CASA B2-07d Maintenance Practices - Structural PDF

MODULE 07 Category B2 Licence CASA B2-07d Maintenance Practices - Structural  Copyright © 2020 Aviation Australia All rights reserved. No part of this document may be reproduced, transferred, sold or otherw...

MODULE 07 Category B2 Licence CASA B2-07d Maintenance Practices - Structural  Copyright © 2020 Aviation Australia All rights reserved. No part of this document may be reproduced, transferred, sold or otherwise disposed of, without the written permission of Aviation Australia. CONTROLLED DOCUMENT 2022-07-26 B2-07d Maintenance Practices Page 2 of 110 CASA Part Part 66 - Training Materials Only  Knowledge Levels Category A, B1, B2 and C Aircraft Maintenance Licence Basic knowledge for categories A, B1 and B2 are indicated by the allocation of knowledge levels indicators (1, 2 or 3) against each applicable subject. Category C applicants must meet either the category B1 or the category B2 basic knowledge levels. The knowledge level indicators are defined as follows: LEVEL 1 Objectives: The applicant should be familiar with the basic elements of the subject. The applicant should be able to give a simple description of the whole subject, using common words and examples. The applicant should be able to use typical terms. LEVEL 2 A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge. Objectives: The applicant should be able to understand the theoretical fundamentals of the subject. The applicant should be able to give a general description of the subject using, as appropriate, typical examples. The applicant should be able to use mathematical formulae in conjunction with physical laws describing the subject. The applicant should be able to read and understand sketches, drawings and schematics describing the subject. The applicant should be able to apply his knowledge in a practical manner using detailed procedures. LEVEL 3 A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner. Objectives: The applicant should know the theory of the subject and interrelationships with other subjects. The applicant should be able to give a detailed description of the subject using theoretical fundamentals and specific examples. The applicant should understand and be able to use mathematical formulae related to the subject. The applicant should be able to read, understand and prepare sketches, simple drawings and schematics describing the subject. The applicant should be able to apply his knowledge in a practical manner using manufacturer's instructions. The applicant should be able to interpret results from various sources and measurements and apply corrective action where appropriate. 2022-07-26 B2-07d Maintenance Practices Page 3 of 110 CASA Part Part 66 - Training Materials Only  Table of Contents Aircraft Defects, Corrosion Inspection, Assessment Techniques and Reprotection (7.18.1) 7 Learning Objectives 7 Aircraft Defects 8 General Defects 8 External Damage 12 Inlets and Exhausts 12 Liquid Systems 12 Gaseous Systems 13 Visual Inspection Techniques 14 Dimensions 14 Tyres 14 Wheels 14 Brakes 14 Landing Gear Locks 15 Indicators 16 External Probes 16 Handles and Latches 17 Panels and Doors 17 Emergency System Indication 18 Pressurised Structure 18 Other Inspections 19 Corrosion Identification, Assessment and Handling 22 Locations of Corrosion in Aircraft 22 Prevention of Corrosion 24 Corrosion Removal 24 Paint Removal 25 Aluminium and Aluminium Alloys 26 Alclad 27 Aluminium Alloy Castings and Forgings, Milled Skin Panels, Etc. 29 Ferrous Metals 30 Magnesium Alloys 32 Blend of a Single Depression 32 Blending Ratios 33 Blend of Multiple Corrosion Areas 33 2022-07-26 B2-07d Maintenance Practices Page 4 of 110 CASA Part Part 66 - Training Materials Only  Using a Dial-Type Depth Gauge 34 Acid Spillage 34 Alkali Spillage 35 Metallic Mercury Corrosion on Aluminium Alloys 35 Mercury Spills/Corrosion Damage 36 Corrosion Protection 40 Aluminium Anti-Corrosive Surface Treatments 40 Ferrous Metal Anti-Corrosive Surface Treatments 41 Paint and Primer 44 Protective Finish Coating 46 Finishing Procedures 48 Surface Finishing Requirements 48 Spray Gun Operation 48 Applying the Finish 49 Corrosion Prevention and Removal 52 Corrosion Prevention 52 Aircraft Surface Cleaning 52 Non-Metal Cleaning 53 