfaces, especially on high performance airplanes, make sure that dynamic and static balances are not adversely affected. Weight distribution and mass balance must be considered to preclude to possibility of induced flutter *flutter- To wave or flap rapidly in an irregular manner: 

Preparation of the structure for covering a.) Battery Box Treatment An Asphaltic, rubber based acid-proof coating should be applied to the structure in the area of a battery by box, by brush, for additional protection from battery acid b.) Worn holes Oversized screw holes or worn size 4 self tapping screw holes through ribs and other structures used to attach fabric may be redrilled a minimum 1-1/2 hole diameter distance from the original hole location with a # 44 (0.086) drill bit.

Fairing Precautions 

Aluminum leading edge replacement fairings installed in short sections may telescope during normal spar bending loads or from thermal expansion and contraction. This action may cause a wrinkle to form in the fabric, at the edge of the lap joint. Trailing edges should be adequately secured to prevent movement and wrinkles.

Dope Protection 

Solvents found in nitrate and butyrate dope will penetrate, wrinkle, lift, or dissolve mostone part wood varnishes and one-part metal primers. All wood surfaces that come in contact with doped fabric should be treated with a protective coating such as aluminum foil, cellulose tape, or dope proof paint to protect them against the action of the solvents in the dope

SEALANT COMPOUND SEALANTS – used to contain fuel, maintain cabin pressure, reduce fire hazards, exclude moisture, prevent corrosion, and fill gaps and smooth discontinuities on the aircraft exterior. SEALING – is a process that confines liquids and gases within a given area or prevents them from entering areas from which they must be excluded.

Categories of Compounds Sealing compounds are divided into two categories, silicone and nonsilicone.  1.Silicone compounds – are usually white, red, or grey in colour and are used in general where heat resistance is required.  2.Nonsilicone compounds – can be any colour and are used where heat resistance is not required. Specification / Classification The classification system for sealants in Boeing material specifications (BMS.s) is as follows:  Class A – Brushcoat Sealant. (Thinned with solvent to provide viscosity suitable for brushing).  Class B – Filleting Sealant. (Relatively heavy consistency with good thixotropic (low-slump) properties).  Class C – Faying Surface Sealant. (Medium consistency for good spreadability).  Class D – Hole-Filling Sealant. (Similar to Class B but with very low slump).  Classes E and F – Sprayable sealant

Identification of Metals

Basic Designation for Wrought and Cast Aluminum Alloys (AANumbering System) Wrought Alloys Alloy Number Major Identifying Elements  1XXX Pure Aluminum (99.00% minimum aluminum)  2XXX Copper  3XXX Manganese  4XXX Silicon  5XXX Magnesium  6XXX Magnesium and Silicon  7XXX Zinc  8XXX Other elements  9XXX Unused series

Cast Alloys Alloy Number Major Identifying Elements  1XXX 99.00 % minimum aluminium  2XXX Copper  3XXX Silicon with added copper and/or magnesium  4XXX Silicon  5XXX Magnesium  6XXX Unused series  7XXX Zinc  8XXX Tin  9XXX Other elements

Aluminum Alloys

Type of Alloy

Classification

Pure (99% above)

1xxx

Copper

2xxx

Manganese

3xxx

Silicon

4xxx

Magnesium

5xxx

Magnesium Silicon

6xxx

Zinc

7xxx

Other Element

8xxx

Temper Designation for Heat Treatable Alloys     

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T1 – Cooled from an elevated temperature shaping process and naturally aged to a substantially stable condition T2 – Annealed T3 – Solution heat treated and cold worked. T4 – Solution heat treated and naturally aged. T42 – Solution heat treated from 0 temper to demonstrate response to heat treatment by the user, and naturally aged to a substantially stable condition T5 – Cooled from an elevated temperature shaping process and artificially aged T6 – Solution heat treated and artificially aged. T62 – Solution heat treated from 0 F temper to demonstrate response to heat treatment by the user, and artificially aged T7 – Solution heat treated and stabilized T8 – Solution heat treated, cold worked, and artificially aged T9 – Solution heat treated, artificially aged, and cold worked T10 – Cooled from an elevated temperature shaping process, cold worked, and artificially aged

Aluminum Association Numbering System

Aluminum Cladding 

Several aluminium alloys as for example 2024 and 7075 are very susceptible tocorrosion. Sheets of such material are clad with a thin layer of pure aluminium with 1 % zinc on both sides as a means of corrosion protection. These layers are permanently welded to the base material in a rolling process at high temperature. Other than electroplated stock, clad material can be formed. The thickness of the clad layers is about 3 or 5 % of the material thickness. An ink print on US sheet metal that reads ALclad, Clad or ALC indicates that such sheet is clad.

Steel Numbering System

Steel Numbering System Type of Steel

Classification

Carbon

1xxx

Nickel

2xxx

Nickel Chromium

3xxx

Molybdenum

4xxx

Chromium

5xxx

Chromium Vanadium

6xxx

Tungsten

7xxx

Silicon Manganese

8xxx

Temper Designation System 

Basic Temper Designation     

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F – As fabricated O – Annealed H – Strain hardened (Non heat treatable products only) W – Solution heat treated T – Heat treated to produce stable tempers other than F, O, or H

Temper Designation for Non Heat Treatable Alloys  H 1 – Strain hardened produced by cold working the metal to the desired

dimension.  H2 – Strain hardened, then partially annealed to remove some of the hardness.  H3 – Strain hardened, then stabilized. 

The degree of hardening is indicated by a second digit following one of the above designations:     

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2 4 6 8 9

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1/4 hard 1/2 hard 3/4 hard full hard extra hard

A third digit may be used to indicate a variation of a two digit number.

Temper Designation System Fabricated (F) – Denotes that the metal has been fabricated to ordered dimensions without any attempt on the part of the producer to control the results of either strain hardening operations or the thermal treatments. There are no mechanical property limit, and the strength levels may vary. Annealed (O)- Applies to wrought that have undergone a thermal treatment to reduce their mechanical property levels to their minimum. Often describe as soft dead metal. Strain Hardened (H)- applies to those wrought products which have had an increase in strength by reduction through strain hardening or cold working operations. H is followed by two or more digits

Temper Designation System Thermally Treated ( T) – Produce temper other than F,O, H. Applies to those products which have had an increase in strength due to thermal treatments, with or without supplementary strain hardening operations. T is always followed by two or more digits. Solution Heat Treated ( W) – An unstable temper applying to the certain of the (7xxx) heat treatable alloys that, after heat treatment spontaneously age harden at room temperature. Only when the period of natural aging is indicated

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%

Heat Treatment Processes METALWORKING PROCESSES  Hot-working ○ Forging ○ Rolling

 Pressing  Hammering  Cold Working ○ Cold Rolling

 Cold Drawing  Extrusions

Heat Treatment Processes for Aluminum 

HEAT TREATMENT – is a series of operations involving the heating and cooling of metals in their solid state. Its purpose is to make the metal more useful, serviceable and safe for a definite purpose.  SOLUTION HEAT TREATMENT – is the process of heating certain aluminum alloys to allow the alloying elements to mix with the base metal.  QUENCHING – rapid cooling by means of water, oil, brine, etc.  SOAKING or HOLDING – held the temperature within about plus or minus 10 degrees Fahrenheit of this temperature and the base metal until the alloying elements is uniform throughout. 

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NATURAL AGING – when an alloy is allowed to cool at room temperature and can take several hours or weeks. ARTIFICIAL AGING – accelerating the aging process by cooling at an elevated temperature. ANNEALING – is the process that softens a metal and decrease internal stresses. STRAIN HARDENING – also referred to as COLD WORKING or WORK HARDENING. This requires mechanically working of metal (stretches, compresses, bends, drawn, etc.) below its critical range

Steps of Heat Treatment The heat treatment takes place in three steps. Step 1: Solution heat treat, that is heating of the material to a specified temperature and holding it there for a specified time. Step 2: Quenching Step 3: Age hardening (precipitation) at room temperature or elevated temperature The quenching must occur rapidly. After quenching the material initially is soft and ductile.

Methods of Heat Treatment

HEAT TREATMENT FOR STEELS:

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ANNEALING – is a form of heat treatment that softens steel and relieves internal stress. It is heated about 50 degrees F above its critical temperature, soaked for specified time then cooled. NORMALIZING – the process of forging, welding, or machining usually leave stresses to the steel that could lead to failure. To normalize, it is heated about 100 degrees F above its critical temperature and held there until the metal is uniformly heat soaked, then removed from the furnace and allowed to cool in still air. HARDENING – is heated above its critical temperature so carbon can disperse uniformly in the iron matrix. TEMPERING – reduces the undesirable qualities of martensitic steel. It is heated to a level considerable below its critical temperature and held there until it becomes heat soaked, then allowed to cool to room temperature in still air.

CASE HARDENING TREATMENTS: 1. CARBURIZING – forms a thin layer of high carbon steel on the exterior of low carbon steel.  PACK CARBURIZING – is done by enclosing the metal in a fire-clay

container and packing it with a carbon-rich material such as charcoal. The container is then sealed, placed in furnace, and heated.  GAS CARBURIZING – is similar to pack carburizing except the carbon monoxide gas combines with gamma iron and forms a high-carbon surface.  LIQUID CARBURIZING – produces a high-carbon surface when a part is heated in a molten salt bath of sodium cyanide or barium cyanide.

2. NITRIDING – differs from carburizing in that a part is first hardened, tempered and then ground to its finished dimensions before it is case hardened. 3. CYANIDING – is a fast method of producing surface hardness on an iron-based alloy of low carbon content.

Hardening of Aluminum Alloys

Forming/Shaping and Forging

Bending Lay-out

Joining of Metals

Aircraft Welding Fusion welding is the blending of compatible molten metals into one common part or joint. Fusing of metals is accomplished by producing sufficient heat for the metals to melt, flow together and mix. The heat is then removed to allow the fused joint to solidify. Non-fusion welding is the joining of metals by adhesion of one metal to another. The most prominent non-fusion welding processes used on aircraft are brazing and soldering, which are covered in detail later in this section.

FUSION WELDING PROCESSES

The three principal methods of fusion welding are gas, electric arc, and electrical resistance. Fusion welding results in superior strength joints because the metal parts are melted together into a single solid object. Since fusion-welded joints are used extensively in high-stress applications, their failure is likely to have catastrophic consequences. To fully appreciate the level of detail that must be exercised when inspecting welded components, you must be aware of the characteristics that define a quality fusion-welded joint.

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OXYACETYLENE WELDING Oxyacetylene welding, often referred to as gas welding, gets its name from the two gases, oxygen and acetylene, that are used to produce a flame. Acetylene is the fuel for the flame and oxygen supports combustion and makes the flame hotter. The combination of these two gases results in sufficient heat to produce molten metal. The temperature of the oxyacetylene flame ranges from 5,600 to 6,300 F



ELECTRIC ARC WELDING Electric arc welding includes shielded metal arc welding, gas metal arc welding, and tungsten inert gas [TIG) arc welding. Although TIG welding is the method that is predominantly used in aircraft fabri cation and repair, a technician is also required to understand the other methods.

SHIELDED METAL ARC WELDING In SMAW welding, a metal wire rod, which is composed of approximately the same chemical composition as the metal to be welded, is clamped in an electrode holder. This holder, in turn, is connected to one terminal of the TR power supply by a heavy gauge electrical cable. The metal to be welded is attached to the other terminal of the power supply through another electrical cable usually equipped with a spring clamp.



GAS METAL ARC WELDING Gas metal arc welding (GMAW), formerly called Metal Inert Gas (MIG) welding, is used primarily in large volume production work. An advantage of GMAW over stick welding is that no slag is deposited on the weld bead. An uncoated filler wire acts as the electrode. It is connected to one terminal on the power supply, and fed into the torch. An inert gas such as argon, helium or carbon dioxide flows out around the wire to protect the weld zone from oxygen. The metal to be welded is connected to the other terminal of the power supply. When power is supplied to the electrode, and it is brought into contact with the work, it produces an arc, which melts the metal and the filler wire.



TUNGSTEN INERT GAS WELDING Tungsten inert gas welding (TIG) is the form of electric arc welding that is used most in aircraft maintenance. TIG welding uses a tungsten electrode that does not act as filler rod. The electrode is connected to an AC or DC electrical power supply to form an arc with the metal being welded. The arc is concentrated on a small area of the metal, raising its temperature to as high as 11,000 F, without excessively heating the surrounding metal. The base metal melts in the area of the arc and forms a puddle into which the filler rod is added.



ELECTRIC RESISTANCE WELDING Many thin sheet metal parts for aircraft, especially stainless steel parts, are joined by one of the forms of electric resistance welding; either spot welding or seam welding. SPOT WELDING When spot welding, two copper electrodes are held in the jaws of a vise-like machine and the pieces of metal to be welded are clamped between them. Pressure is applied to hold the electrodes tightly together while electrical current passes between the electrodes. SEAM WELDING While it would be possible to create a seam with a series of closely spaced spot welds, a better method is to use a seam welder. This equipment is commonly used to manufacture fuel tanks and other components where a continuous weld is needed.

TYPES OF WELDED JOINTS

Weld inspection 

~ A good weld is uniform in width, with even ripples that taper off smoothly into the base metal. There should be no burn marks or signs of overheating, and no oxide should form on the base metal more than 1/2 inch from the weld. Furthermore, a good weld must be free of gas pockets, porosity, and inclusions

Weld inspection 

Penetration is the depth of fusion in a weld, and is the most important characteristic of a good weld. Penetration depends on the thickness of the material to be joined, the size of the filler rod, and welding technique. A typical butt weld should penetrate 100 percent of the thickness of the base metal, while a fillet weld must penetrate 25 to 50 percent

Weld inspection 

~Poor welds display certain telltale characteristics. For example, too much acetylene makes the molten metal boil, causing bumps along the center and craters along the weld's edge. A cold weld has irreg ular edges and considerable variation in depth of penetration, while excessive heat produces a weld with pitting along its edges and long, pointed ripples. If a part is cooled too quickly after being welded, cracks often appear adjacent to the weld. Whenever a welded joint displays any of these defects, all of the old weld must be removed and the joint rewelded

Aircraft Hardware, Cables, and Tools, Equipment

AIRCRAFT RIVETS, BOLTS, NUT, SCREWS AND THREADED FASTENERS

RIVETS

Rivet Head Style

Rivet Head Markings

C = 1.5 D

N = 0.5 D NOTE: As a rule of thumb, to determine fastener diameter to be used will be 3x the thickness of the thickest sheet.

426 – Countersunk Head (100 degrees) 470 – Universal Head



The 2117-T rivet is designated as an “AD” rivet, and has a dimple on the head. A “B” designation is given to a rivet of 5056 material and is marked with a raised cross on the rivet head. Each type of rivet is identified by a part number to allow the user to select the correct rivet. The numbers are in series and each series represents a particular type of head.



Countersunk head rivets (MS20426 supersedes AN426 100-degree) are used where a smooth finish is desired. The 100-degree countersunk head has been adopted as the standard in the United States. The universal head rivet (AN470 superseded by MS20470) has been adopted as the standard for protrudinghead rivets, and may be used as a replacement for the roundhead, flathead, and brazier head rivet. These rivets can also be purchased in half sizes by designating a “0.5” after the main length (i.e., MS20470 AD4-3.5).

Identification marking of rivet MS 20470AD3-5 MS Military 20470 AD 3 5

Complete part number standard number Universal head rivet 2117-T aluminum alloy 3/32nds in diameter 5/16ths in length

Bulbed Cherrylock Rivets. One of the earlier types of mechanical-lock rivets developed were Bulbed Cherrylock blind rivets. These blind rivets have as their main advantage the ability to replace a solid shank rivet size for size.

The CherryMax •It uses one tool to install three standard rivet diameters and their oversize counterparts. •This makes the use of CherryMax rivets very popular with many small general aviation repair shops. •The CherryMax rivets consists of five parts; bulbed blind header, hollow rivet shell, locking (foil) collar, driving anvil, and pulling stem.

An Olympic-Lok •is a light three-piece mechanically locked, spindle-type blind rivet. It carries its stem lock integral to the manufactured head.

Huck rivets The Huck rivet has the ability to tightly draw-up two or more sheets of metal together while being installed.

Common pull-type Pop rivets Produced for nonaircraft related applications, are not approved for use on certificated aircraft structures or components.

AIRCRAFT BOLTS ( Threaded fasteners) GENERAL PURPOSE BOLTS The hex head AN 3 THROUGH AN 20 is an all purpose structural bolt used for general applications involving tension and shear loads where a light drive fit is permissible. Fabricated from SAE 2330 nickel steel and cadmium plated. Identified by a cross or asterisk

AIRCRAFT BOLTS ( Threaded fasteners)

AN BOLT HEAD IDENTIFICATION GENERAL PURPOSE - CROSS OR ASTERISK

EX. AN4-8A

EX. AN4-8

AIRCRAFT BOLTS ( Threaded fasteners) GENERAL PURPOSE BOLTS The AN 73 - AN 81 (MS20073-MS20074) drilled head bolt is similar to the standard hex bolt, but has a deeper head that is drilled to receive wire for safetying. The AN3-AN20 and AN73AN81 series bolts are interchangeable.

AIRCRAFT BOLTS ( Threaded fasteners) CLOSE TOLERANCE BOLTS This type of bolt is machined more accurately than the general purpose bolt. They can be Hex headed - (AN173-AN186) or have a Countersunk head- (NAS80-NAS86) they are used in applications where a tight drive fit is required (the bolt will only move into position only when struck with a 12-14 ounce hammer)

AIRCRAFT BOLTS ( Threaded fasteners)

AN BOLT HEAD IDENTIFICATION CLOSE TOLERANCE - CROSS OR ASTERISK INSIDE A TRIANGLE

AIRCRAFT BOLTS ( Threaded fasteners) CLASSIFICATION OF THREADS NC – American national coarse NF – American national fine UNC – American standard unified coarse UNF – American standard unified fine

AIRCRAFT BOLTS ( Threaded fasteners) THREAD designation Threads are designated by the number of times the incline (threads) rotates around a 1 inch length of given diameter bolt or screw. EX. 4-28 thread indicates that a ¼” dia. Bolt has 28 threads in 1” of its thread length.

AIRCRAFT BOLTS ( Threaded fasteners) THREAD designation Threads are designated by the Class fit (tolerance allowed in manufacturing). Class 1 – Loose fit (Easily turned by the fingers) Class 2 – Free fit (Aircraft Screws) Class 3 – Medium fit (Aircraft Bolts) Class 4 – Close fit (Requires a wrench to turn the nut onto a bolt)

Limits and Fits  



Clearance Fit – in this assembly there is a space between the two parts. The shaft is always smaller than the part it fits into. Interference Fit – in this assembly there is no space between the parts. The shaft is always larger than the part it fits into. This means that force is required to assemble the parts. Transition Fit – this is a range of fits which can be either clearance or interference. The shaft can be larger or smaller than the part it fits into.

AIRCRAFT BOLTS ( Threaded fasteners) AN4-8A • AN means the bolt is manufactured according to Air Force-Navy specs. • 4 identifies the diameter of the bolt shank in 1/16" increments • 8 identifies the length of the shank in 1/8" increments • A means the shank of the bolt is un-drilled (no letter here means a drilled shank)

AIRCRAFT BOLTS ( Threaded fasteners) AN4-H8A

• AN means the bolt is manufactured according to Air Force-Navy specs.

• 4 identifies the diameter of the bolt shank in 1/16" increments • H identifies the head is drilled • 8 identifies the length of the shank in 1/8" increments • A means the shank of the bolt is un-drilled (no letter here means a drilled shank)

AIRCRAFT BOLTS ( Threaded fasteners)

Within a given diameter (i.e. 1/4, 3/8, 1/2, etc.) of any AN/MS/NAS series, all bolts will have the same thread length, no matter how long the bolt. The thread lengths for each series bolt are on the specification prints and in a chart under the "Aerospace Bolt Interchange" heading under Tech Info In all MS and NAS series bolts, the dash number is the grip in 1/16" (0.0625") increments, e.g. -18 = 18 x 0.0625" = 1.125" = 18/16". Thus, to determine the overall length of a bolt, simply add the thread length for that series and diameter to the grip length you desire, e.g. NAS 1306-24: grip is 1.50" + threads: 0.578" = 2.078" overall length. In AN series bolts, you must have a chart or bolt gauge to determine lengths, grips or part numbers. THE DASH NUMBERS DO NOT INDICATE EITHER GRIPS OR OVERALL LENGTHS.

AIRCRAFT BOLTS ( Threaded fasteners)

AIRCRAFT BOLTS ( Threaded fasteners)

BOLT GRIP LENGTH TOO SHORT BOLT GRIP LENGTH CORRECT

BOLT GRIP LENGTH TOO LONG

AIRCRAFT BOLTS ( Threaded fasteners)

Types of Bolts

COUNTERSUNK HEAD BOLT

INTERNAL WRENCHING BOLT

DRILLED HEX HEAD BOLT

CLEVIS BOLT

AIRCRAFT BOLTS ( Threaded fasteners)

HEAD MARKINGS

CLOSE TOLERANCE (STEEL OR ALUMINUM ALLOY)

ALUMINUM ALLOY (62,000 P.S.I.)

STEEL 125,000 P.S.I

CORROSION RESISTANT STEEL (125,000 P.S.I.)

STEEL 150,000 P.S.I

MACHINE SCREW COUNTERSUNK HEAD

grip length

STRUCTURAL SCREW ROUND HEAD

grip length

SELF-TAPPING SCREW

CAUTION CAUTION Self-tapping Screws Self-tapping Screws should should never never be be used used to to replace replace standard standard screws, screws, nuts, nuts, or or rivets rivets originally originally used used in in the the structure. structure.

BRAZIER HEAD

grip length

Certain accepted practices prevail concerning the installation of hardware. A few of these regarding bolt installation follow: 1. In determining proper bolt length - no more than one thread should be hidden inside the bolt hole. 2. Whenever possible, bolts should be installed pointing aft and to the center of an airplane. 3. Use a torque wrench whenever possible and determine torque values based on the size of bolt. 4. Be sure bolt and nut threads are clean and dry. 5. Use smooth, even pulls when tightening. 6. Tighten the nut first - whenever possible.

Certain accepted practices prevail concerning the installation of hardware. A few of these regarding bolt installation follow: 7. A typical installation includes a bolt, one washer and a nut. 8. If the bolt is too long, a maximum of three washers may be used. 9. If more than three threads are protruding from the nut, the bolt may be too long and could be bottoming out on the shank.

Certain accepted practices prevail concerning the installation of hardware. A few of these regarding bolt installation follow: 10. Use un-drilled bolts with fiber lock nuts. If you use a drilled bolt and fiber nut combination, be sure no burrs exist on the drilled hole that will cut the fiber. 11. If the bolt does not fit snugly consider the use of a close tolerance bolt. 12. Don't make a practice of cutting off a bolt that is too long to fit a hole. That can often weaken the bolt and allow corrosion in the area that is cut.

AIRCRAFT NUTS

AIRCRAFT NUTS Aircraft nuts usually have no identification on them but they are made from the same material as bolts. Due to the vibration of aircraft, nuts must have some form of a locking device to keep them in place. The most common ways of locking are cotter pins used in castle nuts, fiber inserts, lockwashers, and safety wire. The aircraft nuts you will most likely encounter are castle nuts, self-locking nuts, and plain nuts. Wing nuts and anchor nuts are also used.