Post-Wash Procedure 54 Non-Destructive Testing (7.18.3) 55 Learning Objectives 55 Non-Destructive Inspections and Testing 56 Non-Destructive Inspection Techniques 56 Selecting a NDI Technique 57 Visual Inspection 58 Acoustic Inspection 68 Electronic Inspection 68 Ultrasonic Inspection 72 Radiography 75 Disassembly, Inspection, Repair and Assembly Techniques (7.18.4) 78 Learning Objectives 78 Disassembly and Re-Assembly Techniques 79 Disassembly and Re-Assembly 79 Complete Airframes 79 Replacement of Major Components / Modules 81 Replacement of Minor Components / Modules 82 Disassembly and Re-Assembly of Major Components 83 2022-07-26 B2-07d Maintenance Practices Page 5 of 110 CASA Part Part 66 - Training Materials Only  Disassembly and Re-Assembly of Minor Components 84 Basic Disassembly and Re-Assembly Techniques 85 Disassembly / Re-Assembly Safe Work Practices 87 Small Part and Component Identification 89 Discarding Parts 89 Freeing Seized Components 91 Use of Correct Tools 92 Murphy’s Law 93 Troubleshooting Techniques (7.18.5) 94 Learning Objectives 94 Troubleshooting Techniques 95 General Troubleshooting Process 95 Identify the Defect 96 Conduct a Visual Inspection 97 Conduct an Operational Check 98 Classify the Defect 100 Isolate the Defect 105 Locate the Defect 107 Correct the Defect 108 Conduct a Final Operational Check 109 2022-07-26 B2-07d Maintenance Practices Page 6 of 110 CASA Part Part 66 - Training Materials Only  Aircraft Defects, Corrosion Inspection, Assessment Techniques and Reprotection (7.18.1) Learning Objectives 7.18.1.1.1 Describe types of defects (Level 2). 7.18.1.1.2 Describe visual inspection techniques (Level 2). 7.18.1.2.1 Describe corrosion removal procedures (Level 2). 7.18.1.2.2 Describe corrosion assessment procedures (Level 2). 7.18.1.2.3 Describe reprotection of corroded areas (Level 2). 2022-07-26 B2-07d Maintenance Practices Page 7 of 110 CASA Part Part 66 - Training Materials Only  Aircraft Defects General Defects An operational aircraft can suffer from many defects and these can be defined as any event or occurrence which reduces the serviceability of the aircraft. The maintenance schedule should specify the inspection areas and the faults expected to be found. In most instances, the inspector is looking for indications of abnormality in the item being inspected. Typical examples are listed as follows. Aviation Australia - David Kingshott General Defects 2022-07-26 B2-07d Maintenance Practices Page 8 of 110 CASA Part Part 66 - Training Materials Only  Metal Parts Applicable to all metal parts, bodies or casings of units in systems; electrical, instrument and radio installations; and metal pipes, ducting, tubes, rods and levers. These are inspected for: Cleanliness and external evidence of damage Leaks and discharge Overheating Fluid ingress Obstruction of drainage or vent holes or overflow pipe orifices Correct seating of panels and fairings and serviceability of fasteners Distortion, dents, scores and chafing Pulled or missing fasteners, rivets, bolts or screws Evidence of cracks or wear Separation of adhesive bonding Failures of welds or spot welds Deterioration of protective treatment and corrosion Security of attachments, fasteners, connections, locking and bonding Aviation Australia - David Kingshott Underfloor corrosion 2022-07-26 B2-07d Maintenance Practices Page 9 of 110 CASA Part Part 66 - Training Materials Only  Rubber, Fabric, Glass Fibre and Plastic Parts This includes coverings, ducting, flexible mountings, seals, insulation of electrical cables and windows. These parts are typically inspected for: Cleanliness Cracks, cuts, chafing, kinking, twisting, crushing, and contraction or sufficient free length Deterioration, crazing, loss of flexibility Overheating Fluid soakage Security of attachment, correct connections and locking. Windscreen crack 2022-07-26 B2-07d Maintenance Practices Page 10 of 110 CASA Part Part 66 - Training Materials Only  Control System Components Cables, chains, pulleys, rods and tubes are inspected for: Correct alignment – no fouling Free movement, distortion, evidence of bowing Scores, chafing, fraying, kinking Evidence of wear, flattening Cracks, loose rivets, deterioration of protective treatment and corrosion Electrical bonding correctly positioned, undamaged