AIRCRAFT NUTS Castle Nuts AN310 and AN320 castle nuts are the most commonly used (see Figure). Castle nuts are fabricated from steel and are cadmium plated. Corrosion resistant castle nuts are also manufactured (AN310C and AC320C - remember, when you encounter a "C" it will designate stainless). Castle nuts are used with drilled shank bolts, clevis bolts and eye bolts. The slots in the nut accommodate a cotter pin for safetying purposes. The thinner AN320 castellated shear nut has half the tensile strength of the AN310 and is used with clevis bolts which are subject to shear stress only. The dash number following the AN310 or AN320 indicates the size bolt that the nut fits. In other words, an AN310-4 would fit a 1/4 inch bolt.

AIRCRAFT NUTS Castle Nuts

AN310 Cad Plated

AN310 Steel

AN320 Shear Cad Plated

AN320 Shear Steel

AIRCRAFT NUTS Self-Locking Nuts Self-locking nuts, as the name implies, do not need a locking device. The most common method of locking is derived from a fiber insert. This insert has a smaller diameter than the nut itself so that when a bolt enters the nut it taps into the fiber insert producing a locking action. This fiber insert is temperature limited to 250-deg. F. The designation of these nuts is AN365 and AN364. This brings us to an example of a cross-reference MS number. An AN365 is also termed MS20365 with the AN364 being MS20364. Both of these nuts are available in stainless. The AN364 is a shear nut not to be used in tension.

AIRCRAFT NUTS Nylon Insert

AN 365 Cad Plated

AN 364 Shear Cad Plated

AN 365 Steel

AIRCRAFT NUTS Self-Locking Nuts An all metal locking nut is used forward of the firewall and in other high temperature areas. In place of a fiber insert, the threads of a metal locking nut narrow slightly at one end to provide more friction. An AN363 is an example of this type of nut. It is capable of withstanding temperatures to 550-deg. F..

AIRCRAFT NUTS Metal Locking Nut

MS21045 Cad Steel Torque Nut (Old AN363)

Molybdenum Dry Lube 450° Low Height Hex Locknut

AIRCRAFT NUTS

The dash number following self-locking nut defines the thread size. Self-locking nuts are very popular and easy to use. They should be used on un-drilled bolts. They may be used on drilled bolts if you check the hole for burrs that would damage the fiber. One disadvantage, self-locking nuts should not be used on a bolt that is connecting a moving part. An example might be a clevis bolt used in a control cable application.

AIRCRAFT NUTS Plain Aircraft Nuts Plain nuts require a locking device such as a check nut or lockwasher. They are not widely used in most aircraft. AN315 is the designation used for a plain hex nut. These nuts are also manufactured with a right hand thread and a left hand thread. The check nut used to hold a plain nut in place is an AN316. If a lockwasher is used a plain washer must be under the lockwasher to prevent damage to the surface.

AIRCRAFT NUTS Other Aircraft Nuts There are a number of other aircraft nuts available. Wing nuts (AN350) are commonly used on battery connections or hose clamps where proper tightness can be obtained by hand. Anchor nuts are widely used in areas where it is difficult to access a nut. Tinnerman nuts, instrument mounting nuts, pal nuts, cap nuts, etc. are all examples of other types that are used.

AIRCRAFT NUTS Tinnerman Nuts

Short U use with Type "B" sheet metal screws

Flat use with Type "B" sheet metal screws

AIRCRAFT NUTS Basics of Aircraft Nut Installation 1. When using a castle nut, the cotter pin hole may not line up with the slots on the nut. The Mechanics General Handbook states "except in cases of highly stressed engine parts, the nut may be over tightened to permit lining up the next slot with the cotter pin hole." Common sense should prevail. Do not over tighten to an extreme, instead, remove the nut and use a different washer and then try to line the holes again. 2. A fiber nut may be reused if you are unable to tighten by hand.

AIRCRAFT NUTS Basics of Aircraft Nut Installation 3. At least one thread should be projecting past the fiber on a fiber nut installation. 4. No self-locking nuts on moving part installations. 5. Do not use AN364 or AN365 fiber nuts in areas of high temperature - above 250' F. 6. Shear nuts are to be used only in shear loads (not tension). 7. Plain nuts require a locking device such as a lockwasher or a check nut.

AIRCRAFT NUTS Basics of Aircraft Nut Installation 8. When using a lockwasher, place a plain washer between the surface of the airplane part and the lockwasher. 9. Shear nuts and standard nuts have different torque values. 10.Use wing nuts only where hand tightness is adequate.

AIRCRAFT WASHERS

AIRCRAFT WASHERS

The main purposes of a washer in aircraft installation are to provide a shim when needed, act as a smooth load bearing surface, and to adjust the position of castle nuts in relation to the drilled hole in a bolt. Also, remember that plain washers are used under a lockwasher to prevent damage to a surface.

AIRCRAFT WASHERS AN960 washers are the most common. They are manufactured in a regular thickness and a thinner thickness (one half the thickness of regular). The dash number following the AN960 indicates the size bolt for which they are used. The system is different from others we have encountered. As an example, an AN960616 is used with a 3/8" bolt.

AIRCRAFT WASHERS

AN960 Cad plated Standard Thickness Flat Washers (Heavy)

Part No.

Size

AN960-6/50

6

AN960-8/50

8

AN960-10/50

3/16

AN960-416/50

1/4

AN960-516/50

5/16

AN960-616/50

3/8

AN960-716/50

7/16

AN960-816/50

1/2

AN960-916

9/16

AN960-1016

5/8

AN960-1216

3/4

AIRCRAFT WASHERS

AN960-6L/50

AN960 Cad plated Half Thickness Flat Washers (Light)

AIRCRAFT WASHERS

AN960C STAINLESS STEEL Standard Thickness Flat Washers (Heavy) AN960C-6/50

AN960C STAINLESS STEEL Half Thickness Flat Washers (Light) AN960C-6L/50

AIRCRAFT WASHERS

Internal Tooth Cad Plated Steel Lock Washer MS35333-36 B

Internal Tooth Stainless Steel Lock Washer MS35333-70 B

AIRCRAFT WASHERS

Stainless 82° Cup Interior Finishing Washer FCW4

Stainless 100° Countersunk Washer DD06SS

AIRCRAFT WASHERS

AN970 Cad Plate Large Area Washer

Natural Color Nylon Flat Washer NW-4 B

Control Cables and Cable Assemblies

NONFLEXIBLE CABLE In areas where a linkage does not pass over any pulleys nonflexible cable can be used. It is available in either a 1 x 7 or 1 x 19 configuration. The 1x7 cable is made up of one strand comprised of seven individual wires, whereas the 1 x 19 consists of one strand made up of 19 individual wires. Nonflexible cable is available in both galvanized carbon steel and stainless steel.

FLEXIBLE CABLE Flexible steel cable made up of seven strands of seven wires each is called 7 x 7 or flexible cable, To check the tension of aircraft control cables a and is available in 1/16 and 3/32 inch sizes in both galvanized carbon steel and stainless steel. Both types are preformed which means that when the cable is manufactured each strand is formed into a spiral shape. This process keeps strands together when the cable is wound and also helps prevent the cable from spreading out when cut. Furthermore, preforming gives cable greater flexibility and relieves bending stresses when the strands are woven into the cable.

EXTRA-FLEXIBLE CABLE The most widely used cable, 7 x 19, is available in sizes from 1/8 inch up. It is extra flexible and is made of 133 individual wires wound in seven strands, each strand having 19 wires. These cables are preformed and are available in both galvanized and stainless steel. Galvanized cable is more resistant to fatigue than stainless steel, but in applications where corrosion is a factor, stainless steel is used

ATTACHING CABLES At one time, most cables were attached to bellcranks, control surfaces, and flight controls with woven splices, such as the Army-Navy five-tuck splice or the Roebling roll. Because both types of woven splices require a great deal of hand work and develop only 75 percent of the cable strength, this method of attaching cables has almost been completely replaced.

SWAGED TERMINALS The cable fittings used most in large aircraft manu facture are MS-type swaged cable terminals. To install these terminals, cut the cable and insert it into the end of a terminal. Then, use either a hand or power swaging tool to force the metal of the ter minal down into the cable. This forms a joint that is at least as strong as the cable itself.

To ensure that a terminal is properly swaged, a measurement is made of the swaged terminal with a go/no-go gauge. The swaging process must decrease the terminal's diameter to the extent that the go end of a go/no-go gauge passes over the swaged terminal, but the no-go end does not. As an inspection aid to ensure the cable does not pull out of the terminal, a small mark of paint is placed over the terminal end and onto the cable. A broken paint mark indicates the cable has slipped inside the terminal.

NICOPRESS OVAL SLEEVES Many light aircraft use Nicopress sleeves that are squeezed onto control cables to form terminal ends. A nicopress sleeve is made of copper and has two holes to accommodate a control cable. When a cable is wrapped around an AN100 thimble and properly squeezed with the correct Nicopress squeezer, the terminal develops at least the strength of the cable.

TURNBUCKLES Turnbuckles are a type of cable fastener that allows cable tension to be adjusted. A complete turnbuckle assembly consists of two ends, one with right-hand threads and the other having left-hand threads, with a brass barrel joining them. Minor cable adjustment is made by rotating the turnbuckle which effectively lengthens or shortens the cable's length.

Part 2

AIRCRAFT CONSTRUCTION, REPAIR AND MODIFICATION

Aircraft Structural Components Metal Structures Non-Metal Structures Composite Materials

Structural Design Philosophies Fail Safe – relies upon a duplication of certain structural members to ensure that if one member failed, the other would assume the load of the failed member.

Damage Tolerance – requires an evaluation of the structure to ensure that should serious damage, that is cracking or partial failure, occur within the operational life of the aircraft, the remaining structure can withstand reasonable loads without failure until the damage is detected.

Fatigue – This phenomenon of fracturing after a series of cyclic loads, maybe much less than the ultimate load.

Safe Life – The period during which it is considered that failure of a component is extremely unlikely. Life may be expressed in flying hours, elapsed time, number of flights or number of applications of load.

AIRFRAME 

FUSELAGE – the main structure or body of the airplane.  TRUSS – is a rigid framework made up of members such as beams, struts,

and bars to resist deformation by applied load.  SEMI-MONOCOQUE – consist of a framework of vertical and longitudinal members covered with a structural skin that carries the large percentage of the stresses.  MONOCOQUE – it involves the construction of a metal tube or cone without structural member. ○ Bulkhead – the vertical members of the fuselage frames. Structural partitions that ○

○ ○ ○ ○

runs perpendicular to the longerons. Frame - lateral fuselage or nacelle member giving cross-sectional shape which is often circular. Also known as FORMERS or RINGS that maintains the uniform shape of the structure. Stringers – (for semi-monocoque) the longitudinal members serves for stiffening the metal skin and prevent it from bulging or buckling under severe stresses. Gusset or Gusset Plates – to reinforce the intersecting structural members and to transfer stresses from one member to another. Longeron – the main longitudinal member of a fuselage or nacelle. Skin – the smooth outer cover of the aircraft. The materials used for the skin covering is usually sheets aluminum alloy.

AIRFRAME 

CRACK STOPPER - A reinforcing member normally placed at right angles to the path of an anticipated crack which will reduce the rate of further propagation.



AERODYNAMIC LOADING - The loads imposed on an aircraft in flight.



STATIC LOADING - The loads imposed on an aircraft when stationary.



STATION NUMBERS - Numbers allocated to certain components, e.g. frames and ribs, to indicate their positions within the structure. The numbers may represent in inches the distance from a datum point which could be the fuselage, nose or the wing root.

FUSELAGE



WING – aerofoil structure that produces lift of an airplane . ○ CANTILEVER – no external bracing is needed. ○ SEMI-CANTILEVER – uses external bracing (strut, wires, etc.)  Spars – it is the principal structural members of the wing.  Ribs – used to give the shape of the wing and to transmit the load from the skin to the spars.  Wing Tip – smooth out the wing tip airfoil to give wing a finish look.  Fairing/Fillets – used to smooth the airflow over the angles formed by the wings and other structural units with the fuselage, shaped rounded panels or metal skin are attached  Tie rod (Tension rod) – members taking tensile load.  Strut – members taking compression load



EMPENNAGE – the complete tail assembly of an aircraft. HORIZONTAL TAIL  Horizontal stabilizers – fixed surface  Elevators – movable surface

VERTICAL TAIL  Vertical stabilizers – fixed surface (FIN)  Rudder – movable surface

WING

VERTICAL STABILIZER

HORIZONTAL STABILIZER



FLIGHT CONTROL SURFACES  PRIMARY GROUP ○ AILERON – attached to both wings of an aircraft that goes up and down, thus, causing the aircraft to roll at longitudinal axis. ○ RUDDER – hinged to TE of the vertical stabilizer to turn about its vertical axis. ○ ELEVATOR – attached to the TE of the horizontal stabilizer use for control on pitch up and pitch down at its lateral axis.  SECONDARY or AUXILIARY GROUP – their purpose is to reduce the force

required to actuate the primary controls, to trim and balance the aircraft in flight, to reduce speed or shorten the landing run and the change the speed of the aircraft flight. ○ Trim tabs – used to make fine adjustments to the flight path of the aircraft. ○ Balance tabs – movement of the main control surface will give an opposite ○ ○ ○ ○



movement to the tab. Servo tabs – referred to as flight tabs. Flaps – use to increase area of wing for the purpose of increasing lift. Spoilers – a device designed to reduce the lift of the wing. Use for speed brakes. Leading edge devices – a high lift device (SLATS) normally used on large transport category.

LANDING GEAR - is the assembly that supports the aircraft during landing or while resting or moving about on the ground.

FLIGHT CONTROL SURFACE

Repair Basic

Structural Repair Manual (SRM) – ATA Chapters The manual is divided into the following chapters:  Chapter 51 - Structures, General  Chapter 52 – Doors  Chapter 53 - Fuselage  Chapter 54 - Nacelles and Pylons  Chapter 55 - Stabilizers  Chapter 56 - Windows  Chapter 57 - Wings

Structural Repair Manual (SRM) 

Page Block (PB) 001 – IDENTIFICATION ○ Pages 1-99, Figures 1-99, Tables 1-99



Page Block (PB) 101 – ALLOWABLE DAMAGE ○ Pages 101-199, Figures 101-199, Tables 101-199



Page Block (PB) 201 – REPAIRS ○ Pages 201-299, Figures 201-299, Tables 201-299

Aircraft Maintenance Manual (AMM)         

Page Block (PB) 001 – Description and Operation Page Block (PB) 201 – Maintenance Practice Page Block (PB) 301 – Servicing Page Block (PB) 401 – Removal and Installation Page Block (PB) 501 – Adjustment and Test Page Block (PB) 601 – Inspection and Check Page Block (PB) 701 – Cleaning and Painting Page Block (PB) 801 – Repair Page Block (PB) 901 – Deactivation/ Activation

Aircraft Zoning System Major Zones ○ 100 - Lower Half of Fuselage ○ 200 - Upper Half of Fuselage ○ 300 - Empennage and Body Section 48 ○ 400 - Power Plants and Nacelle Struts ○ 500 - Left Wing ○ 600 - Right Wing ○ 700 - Landing Gear and Landing Gear Doors (Fixed) ○ 800 - Doors



TYPES OF MAINTENANCE, REPAIR, AND ALTERATIONS PREVENTIVE MAINTENANCE – is defined as simple or minor preservation operations and the replacement of small standard parts not involving complex assembly operations. Operations classed as preventive maintenance are as follows: ○ ○ ○ ○ ○ ○ ○ ○ ○

○

○

Removal, installation, and repair of landing gear tires. Replacing elastic shock-absorber cords on landing gear. Servicing landing-gear shock struts by adding oil, air, or both. Servicing landing gear wheel bearings, such as cleaning and greasing. Replacing defective safety wiring or cotter keys. Lubrication not requiring disassembly other than removal of non-structural items such as cover plates, cowlings, and fairings. Making simple fabric patches not requiring rib stitching or the removal of structural parts or control surfaces. Replenishing hydraulic fluid in the hydraulic reservoir. Refinishing decorative coating of fuselage, wings, tail group surfaces (excluding balanced control surfaces), fairings, cowling, landing gear, cabin, or cockpit interior when removal or disassembly of any primary structure or operating system is not required. Applying preservative or protective material to components where no disassembly of any primary structure or operating system is involved and where such coating is not prohibited or is not contrary to good practices. Repairing upholstery and decorative furnishings or the cabin or cockpit interior when the repairing does not require disassembly of any primary structure or operating system or affect the primary structure of the aircraft.

Making small simple repairs to fairings, non-structural cover plates, cowlings, and small patches and reinforcements not changing the contour so as to interfere with the proper airflow. ○ Replacing side windows where that work does not interfere with the structure of any operating system such as controls, electrical equipment, etc. ○ Replacing safety belts. ○ Replacing seats or seat parts with replacement parts approved for the aircraft, not involving disassembly of any primary structure or operating system. ○ Troubleshooting and repairing broken circuits in landing light wiring circuits. ○ Replacing bulbs, reflectors, and lenses of position and landing lights. ○ Replacing wheels and skis where no weight and balance computation is involved. ○ Replacing any cowling not requiring removal of the propeller or disconnecting of flight controls. ○ Replacing or cleaning spark plugs and setting of spark plug gap clearance. ○ Replacing any nose connections except hydraulic connections. ○ Replacing pre-fabricated fuel lines. ○ Cleaning fuel and oil strainers. ○ Replacing batteries and checking fluid level and specific gravity. ○ Removing and installing glider wings and tail surfaces that are specifically designed for quick removal and installation and when such removal and installation can be accomplished by the pilot. The holder of a pilot certificate issued under FAR Part 61, may perform preventive maintenance on any aircraft owned or operated by him that is not used in air carrier service. Preventive maintenance may also be performed by certificated mechanics, repair stations, repairmen, air carriers, and others authorized by the FAA. A person who plans to perform preventive maintenance must ascertain that the operation falls within this category and that he is authorized to perform the work. ○

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Routine maintenance task ○Towing ○Taxiing ○Parking ○Mooring ○Brake Service ○360 degree inspection ○Landing Gear service ○Propeller Service ○Tire Inflation ○Battery Service ○Replenishing ○Refueling ○Defueling ○Replenishment of oil ○Oil change

AIRCRAFT CLEANING 

~ Appendix D of FAR Part 43 requires that the airframe and engine be cleaned before performing an annual or 100 hour inspection. . Dirt can cover up cracked or damaged components

EXTERIOR CLEANING ~ pitot tubes and static openings should always be plugged or taped prior to cleaning an aircraft to prevent water ingestion. ~ wheel and brake assemblies should be covered to keep out cleaning agents ~ is extremely important to use the cleaning compounds and other chemicals that are recommended by the aircraft manufacturer, or are MIL SPEC approved for the particular application.

EXTERIOR CLEANING ~ Hydrogen embrit-tlement results when a chemical reaction produces hydrogen gas that is absorbed into a metal ~Avoid washing an aircraft in the sun to help prevent the surface from drying before the cleaner has time to penetrate the film and dirt. For the main part of the aircraft exterior, use a 1:5 or a 1:3 mixture of water and an emulsion-type cleaner that meets MIL-C-15769 specifications. Brush or spray the mixture onto the surface and allow it to stand for a few minutes, then rinse it off with a high-pressure stream of warm water.

EXTERIOR CLEANING ~ The engine cowling and wheel well area usually have grease or oil deposits that require special treatment. Typically, these areas must be soaked with a 1:2 mixture of emulsion cleaner and water. After allowing the cleaner to remain on the surface for a few minutes, scrub the heavily soiled areas with a soft bristle brush to completely loosen the dirt, and rinse it with a high-pressure stream of warm water. ~Stubborn exhaust stains may require a 1:2 mixture of cleaner with Varsol or kerosene. Mix these ingredients into a creamy emulsion and apply it to the surface. Let it stand for a few minutes, then work all of the loosened residue with a bristle brush and hose it off with a high-pressure stream of warm water

EXTERIOR CLEANING ~ to remove oil, grease, or soft preservative compounds, dry-cleaning solvent, or naphtha, is often used. ~ Aliphatic naphtha is a hydrocarbon solvent that dissolves oil and grease but does not harm rubber or acrylic components. Aromatic naphtha, on the other hand, attacks rubber and acrylic compounds. ~ Chemical cleaners must be used with great care in cleaning assembled aircraft. The danger of entrapping a potentially corrosive solvent in faying surfaces and crevices counteracts any advantages in their speed and effectiveness. ~, caustic cleaners can cause corrosion on aluminum or magnesium alloys

EXTERIOR CLEANING ~ Magnesium engine parts should be washed with a commercial solvent and decarbonized, and then scraped or grit blasted. Before they are painted, magnesium engine parts should be wiped down with a dichromate solution to improve paint adhesion. ~When cleaning aluminum, you should always use cleaners which are relatively neutral and easy to remove. If you must use an abrasive to remove cor rosion products from aluminum structure, use aluminum wool or aluminum oxide sandpaper. Carborundum paper, crocus cloth, and steel wool must be avoided

EXTERIOR CLEANING ~ Magnesium engine parts should be washed with a commercial solvent and decarbonized, and then scraped or grit blasted. Before they are painted, magnesium engine parts should be wiped down with a dichromate solution to improve paint adhesion. ~When cleaning aluminum, you should always use cleaners which are relatively neutral and easy to remove. If you must use an abrasive to remove cor rosion products from aluminum structure, use aluminum wool or aluminum oxide sandpaper. Carborundum paper, crocus cloth, and steel wool must be avoided

NON METALIC CLEANING ~, the slightest amount of dust on a plastic or plexiglass surface can scratch the finish if rubbed with a dry cloth. ~ washing a plastic window, rinse the area with water first. Once clean, dry the window with a soft cloth to prevent streaking

NON METALIC CLEANING ~ Oil and hydraulic fluid attack and rapidly destroy the rubber in aircraft tires. . The tire should then be washed with soap and water Because most cleaning solvents are petroleumbased, soap and water are the only approved solution for cleaning tires. ~ Rubber deice boots have a conductive coating to help dissipate static charges. ~ as radomes are painted with special materials that are transparent to radio signals. These areas should be cleaned gently and never subjected to abrasives or stiff brushes

POWERPLANT CLEANING ~ Prior to washing an engine, inspect it for excessive oil leakage. Then, apply a soap or solvent solution to the engine and allow it to set for several minutes. Heavy accumulations of dirt and grease can be scrubbed with a bristle brush to loosen them. The engine is then rinsed and allowed to dry. If volatile solvent was used, be sure it has dried before starting the engine to minimize the risk of fire. Lubricate all controls and rod ends in the engine compartment, and remove protective covers that were installed on electrical components.

POWERPLANT CLEANING ~ electrical components in the engine compartment must be protected from solvent and soap. This includes wrapping the magnetos so no water can get in the vents. If the powerplant is located over a landing gear, the gear's brake and tire assemblies should be covered in plastic. If you use a high-pressure water spray, avoid spraying the starter, alternator, and air intakes with solvent or water rinse.

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CLASSIFICATION OF ALTERATIONS:  MAJOR ALTERATIONS – is an alteration not listed in the aircraft, aircraft engine, or propeller

specifications (1) that might appreciably affect weight, balance, structural strength, performance, power plant operation, flight characteristics and other factors of airworthiness or (2) that is not done according to accepted practices or cannot be done by elementary operations. Alterations of the following parts and alterations of the following types, when not listed in the aircraft specifications issued by the FAA, are airframe major alterations: - Wings - Tail Surfaces - Fuselage - Engine mounts - Control system - Landing gear - Hull or floats - Elements of an airframe, including spars, ribs, fittings, shock absorbers, bracing cowlings, fairings, and balance weights - Hydraulic and electrical actuating systems or components - Rotor blades - Changes to the empty weight or empty balance which result in an increase in the maximum certificated weight or center-of-gravity limits of the aircraft. - Changes in the basic design of the fuel, oil, cooling, cabin pressurization, electrical, hydraulic, deicing, or exhaust systems. - Changes to the wing or to fixed or movable control surfaces which affect flutter and vibration characteristics.