and secure Attachments, end connections and locking secure. Aviation Australia - David Kingshott Control Cable Inspection 2022-07-26 B2-07d Maintenance Practices Page 11 of 110 CASA Part Part 66 - Training Materials Only  Electrical Components Actuators, alternators and generators, motors, relays, solenoids and contactors. Such items are inspected for: Cleanliness, obvious damage Evidence of overheating Corrosion and security of attachments and connections Cleanliness, scoring and worn brushes, adequate spring tension after removal of protective covers Overheating and fluid ingress Cleanliness, burning and pitting of contacts Evidence of overheating and security of contacts after removal of protective covers. External Damage Damage to the outside of the airframe can occur by interference between moving parts such as flying controls and flaps, although this is quite rare. The most common reason for airframe damage is being struck by ground equipment or by severe hail in flight. During ground servicing, many vehicles need to be manoeuvred close to the airframe and some have to be in light contact with it to work properly. Contact with the airframe by any of these vehicles can cause dents or puncture the pressure hull, resulting in a time-consuming repair. Inlets and Exhausts Any inlet or exhaust can be a potential nest site for wildlife. The damage done by these birds, rodents and insects can be very expensive to rectify. Other items that have been known to block access holes include branches, leaves and polythene bags. A careful inspection of all inlets and exhausts must be made to ensure that nothing is blocking them. A blocked duct can result in overheating equipment or major damage to the internal working parts of the engine. 2022-07-26 B2-07d Maintenance Practices Page 12 of 110 CASA Part Part 66 - Training Materials Only  Liquid Systems Liquid systems usually have gauges to ascertain the quantity in that particular system. A physical quantity check is often done in addition to using the gauges, as the gauges are not always reliable. These systems usually include oil tanks for the engine; APU and Integrated Drive Generators (IDGs); and the hydraulics, fuel and potable water tanks. The cause of a lower-than-expected level should be immediately investigated, bearing in mind that some systems consume specific amounts of fluids during normal operation. The consumption rate must be calculated before instigating any troubleshooting. A low hydraulic system should not be replenished without first investigating the cause of the leak. External leaks of oil and fuel systems are normally easy to locate. An external leak is usually rectified by simply replacing the component, seal or pipework at fault and completing any tests required by the AMM. If the leak is internal, then a much more thorough inspection of the component must be made, as the problem is more difficult to find. The symptoms are usually signalled by slower movement of services or by erratic operation of services due to the return line being pressurised. Gaseous Systems Gaseous systems include gases such as oxygen, nitrogen and air. If the gas is used by a system during flight, a leak is very hard to confirm unless a physical check is carried out. A leak from an oxygen system is extremely dangerous due to the chances of an explosion if it comes into contact with oil or grease. Once the leak has been cured, the system can be re-charged and leak tested. Pneumatic systems contain high-pressure air of a stated pressure, and should have the same pressure at the end of the flight as at the start. If the pressure is low at the end of the flight, then the compressor or a leak in the ducting/pipes could be suspected. If the pressure falls between flights, it is probably due to a slow leak in the storage system, and this can be investigated using leak-detecting fluids. 