The following alterations of a power plant, when not listed in the engine specifications issued by the FAA, are power plant major alterations. - Conversion of an aircraft engine from one approved model to another, involving any changes in compression ratio, propeller reduction gear, impeller gear ratios, or the substitution of major engine parts which requires extensive rework and testing of the engine. - Changes to the engine by replacing aircraft engine structural parts with parts not supplied by the original manufacturer or parts not specifically approved by the FAA administrator. - Installation of an accessory which is not approved for the engine. - Removal of accessories that are listed as required equipment on the aircraft or engine specification. - Installation of structural parts other than the type of parts approved for the installation. - Removal of accessories that are listed as required equipment on the aircraft or engine specification. - Installation of structural parts other than the type of parts approved for the installation. - Conversions of any sort for the purpose of using fuel of a rating or grade other than that listed in the engine specifications.

Minor Alterations – of either an airframe or a power plant are alterations other than major alterations.

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CLASSIFICATION OF REPAIRS  Repairs of airframes and power plants are classified as either major or minor depending upon the

type and effect of the repair. A major repair is one which, if improperly done, might appreciably affect the weight, balance, structural strength, performance, power plant operation, flight characteristics, or other qualities affecting airworthiness; or one which is not done according to accepted practices or cannot be done by elementary operations.  Repairs to the following parts of an airframe and repairs of the following types, involving the strengthening, reinforcing, splicing, and manufacturing of primary structural members, or their replacement (when replacement is by fabrication such as riveting or welding), are airframe major repairs: - Box beams - Monocoque or semi-monocoque wings or control surfaces - Wing stringers or chord members - Spars - Spar flanges - Members of truss-type beams - Thin sheet webs of beams - Keel and chine members or boat hulls or floats - Corrugated sheet compression members which act as flange material of wings or tail surfaces - Wing main ribs and compression members - Wing or tail surface brace struts - Engine mounts - Fuselage longerons - Members of the side truss, horizontal truss, or bulkheads - Main seat support braces and brackets - Landing gear brace struts

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Axles Wheels Skis and ski pedestals Parts of the control system such as control columns, pedals, shafts, brackets, or horns Repairs involving the substitution of material The repair of damaged areas in metal or plywood stressed covering exceeding 6 in. in any direction The repair of portions of skin sheets by making additional seams The splicing of skin sheets The repair of three or more adjacent wing or control surface ribs or the leading edge of wings and control surfaces between such adjacent ribs Repair of fabric covering involving an area greater than that required to repair two adjacent ribs Replacement of fabric on fabric covered parts such as wings, fuselages, stabilizers, and control surfaces Repairing of removable or integral fuel tanks and oil tank, including re-bottoming the tanks

 Repairs of the following parts of an engine and repairs of the following

types are powerplant major repairs: -

Separation or disassembly of a crankcase or crankshaft of a reciprocating engine equipped with an integral superchargers Separation or disassembly of a crankcase or crankshaft of a reciprocating engine equipped with other than spur-type propeller reduction gearing Special repairs to structural engine parts by welding, plating, metalizing, or other methods.

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Annual and 100-hr inspections  According to the provisions of FAR, Part 91, no person may operate an aircraft unless, within the

preceding 12 calendar months, it has had an annual inspection and has been approved for return to service by an authorized person. An inspection for the issuance of an Airworthiness Certificate will serve as a substitute for the annual inspection.  A 100-hr inspection – is similar to the annual inspection; however, it may not be substituted for the annual inspection unless it is performed by a person certificated or otherwise authorized to make annual inspections and is entered as an annual inspection in the aircraft maintenance records (log book). A 100-hr inspection is required on every aircraft used for carrying persons for hire other than the crew or for giving flight instruction. This means the aircraft must undergo a complete inspection, as set forth in FAR, Part 43, within every 100 hrs of operating time. After the 100-hr limitation may be exceeded by not more than 10 hr if necessary to reach a place at which the inspection can be made. The excess time, however, is included in computing the next 100 hr of time in service.

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Progressive inspection  A progressive inspection – requires the setting up of a schedule, specifying the intervals in

hours or days when routine and detailed inspections will be performed, including instructions for exceeding an inspection interval by not more than 10 hr while enroute, and for changing an inspection interval because of service experience. Progressive inspection are usually established by air carriers in order to provide for better utilization of aircraft. Approval for such an inspection system requires that a properly authorized person or agency supervise the inspection procedures and that an inspection procedures manual be available and readily understandable to pilot and maintenance personnel. Aircraft subject to an approved progressive inspection system need not undergo the 100-hr inspection otherwise required.

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CONTINUOUS AIRWORTHINESS INSPECTION PROGRAMS OF LARGE & TURBINE-POWERED MULTI-ENGINE AIRPLANES:  AIRCRAFT INSPECTIONS – this element deals with the routine inspections, servicing, and tests

performed on the aircraft at prescribed intervals. It includes detailed instructions and standards (or related references) by work forms, job cards, and other records which also serve to control the activity and to record and account for the tasks that comprise this element. Each airline is free to develop its own terminology, which is assigned to the different parts the inspection program. The use of terms such as A-check and D-Check as is illustrated in Figure 16-5 is common in a continuous inspection program. Figure 16-5 provides an example of the continuous inspection program.  SCHEDULED MAINTENANCE – this element concerns maintenance tasks performed at prescribed intervals. Some are accomplished concurrently with the inspection tasks that are part of the inspection elements and may be included on the same form. Other tasks are accomplished independently. The scheduled tasks include the replacement of life-limited items and components requiring replacement for periodic overhaul; special inspections such as X-rays, checks, or tests for on-condition items; lubrications; and so on. Special work forms can be provided for accomplishing these tasks or they can be specified by a work order or some other document. In any case, instructions and standards for accomplishing each task should be provided to ensure that it is properly accomplished and that it is recorded and signed for.  UNSCHEDULED MAINTENANCE – this element provides instructions and standards for the accomplishment of maintenance tasks generated by the inspection and scheduled maintenance elements, pilot reports, failure analyses, or other indications of a need for maintenance. Procedures for reporting, recording, and processing inspection findings, operational malfunctions, or abnormal operations, such as hard landings, are an essential part of this element. A continuous aircraft logbook can serve this purpose for occurrences and resultant corrective action between scheduled inspections. Inspection discrepancy forms are usually used for processing unscheduled maintenance tasks in conjunction with scheduled inspections. Instructions and standards for unscheduled maintenance are normally provided by the operator’s technical manuals. The procedures to be followed in using these manuals and for recording and certifying unscheduled maintenance are included in the operator’s procedural manual.

Type

When

Who

No. 1 service "walk-around"

No. 2 service

Before each flight Mechanic and pilot During overnight layovers at maintenance locations; at least every 45 hours of domestic flying or 65 hours of international flying Mechanics

A-check Approximately every 200 flying hours, or about every 15 to 20 days depending on type of aircraft 3-5 Mechanics Heaviest level of routine line B-/M-/L-checks maintenance; approximately every 550 flying hours or every 40-50 days; work performed overnight 12-80 Mechanics

C-check

Every 12-15 months, depending on aircraft type; airplane out of service for 3-5 days

From 150-200 mechanics and inspectors depending on aircraft type

D-check Most intensive inspection; every 4-5 From 150-300 mechanics years, depending on aircraft type; and inspectors airplane out of service up to 30 days depending on aircraft type

Figure 16-5

What Exterior check of aircraft and engines for damage and leakage; includes specific checks such as brake and tire wear Same as No. 1 service plus specific checks including oils, hydraulics, oxygen, and unique needs by aircraft type More detailed check of aircraft and engine interior including specific checks, services, and lubrications of systems such as ingnition, generators, cabins, air conditioning, hydraulics, structure, landing gear Similar to A-check but in greater detail, which specific aircraft and engine needs such as torque tests, internal checks, and flight controls Detailed inspection and repair of aircraft, engines, components, systems and cabin, including operating mechanisms, flight controls, and structural tolerances Major structural inspections for detailed needs which include attention to fatigue corrosion; aircraft is dismantled, repaired and rebuilt as required; systems and parts are tested, repaired or replaced

Testing and Inspection a. Testing of Metals - Hardness Tests b. Non-Destructive Test and Inspection

NON- DESTRUCTIVE INSPECTION

APPROVED PROCEDURE 

Title 14 CFR, part 43 requires that all maintenance be performed using methods, techniques, and practices prescribed in the current manufacturer’s maintenance manual or instructions for continued airworthiness prepared by its manufacturer, or other methods, techniques, and practices acceptable to the administrator.

FLAWS

Inspection personnel should know where flaws occur or can be expected to exist and what effect they can have in each of the NDI test methods. Misinterpretation and/or improper evaluation of flaws or improper evaluation of NDI can result in serviceable parts being rejected and defective parts being accepted.

A). CORROSION This is the electrochemical deterioration of a

metal resulting from chemical reaction with the surrounding environment. Very common Extremely critical defect

CORROSION

B). INHERENT FLAWS This group of flaws is present in metal as the

result of its initial solidification from the molten state, before any of the operations to forge or roll it into useful sizes and shapes have begun. The following are brief descriptions of some inherent flaws.

B). INHERENT FLAWS PRIMARY PIPE BLOWHOLES SEGREGATION POROSITY  INCLUSIONS SHRINKAGE

PRIMARY PIPE

BLOWHOLES

SEGREGATIO N

POROSITY

POROSITY

INCLUSION

SHRINKAG E

CAVITY SHRINKAGE

FILAMENTARY SHRINKAGE

SPONGE SHRINKAGE

C.) PRIMARY PROCESSING FLAWS Flaws which occur while working the metal down

by hot or cold deformation into useful shapes such as bars, rods, wires and forged shapes are primary processing flaws. The following are brief descriptions of some primary processing flaws:

C.) PRIMARY PROCESSING FLAWS  SEAMS- are surface flaws, generally long, straight

and parallel to the longitudinal axis of the material  LAMINATIONS- Metal defects with separation or weakness generally aligned parallel to the worked surface of the metal.  CUPPING- series of internal metal ruptures created when the interior metal does not flow as rapidly as the surface metal during drawing or extruding processes

C.) PRIMARY PROCESSING FLAWS  COLD SHUT  INCOMPLETE WELD PENETRATION  INCOMPLETE WELD FUSION  SLAG INCLUSION

INCOMPLETE WELD PENETRATION

INCOMPLETE WELD FUSION

D.) SECONDARY PROCESSING/FINISHING FLAWS  This category includes those flaws associated with

the various finishing operations, after the part has been rough-formed by rolling, forging, casting or welding. Flaws may be introduced by heat treating, grinding, and similar processes. The following are brief descriptions of some secondary processing or finishing flaws.

D.) SECONDARY PROCESSING/FINISHING FLAWS  MACHINING TEARS  HEAT TREATING CRACKS  GRINDING CRACKS  PLATING CRACKS  ETCHING CRACKS

E.) IN-SERVICE FLAWS  These flaws are formed after all fabrication has been

completed and the aircraft, engine, or related component has gone into service. These flaws are attributable to aging effects caused by either time, flight cycles, service operating conditions, or combinations of these effects. The following are brief descriptions of some in-service flaws.

E.) IN-SERVICE FLAWS  STRESS CORROSION  OVERSTRESS CRACKS  FATIGUE CRACKS  UNBONDS/ DISBONDS  DELAMINATION

SELECTING THE NDI METHODS

The NDI method and procedure to be used for any specific part or component will generally be specified in the aircraft or component manufacturer’s maintenance or overhaul manuals, SSID’s, SB’s, or in AD’s.

A.) APPROPRIATE METHOD The appropriate NDI method may consist of several separate inspections. Making the correct NDI method selection requires an understanding of the basic principles, limitations, and advantages and disadvantages of the available NDI methods and an understanding of their comparative effectiveness and cost.

B.) OTHER FACTORS (1) The critical nature of the component; (2) The material, size, shape, and weight of the part; (3) The type of defect sought; (4) Maximum acceptable defect limits in size and distribution; (5) Possible locations and orientations of defects; (6) Part accessibility or portability; and (7) The number of parts to be inspected.

B.) DEGREE OF INSPECTION The degree of inspection sensitivity required is an important factor in selecting the NDI method. Critical parts that cannot withstand small defects and could cause catastrophic failure require the use of the more sensitive NDI methods. Less critical parts and general hardware generally require less-sensitive NDI methods.

B.) MATERIAL SAFETY DATA SHEETS The various materials used in NDI may contain chemicals, that if improperly used, can be hazardous to the health and safety of operators and the safety of the environment, aircraft, and engines. Information on safe handling of materials is provided in MSDS. MSDS, conforming to Title 29 of the Code of Federal Regulations (29 CFR), part 1910, section 1200, or its equivalent, must be provided by the material supplier to any user and must be prepared according to FEDSTD-313.

VISUAL INSPECTION

TYPES OF INSPECTION

Nondestructive testing methods are techniques used both in the production and in-service environments without damage or destruction of the item under investigation. Examples of NDI methods are as follows:

TYPES OF INSPECTION a. Visual inspection b. Magnetic particle c. Penetrants d. Eddy current e. Radiography f. Ultrasonic g. Acoustic emission h. Thermography i. Holography j. Shearography k. Tap testing

VISUAL INSPECTION

VISUAL INSPECTION  is the oldest and most common form of NDI for

aircraft. Approximately 80 percent of all NDI procedures are accomplished by the direct visual methods.  This inspection procedure may be greatly enhanced by the use of appropriate combinations of magnifying instruments, borescopes, light sources, video scanners, and other devices discussed in this AC.

SIMPLE VISUAL INSPECTION  It should be emphasized that the eye- mirror-

flashlight is a critical visual inspection process. Aircraft structure and components that must be routinely inspected are frequently located beneath skin, cables, tubing, control rods, pumps, actuators, etc.

A). Flashlights Flashlights used for aircraft inspection should be suitable for industrial use and, where applicable, safety approved by the Underwriters Laboratory or equivalent agency as suitable for use in hazardous atmospheres such as aircraft fuel tanks.

A). Flashlights (1) Standard incandescent (for long- battery life). (2) Krypton (for 70 percent more light than standard bulbs). (3) Halogen (for up to 100 percent more light than standard bulbs). (4) Xenon (for over 100 percent more light than standard bulbs).

B.) Inspection Mirrors An inspection mirror is used to view an area that is not in the normal line of sight. The mirror should be of the appropriate size to easily view the component, with the reflecting surface free of dirt, cracks, worn coating, etc., and a swivel joint tight enough to maintain its setting.

C.) Simple Magnifiers A single converging lens, the simplest form of a microscope, is often referred to as a simple magnifier. Magnification of a single lens is determined by the equation M = 10/f. In this equation, “M” is the magnification, “f” is the focal length of the lens in inches, and “10” is a constant that represents the average minimum distance at which objects can be distinctly seen by the unaided eye. Using the equation, a lens with a focal length of 5 inches has a magnifi- cation of 2, or is said to be a two-power lens.

BORESCOPES 

A borescope is an optical device similar in principle to a telescope in that it enlarges objects like a magnifying glass.



~ A fiberoptic borescope is similar to a standard borescope, but has a flexible, articulated probe that can bend around corners. The maximum length borescopes is four feet

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With a video scope, light is carried to an object by fiberoptics or light-emitting diodes. The image is viewed through a lens by a light sensitive chip and transmitted to a video processor where the electronic signal from the chip is assembled and output to a monitor and VCR as appropriate

BORESCOPES These instruments are long, tubular, precision optical instruments with built-in illumination, designed to allow remote visual inspection of internal surfaces or otherwise inaccessible areas. The tube, which can be rigid or flexible with a wide variety of lengths and diameters, provides the necessary optical connection between the viewing end and an objective lens at the distant, or distal tip of the borescope.

BORESCOPES USES •

• • • •

to reduce or eliminate the need for costly tear-downs. (Aircraft turbine engines have access ports that are specifically designed for borescopes.) used to determine the airworthiness of difficult-to-reach components. used to inspect interiors of hydraulic cylinders and valves for pitting, scoring, porosity, and tool marks; inspect for cracked cylinders in aircraft reciprocating engines; inspect turbojet engine turbine blades and combustion cans;

BORESCOPES USES • • •

verify the proper placement and fit of seals, bonds, gaskets, and subassemblies in difficult to reach areas; and assess Foreign Object Damage (FOD) in aircraft, airframe, and power plants. used to locate and retrieve foreign objects in engines and airframes.

OPTICAL DESIGNS

Typical designs for the optical connection between the borescope viewing end and the distal tip are:

VISUAL INSPECTION PROCEDURES

1. Preliminary Inspection Perform a preliminary inspection of the overall general area for cleanliness, presence of foreign objects, deformed or missing fasteners, security of parts, corrosion, and damage. If the configuration or location of the part conceals the area to be inspected, use visual aids such as a mirror or borescope.

2. Corrosion Treatment Treat any corrosion found during preliminary inspection after completing a visual inspection of any selected part or area.

Inspection (1) Surface cracks. When searching for surface cracks with a flashlight, direct the light beam at a 5 to 45 degree angle to the inspection surface, towards the face. (See figure 5-2.) Do not direct the light beam at such an angle that the reflected light beam shines directly into the eyes. Keep the eyes above the reflected light beam during the inspection. Determine the extent of any cracks found by directing the light beam at right angles to the crack and tracing its length. Use a 10-power magnifying glass to confirm the existence of a suspected crack. If this is not adequate, use other NDI techniques, such as penetrant, magnetic particle, or eddy current to ver.ify cracks

8.2 Inspection (2) Other surface discontinuities. Inspect for other surface discontinuities, such as: discoloration from overheating; buckled, bulging, or dented skin; cracked, chafed, split, or dented tubing; chafed electrical wiring; delaminations of composites; and damaged protective finishes.

9. Recordkeeping Document all discrepancies by written report, photograph, and/or video recording for appropriate evaluation. The full value of visual inspection can be realized only if records are kept of the discrepancies found on parts inspected. The size and shape of the discontinuity and its location on the part should be recorded along with other pertinent information, such as rework per- formed or disposition. The inclusion on a report of some visible record of the discontinuity makes the report more complete.

Non-Destructive Testing (NDT)       

1. Visual Inspection 2. Tap Test 3. Liquid Penetrate Inspection 4. Magnetic Particle Inspection 5. Eddy Current Inspection 6. Ultrasonic Inspection 7. X-ray Inspection

COIN TAP TEST Although it is one of the most simple tests available, the coin tap test is also one of the most effective on laminated, bonded, and honeycomb materials. ~ Undamaged material produces a solid ringing sound, while a damaged area. makes a hollow thud. Impact damage to laminated structure, such as ice and rain impingement on radomes, is quickly and easily found using the coin tap test.

LIQUID PENETRANT INSPECTION ~ inspection suitable for locating cracks, porosity, or other types of faults open to the surface. ~usable on ferrous and non-ferrous metals, as well as nonporous plastic material ~Dye penetrant inspection is based on the principle of capillary attraction. The area being inspected is covered with a penetrating liquid that has a very low viscosity and low surface tension. ~ After sufficient time, the excess penetrant is washed off and the surface is covered with a developer ~. The developer, by the process of reverse capillary action, blots the penetrant out of cracks or other faults forming a visible line in the developer. ~. If an indication is fuzzy instead of sharp and clear, the probable cause is that the part was not thoroughly washed before the developer was applied

LIQUID PENETRANT INSPECTION ~ When performing a liquid penetrant inspection, the penetrant is spread over the surface of the material being examined, and allowed sufficient time for capillary action to take place. (B) The excess penetrant is then washed from the surface, leaving any cracks and surface flaws filled. (C) An absorbent developer is sprayed over the surface where it blots out any penetrant. The crack then shows up as a bright line against the white developer.

LIQUID PENETRANT INSPECTION 

~ There are two types of dyes used in liquid penetrant inspection: fluorescent and colored. An ultraviolet light is used with the fluorescent penetrant and any flaw shows up as a green line.



~ using liquid penetrant it is important that the surface be free of grease, dirt, and oil. The best method of cleaning a surface is with a volatile petroleum-based solvent. If vapor degreasing is not practical, the part is cleaned by scrubbing with a solvent or a strong detergent solution. Parts to be inspected with liquid penetrant should not be cleaned by abrasive blasting, scraping, or heavy brushing

LIQUID PENETRANT INSPECTION  



PENETRANT APPLICATION ~ Penetrant is typically applied to a surface by immersing the part in the liquid or by swabbing or brushing a penetrant solution onto the part's surface ~The amount of time required for a penetrant to cure is called its dwell time and is determined by the size and shape of the discontinuities being looked for. For example, small, thin cracks require a longer dwell time than large and more open cracks. Dwell time is decreased if a part is heated. However, if the part gets too hot the penetrant evaporates

LIQUID PENETRANT INSPECTION REMOVAL OF SURFACE PENETRANT ~ Liquid penetrants are typically removed using either water, an emulsifying agent, or a solvent. with water that is sprayed at a pressure of 30 to 40 psi, spray nozzle is held at a 45 ~ Some penetrants are neither water soluble nor emulsifiable, but instead are solvent-removeable. ~ When using this type of penetrant, excess penetrant is removed with an absorbent towel, and the part's surface is then wiped with clean towels dampened with solvent. The solvent should not be sprayed onto the surface nor should the part be immersed in the solvent, since this will wash the penetrant out of faults or dilute

LIQUID PENETRANT INSPECTION Three kinds of developers ~Dry developer is a loose powder material such as talcum that adheres to the penetrating liquid and acts as a blotter to draw the penetrant out of any surface faults ~Wet developer typically consists of a white powder mixed with water that is either flowed over a surface, or a part is immersed in it. The part is then air-dried a ~ Nonaqueous developer consists of a white chalk-like powder sus pended in a solvent that is normally applied from a pressure spray can, or sprayed onto a surface with a paint gun.

LIQUID PENETRANT INSPECTION ~ Colored dye is used in this penetrating liquid so that examination under white light can be accomplished. (B) right A fluorescent dye is used in this penetrant inspection and then the part is examined under black or ultraviolet light where any fault appears as a vivid green mark.

MAGNETIC PARTICLE INSPECTION ~ made of iron or iron alloys ~, a part is magnetized and an oxide containing magnetic particles is poured or sprayed over the part's surface. Any discontinuities in the material, either on or near the surface, create disruptions in the magnetic field around the part ~ useful for detecting cracks, splits, seams, and voids that form when a metal ruptures. It is also useful for detecting cold shuts and inclusions of foreign matter that occurred when the metal was cast or rolled.

MAGNETIC PARTICLE INSPECTION MAGNETIC ORIENTATION ~, the part must be magnetized in such a way that the lines of flux are perpendicular to the fault ~ To ensure that the flux lines are nearly perpendicular to a flaw, a part should be magnetized both longitudinally and circularly. ~ longitudinal magnetization, the magnetizing current flows either through a coil in which the part is placed, or through a coil around a soft iron yoke. ~CIRCULAR MAGNETIZATION . Current is sent through the part by placing it between the heads of magnetizing equipment.

MAGNETIC PARTICLE INSPECTION ~ Large flat objects are circularly magnetized by using test probes that are held firmly against the surface with current passed through them. ~ Either circular or longitudinal magnetization can reveal defects that are 45 degrees to the magnetic field.

MAGNETIC PARTICLE INSPECTION METHODS OF MAGNETIZATION ~ magnetic particle inspection employs direct current magnetization, half-wave rectified DC magnetiza tion, or alternating current magnetization. DIRECT CURRENT. Pure direct current at voltages from 110 to 440 has excellent penetrating qualities and is suitable for magnetizing parts in coils and with yokes. However, DC has the disadvantage of being difficult to change its value as required for inspecting objects of different sizes.

MAGNETIC PARTICLE INSPECTION

HALF-WAVE RECTIFIED DC,, can be rectified to DC with a half-wave rectifier. In addition, by controlling the AC input the DC output can be adjusted to any value. Half-wave DC has the identical penetrating qualities as straight DC, and its pulsating nature helps distribute the magnetic particles so they arrange themselves over any fault ALTERNATING CURRENT..The principle of magnetization is based on the magnetic domains of a material aligning with the external magnetizing force.

MAGNETIC PARTICLE INSPECTION TESTING MEDIUM ~ magnetic particle inspection is ferromagnetic. In other words, the material is finely divided, has a high permeability, and a low retentivity. Furthermore, for operator safety it is nontoxic. ~ these materials are extremely fine iron oxides that are dyed gray, black, red, or treated with a dye that causes them to fluoresce when illuminated with ultraviolet light. ~ iron oxides are often used dry, but can be mixed with kerosene or some other light oil and sprayed over a surface.

MAGNETIC PARTICLE INSPECTION ~ Dry particles are typically applied with hand shakers, spray bulbs, or powder guns. ~ Wet particles are flowed over a part as a bath. ~ Measuring particle concentration is accomplished by collecting a sample of the agitated bath in a centrifuge tube.

MAGNETIC PARTICLE INSPECTION TESTING METHOD The two methods you must be familiar with are the residual magnetism method and the continuous magnetism method.

RESIDUAL MAGNETISM- When a part is magnetized and the magnetizing force is removed before the testing medium is applied, . The residual procedure is only used with steels that have been heat-treated for stressed applications . CONTINUOUS MAGNETISM -magnetization requires that a part be subjected to the magnetizing force when the testing medium is applied. The continuous process of magnetization is most often used to locate invisible defects since it provides a greater sensitivity in locating subsurface discontinuities

MAGNETIC PARTICLE INSPECTION INSPECTION~ gray, black, or red dye is used, the inspection is made in white light. However, if a fluorescent dye is used, the part is inspected using a black light in a dark booth.

MAGNETIC PARTICLE INSPECTION ~FATIGUE CRACKS give sharp, clear patterns, generally uniform and unbroken throughout their length. These cracks are often jagged in appearance. are only found in parts that were in service. These cracks are usually in highly stressed areas of a part where a stress concentration exists. HEAT-TREAT CRACKS, have a smooth outline, and are usually less clear with less buildup than fatigue cracks. a characteristic form, consisting of short jagged lines grouped together SHRINK CRACKS, give a sharp, clear pattern and the line is usually very jagged GRINDING CRACKS are fine, sharp, and seldom have a buildup because of their limited depth.

MAGNETIC PARTICLE INSPECTION SEAMS Indications of seams are typically straight, sharp, and fine. They are often intermittent and sometimes have very little buildup. HAIRLINE CRACKS are very fine seams in which the faces are forced very close together during fabrica tion.. Discontinuities of this type are normally considered detrimental only in highly stressed parts. INCLUSIONS,,are nonmetallic materials that have been trapped in the solidifying metal during the manu facturing process.

MAGNETIC PARTICLE INSPECTION DEMAGNETIZATION before a part is returned to service, it is required to be thoroughly demagnetized. ~ AC DEMAGNETIZATION the part is subjected to a magnetizing force opposite the force used to magnetize it. the magnetizing force was AC, the domains alternate in polarity, and if the part is slowly removed from the field while current is still flowing, the reversing action progressively becomes weaker. ~ DC DEMAGNETIZATION AC current does not penetrate a surface very deeply. a part is placed in a coil and subjected to more current than initially used to magnetize the part

EDDY CURRENT INSPECTION ~ Eddy current inspection is based on the principle of current acceptance. In other words, it determines the ease with which a material accepts induced current.

~. The ease with which a material accepts the induced eddy currents is determined by four properties: its conductivity, permeability, mass, and by the presence of any voids or faults.

EDDY CURRENT INSPECTION ~ The conductivity of a metal varies with alloy type, grain size, degree of heat treatment, and tensile strength ~, a comparison probe and a test probe are held on a reference, or sample material and the meter is balanced to a null indication. ~ The permeability of a material is the measure of its ability to accept lines of magnetic flux.

EDDY CURRENT INSPECTION ~ eddy current meter indirectly measures current flowing in the test probe, and the probe current is proportional to the current induced into a test spec imen. The mass of the material being tested deter mines the ease with which eddy currents flow.

~probe passes over an area containing corrosion or some sort of discontinuity, the meter needle deflects, indicating a decrease in mass. However, as the probe is moved over a surface free from faults, it remains steady

EDDY CURRENT INSPECTION ABSOLUTE METHOD OF INSPECTION In the absolute method of inspection, bridge-type eddy current equipment is used to identify a mater-ial's characteristics by measuring the amount of probe current that flows when current is induced into a test specimen.

COMPARISON METHOD OF INSPECTION The comparison method of inspection uses a double-coiled probe. Instead of zeroing to a standard piece of material, the comparison method indicates differences in characteristics between the material under the reference probe and that under the test probe.

EDDY CURRENT INSPECTION ~ Several eddy current instruments which use a two dimensional display, such as an X-Y oscilloscope, are available without the limitations associated with the meter type instrument

ULTRASONIC INSPECTION

~ that can be used on plastics, ceramics, and most metals. . Ultrasonic energy bounces off a fault and is reflected on the CRT screen as a peak on the base line between the peaks representing the front and back surfaces. ~ Under normal conditions, these sound waves propagate longitudinally from the source of vibration and are called longitudinal waves. However, a second type of wave propagation occurs at right angles to the direction of the sound. This type of wave propagation occurs only in materials made of tightly bonded molecules, such as solids, and are called transverse, or shear waves. Shear waves that travel along the surface of a material, and do not appreciably extend into the material, are known as surface, or Rayleigh, waves.

ULTRASONIC INSPECTION ~ Ultrasonic waves used for nondestructive inspection vary in frequency from 200 kilohertz to 25 megahertz, and are either reflected, focused, or refracted ~ but for sonar operation and for ultrasonic cleaning. Some materials produce electricity when they are struck, pressed, bent, or otherwise distorted. Materials that possess this property are called piezo electric materials. In addition to producing current, piezoelectric materials vibrate when subjected to alternating current. This makes these types of mate rials useful as transducers

ULTRASONIC INSPECTION ENERGY INTRODUCED INTO TEST MATERIALThere are three basic ways in which ultrasonic energy is introduced into the test specimen. The first is by direct contact on only one side of the material. The energy is transmitted from this point and the return echo is received from the same side. The second method uses a transducer on both sides of the material; one introduces a pulse into the material, and the other receives the signal and sends it to the CRT. The third way of inducing sound energy into a material is the immersion method. the test specimen is immersed in water and the transducer beams its energy . The desired orientation is achieved through the use of acrylic wedges inserted between the transducer and the material's surface

ULTRASONIC INSPECTION FAULT INDICATIONS Two basic systems are used in ultrasonic inspection. They are the pulse-echo system and resonance system. With the pulse-echo system a cathode ray oscilloscope is used in conjunction with a CRT as a fault indicator. . A time based signal produces a straight line across a CRT screen and when a pulse of energy is sent into the material, a pip, or peak, occurs on this horizontal line. ~ Because ultrasonic test equipment indicates the thickness of a material, it is an efficient means of inspecting for corrosion on the inside of a structure

ULTRASONIC INSPECTION ~ resonance system. Like the pulse-echo system, the resonance system is also used to measure the thickness of material However, its principle of operation differs from that of the pulse-echo system in that the resonance system depends on matching the oscillator's frequency to the resonance point of the material being tested.

RADIOGRAPHIC INSPECTION Radiographic inspection allows a photographic view inside a structure. In other words, this method uses certain sections of the electromagnetic spectrum to photograph an object's interior.

RADIOGRAPHIC INSPECTION ~ X-ray and gamma ray radiation are forms of high energy, short wavelength electromagnetic waves. amount of absorption is proportional to the density of the material. ~ An x-ray generator consists of a tube containing a heavy insulating envelope. A coil at one end of the tube serves as a cathode that emits electrons when it is heated with electrical current.

RADIOGRAPHIC INSPECTION SET-UP AND EXPOSURE For a permanent record of a radiographic inspection, a sheet of photographic film is placed on one side of the object being inspected, and the radiation source on the other. ~ The denser the specimen, the less radiation passes through, and the less the film is exposed. The specimen is then exposed to the radiation source.

RADIOGRAPHIC INSPECTION Factors that determine the proper exposure include, but are not limited, to the following:        

Material thickness and density Shape and size of the object Type of defect to be detected Characteristics of the equipment used Exposure distance Exposure angle Film characteristics Types of intensifying screen, if used

RADIOGRAPHIC INSPECTION ~ Photographic film is composed of flexible transparent plastic sheets coated with a thin layer of gelatin. This gelatin contains an emulsion of extremely fine silver bromide grains. ~ The exposed film is then treated with developer which reduces only the silver bromide grains that were touched by the radiation into clumps of black metallic silver. After all of the affected emulsion is converted, the developing action is stopped with an acid stop bath. ~ Because of this, less-dense areas or places with the most radiation exposure are dark. Those places where the density is the greatest get the least radiation and are the clearest.

RADIOGRAPHIC INSPECTION ~ Atypical camera is made of lead to contain the isotope's gamma radiation. To expose a film and obtain an x-ray, the cover is raised and the control rod is extended to expose the source and provide a wider angle of coverage. ~ FLUOROSCOPY For high-speed radiographic inspection where no permanent record is required, a fluoroscope is used. main advantage of fluoroscopy is that objects are viewed in real time. Furthermore, the test piece can be moved or rotated in front of the screen by handling devices. Moving the object closer to the x-ray tube for magnification is also possible.

RADIOGRAPHIC INSPECTION

Corrosion Protection and Control a. Types of Corrosion b. Corrosion Protection and Removal

CORROSION

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CORROSION is a natural phenomenon which attacks metal by chemical or electrochemical action and converts it into a metallic compound. The corrosion occurs because of the tendency of metals to return to their natural state. Steel: Corrosion of steel is easily recognized because the corrosion product is red rust. Aluminum: Aluminum and its alloys exhibit a wide range of corrosion such as crevice, stress, and fretting corrosion

Four Conditions must exist for corrosion to start

Causes of Corrosion

Causes of Corrosion ACIDS AND ALKALIS Almost all acids and alkalis form effective electrolytes as they react with metals to form metallic salts, ~ Ferrous metals are subject to damage from both acids and alkalis, but aluminum is more vulnerable to strong alkaline solutions than it is to acids.

Causes of Corrosion SALTS . In general, salts are the result of a metallic element combining with a nonmetal. The resulting compound is almost always a good electrolyte, and can promote corrosive attack. Magnesium is particularly vulnerable to corrosive attack from an electrolyte formed by salt solutions. MERCURY Mercury attacks aluminum by a chemical reaction known as amalgamation. In this process, the mercury attacks along the grain boundaries within the alloy, and in a very short time completely destroys it WATER Pure water reacts with metals to form corrosion or oxidation, but water holding a concentration of salts or other contaminants causes much more rapid corrosion.

CORROSION DETECTION 

~ Another means of corrosion detection is through the use of ultrasonic equipment. As discussed in the previous chapter, there are two types of ultrasonic indications used for corrosion detection: the pulse-echo and the resonance method.

CORROSION-PRONE AREAS              

ENGINE EXHAUST AREA BATTERY COMPARTMENTS AND VENTS LAVATORIES AND FOOD SERVICE AREAS WHEEL WELLS AND LANDING GEAR EXTERNAL SKIN AREAS ENGINE INLET AREAS FUEL TANKS PIANO HINGES BILGE AREAS LANDING GEAR BOXES ENGINE MOUNT STRUCTURE CONTROL CABLES WELDED AREAS ELECTRONIC EQUIPMENT

Types of Corrosion

INTERGRANULAR CORROSION

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STRESS CORROSION - Stress corrosion cracking is an inter-granular cracking of the metal which is caused by a combination of stress and corrosion. Stress may be caused by internal or external loading. Internal stress are produced by non-uniform deformation during cold working, by unequal cooling from high temperatures, and by internal structural rearrangement involving volume changes.

FATIGUE CORROSION - Fatigue corrosion is caused by the combined effects of cyclic stress and corrosion.

Electrode Potential of Metals

ANODIC – will give up electrons (corrode easily) CATHODIC – least to corrode

TREATMENT OF CORROSION

TREATMENT OF CORROSION Regardless of the type of corrosion or the metal involved, corrosion treatment requires three basic steps: 1. 2. 3.

Remove as much of the corrosion as possible. Neutralize any residual material. Restore the protective surface film.

CORROSION REMOVAL ~ All corrosion products must be removed as soon as they are discovered, because corrosion continues as long as the deposits remain on the surface. ~ Corrosion under a paint film cannot be thoroughly inspected without first removing all of the paint. ~ One thing to keep in mind is never use a caustic paint remover.

CORROSION REMOVAL ~ All corrosion products must be removed as soon as they are discovered, because corrosion continues as long as the deposits remain on the surface. ~ Corrosion under a paint film cannot be thoroughly inspected without first removing all of the paint. ~ One thing to keep in mind is never use a caustic paint remover.

CORROSION REMOVAL ~ When stripping large areas, spread a sheet of polyethylene plastic over the wet paint remover to slow its drying time. ~After all of the finish has swelled up and broken away from the surface, it should be rinsed off with hot water or with live steam. A stiff nylon bristle brush may be required around rivet heads and along seams to get all of the stubborn paint that adheres to these places

TREATMENT OF ALUMINUM ALLOYS ~. Treatment includes the mechanical removal of as much of the corrosion as practicable, the neutralization of residual materials by chemical means, and, finally, the restoration of the permanent surface coating.

TREATMENT OF ALUMINUM ALLOYS MECHANICAL CORROSION REMOVAL

~ After the paint is removed from a corroded area, all traces of corrosion must be removed from the surface. Very mild corrosion may be removed by using a neutral household abrasive cleaner, such as Bon-Ami, but be sure that the abrasive does not contain chlorine. Nylon scrubbers, such ( as "ScotchBrite" pads, can also be used to remove mild corrosion. More severe corrosion can be removed by brushing with aluminum wool or with an aluminum wire brush. Under no circumstances should you use a steel wire brush or steel wool since traces of the steel can become embedded in the aluminum and lead to severe corrosion

TREATMENT OF ALUMINUM ALLOYS MECHANICAL CORROSION REMOVAL ~ Blasting the surface with glass beads smaller than 500 mesh can be used to remove corrosion from pits. After using abrasives or brushing, examine the metal with a five- to tenpower magnifying glass to ensure that all traces of the corrosion have been removed. ~Severely corroded aluminum alloys must be given more drastic treatment to remove all corrosion. In these situations rotary files or power grinders using rubber wheels impregnated with aluminum oxide are used to grind out corrosion damage

TREATMENT OF ALUMINUM ALLOYS MECHANICAL CORROSION REMOVAL ~ After an examination with a five- or ten-power magnifying glass shows no trace of corrosion remaining, remove about two thousandths of an inch more material to be sure that the ends of the intergranular cracks have been reached. Finish by sanding the area smooth with 280-grit, then 400-grit, sandpaper. Clean the area with solvent or an emulsion cleaner, and neutralize the surface with an inhibitor.

TREATMENT OF ALUMINUM ALLOYS CHEMICAL NEUTRALIZATION After removing all corrosion, treat the surface with a five percent chromic acid solution to neutralize any remaining corrosion salts. After the acid has been on the surface for at least five minutes, it should be washed off with water and allowed to dry.

TREATMENT OF ALUMINUM ALLOYS CHEMICAL NEUTRALIZATION PROTECTIVE COATING 

CLADDING

SURFACE OXIDE FILM ~ The process of applying an oxide film is performed in the factories by an electrolytic process known as anodizing. ~ The anodizing process is an electrolytic treatment in which a part is bathed in a lead vat containing a solution of chromic acid and water. This process forms an oxide film on the part that protects the alloy from further corrosion. 

TREATMENT OF ALUMINUM ALLOYS CHEMICAL NEUTRALIZATION 

SURFACE OXIDE FILM

~After the oxide film has formed, the part is washed in hot water and air-dried. Aluminum treated by this process is not appreciably affected with regard to its tensile strength, its weight, or its dimensions ~ In addition to preventing corrosion, the anodic film produced by the anodizing process also acts as an electrical insulator ~ When small parts are fabricated in the field, or when the protective anodizing film has been damaged or removed, the part can have a protective film applied through chemical rather than an electrolytic process. This process is known as alodizing and uses a chemical that meets specification MIL-C-5541 and is available under several proprietary names, such as Alodine 1201.

TREATMENT OF ALUMINUM ALLOYS CHEMICAL NEUTRALIZATION 

ORGANIC FILM

~ paint to have a rough surface to which it can adhere. An aluminum surface is typically roughened with a mild chromic acid etch, or by the formation of an oxide film through anodizing or alodizing. ~ The surface can also be mechanically roughened by carefully sanding it with 400-grit sandpaper. When sandpaper is used, it is absolutely imperative that every bit of sanding dust be removed with a damp rag before the primer is applied. Shop rags or hand towels obtained from a commercial service do not normally make good rags for washing surfaces prior to painting. These rags, though clean, frequently contain silicone or other surface contaminants that are incompatible with finishing materials.

TREATMENT OF ALUMINUM ALLOYS CHEMICAL NEUTRALIZATION

~ After removing dust and contaminants, perform a final cleaning of the surface with aliphatic naphtha or an approved prep solvent. ~ Zinc chromate primer has been used for years with laquer and enamel. It is an inhibiting primer, meaning that the film is slightly porous and water can enter it causing chromate ions to be released and held on the surface of the metal. ~ A wash primer is used in aircraft factories for priming new aircraft before they are painted. This two-part primer consists of a resin and an alcohol-phosphoric acid catalyst. ~ Epoxy primers are one of the most popular primers for use under polyurethane finishes because they provide maximum corrosion protection

TREATMENT OF FERROUS METALS MECHANICAL CORROSION REMOVAL Unlike aluminum, the oxide film that forms on ferrous metals is porous and attracts moisture. Therefore, if any trace of iron oxide remains on an iron alloy, it continues to convert the metal into corrosion. ~The most effective method of removing rust is by mechanical means. Abrasive paper and wire brushes can be used, but the most thorough means of removing all corrosion from unplated steel parts is by abrasive blasting. Abrasive blasting is typically done using sand, aluminum oxide, or glass beads. If a part has been plated, either with cadmium or with chromium, exercise care to protect the plating, since it is usually impossible to restore it in the field

TREATMENT OF FERROUS METALS MECHANICAL CORROSION REMOVAL ~ A fine stone, fine abrasive paper, or even pumice typically works well. Wire brushes should not be used since they cause minute scratches which can produce stress concentrations that potentially weaken a part. ~ After all corrosion has been removed, any rough edges caused by pitting must be faired with a fine stone, or with 400-grit abrasive paper. The surface should then be primed as soon as possible. ~ Zinc chromate primer is used to protect most freshly cleaned steel surfaces

TREATMENT OF FERROUS METALS SURFACE TREATMENT NICKEL OR CHROME PLATING This plating process produces an airtight coating over the surface that excludes moisture from the base metal. ~There are two types of chrome plating used in aircraft con struction: decorative and hard chrome ~ Decorative chrome is used primarily for its appearance and surface protection, while hard chrome is used to form a wear-resistant surface on piston rods, cylinder walls, and other parts which are subject to abrasion. 

TREATMENT OF FERROUS METALS SURFACE TREATMENT 

CADMIUM PLATING

Almost all steel aircraft hardware is cadmium-plated. This soft, silvery-gray metal is electroplated onto the steel to a minimum thickness of 0.005 inch. It provides an attractive finish as well as protection against corrosion ~ When the cadmium plating on a part is scratched through to the steel, galvanic action takes place and the cadmium corrodes. ~ similar to those which form on aluminum in that they are dense, airtight, and watertight.

TREATMENT OF FERROUS METALS SURFACE TREATMENT GALVANIZING Steel parts such as firewalls are typically treated with a coating of zinc in a process called galvanizing. ~ zinc corrodes and forms an airtight oxide film. Steel is galvanized by passing it through vats of molten zinc and then rolling it smooth through a series of rollers. 

TREATMENT OF FERROUS METALS SURFACE TREATMENT 

METAL SPRAYING

Aircraf1t engine cylinders are sometimes protected from corrosion by spraying molten aluminum on their surface ~To accomplish this process, a steel cylinder barrel is sandblasted absolutely clean, then aluminum wire is fed into an acetylene flame where the wire is melted and blown onto the surface by high-pressure compressed air

TREATMENT OF FERROUS METALS SURFACE TREATMENT 

ORGANIC COATINGS

~ Dry abrasive blasting typically removes all of the surface oxides and roughens the surface enough to provide a good bond for the paint. However, parts which have been cadmium-plated must normally have their surface etched with a five percent solution of chromic acid before the primer adheres.

TREATMENT OF MAGNESUIM ALLOYS ~Magnesium alloys do not naturally form a protective film on their surfaces the way aluminum does, so special care must be taken so that the chemical or electrolytic film applied dur ing the manufacturing process is not destroyed

TREATMENT OF MAGNESUIM ALLOYS MECHANICAL REMOVAL OF CORROSION ~ Therefore, magnesium corrosion typically raises paint or, if it forms between lap joints, it swells the joints. When corrosion is found on a magnesium structure, all traces must be removed and the surface treated to inhibit further corrosion.

TREATMENT OF MAGNESUIM ALLOYS MECHANICAL REMOVAL OF CORROSION Since magnesium is anodic to almost all of the commonly used aircraft structural metals, corrosion should not be removed with metallic tools. Any metallic tool can leave contaminants embedded in the metal that cause further damage. Therefore, stiff nonmetallic bristle brushes or nylon scrubbers are used to remove corrosion. If corrosion exists in the form of deep pits the corrosion must be cut out with sharp carbide-tipped cutting tools or scrapers. If abrasive blasting is used to remove corrosion from magnesium, use only glass beads which have been used for nothing but magnesium. Many engine parts are made of magnesium, and these parts require special cleaning procedures. Because of the high temperatures and contaminants in engine compartments, carbon deposits build up on engine cases and become baked on. These contaminants are removed through a process called decarbonization.

TREATMENT OF MAGNESUIM ALLOYS MECHANICAL REMOVAL OF CORROSION A decarbonizing unit consists of a heated tank and a decarbonizing agent, either water soluble or hydrocarbon based. Parts are immersed in the heated liquid which loosens the accumulated carbon. Complete removal, however, sometimes requires brushing, scraping, or grit blasting. Magnesium parts must not be placed in the decar bonizing tank with steel parts, and metallic cleaning materials such as brushes or abrasives are not to be used.

TREATMENT OF MAGNESUIM ALLOYS SURFACE TREATMENT After all of the corrosion has been removed, a chromic acid pickling solution, which conforms to MIL-M-3171A Type 1 (Dow No. 1), is applied. A satisfactory substitute for this solution may be made by adding about 50 drops of sulfuric acid to a gallon of 10 percent chromic acid solution. Apply this to the surface with rags and allow it to stand for about ten or fifteen minutes, then rinse the part thoroughly with hot water.

CORROSION PREVENTION As stated earlier, the best way to prevent the formation of corrosion is to eliminate one or more of its basic requirements. This is typically done by removing the electrode potential difference within the metal or preventing the introduction of an electrolyte. DISSIMILAR METAL INSULATION It is often necessary for different metals be held in contact with each other. When this is the case, dissimilar metal or galvanic corrosion can take place. In order to minimize this danger, the areas to be joined are sprayed with two coats of zinc chromate primer, and a strip of pressure-sensitive vinyl tape is placed between the surfaces before they are assembled.