2022-07-26 B2-07d Maintenance Practices Page 13 of 110 CASA Part Part 66 - Training Materials Only  Visual Inspection Techniques Dimensions There are a number of places where checking the measurement of a component can establish its serviceability. Landing gear oleo shock struts can be checked for correct inflation by measuring their extension. If the dimension is less than quoted in the manual, then they may be low on pressure and further checks will be required. These checks are usually only done during line maintenance, with pressure checks required for troubleshooting or hangar maintenance. Tyres Tyres serviceability is indicated by the depth of the groove in the tyre tread. The AMM explains what constitutes a worn or damaged tyre. Apart from normal wear, other defects that can affect a tyre are cuts, blisters, creep and low pressure. Some tyres can be re-treaded a number of times after they have reached their wear limits, but the re- tread can be completed only if the complete tyre has not been damaged badly. Creep is the movement of the tyre around the rim in very small increments due to heavy braking action. This movement is dangerous if the tyre is fitted with a tube, as the movement can tear the charging valve out of the tube, causing a rapid loss of pressure. As an indicator, small white marks are painted across the wheel rim and the tyre side wall so that if creep takes place, the marks will move in opposite directions indicating creep. Wheels Defects in aircraft wheels are usually due to impact damage from heavy landings or from items on the runway hitting the wheel rim. Other problems can arise from corrosion starting as a result of impact damage and shearing of wheel bolts, which hold the two halves of a split wheel together. Wheels are usually inspected thoroughly during tyre replacement. 2022-07-26 B2-07d Maintenance Practices Page 14 of 110 CASA Part Part 66 - Training Materials Only  Brakes Brake units are normally attached to the axle of an undercarriage leg. During operation, they absorb large amounts of energy as heat. This results in the brake rotors and stators wearing away, and if they become too hot, the stator material may break up. Inspection of brake units between flights is essential to check for signs of excessive heating and to ensure that they have not worn beyond their limits. Wear reduces the total thickness of the brake pack, which means it can be monitored by measuring the thickness of the pack. Once the amount of wear reaches a set figure, the brake pack is overhauled. If the pads are breaking up, there will be signs of debris, excessive amounts of powder and, in extreme cases, scoring of the discs. This requires immediate replacement of the complete brake unit. Brake units 2022-07-26 B2-07d Maintenance Practices Page 15 of 110 CASA Part Part 66 - Training Materials Only  Landing Gear Locks Landing gear locks are normally fitted to the aircraft’s undercarriage as a safety device to prevent landing gear from inadvertently collapsing. They are usually fitted when the aircraft is to stay on the ground for some time, and removed before the next flight. These flags are designed to attract attention to ensure they are not left in position when the aircraft departs. Indicators The most common type of indicator is the ‘blow-out’ disc used in fire extinguishing and oxygen systems. It shows that a high-pressure gas bottle has discharged its contents overboard, blowing the disc from its flush housing in the aircraft’s skin. The reason for the ruptured disc could be fire extinguisher operation or discharge of the extinguishant due to excessive pressure. Indicators 2022-07-26 B2-07d Maintenance Practices Page 16 of 110 CASA Part Part 66 - Training Materials Only  External Probes Several different types of probes project into the airflow to send information to the flight deck. These can include the pitot/static probes and the angle-of-attack (AOA) probes. Probes are designed to project outwards from the aircraft skin, and this makes them vulnerable to physical damage. Probes need to be regularly inspected for signs of physical damage, discoloration, or obstruction. External probes Handles and Latches Handles and latches usually wear through constant use. The handles and latches of cargo bays and baggage holds, which are operated every time the aircraft lands, are particularly prone to wear. Technicians have to be aware that all panel fasteners wear slowly and must be secured in flight. Most fasteners have a ‘positive’ form of closing or locking, while the more important installations use an indication system (such as painted lines and flush-fitting catches) to ensure correct closure. These must be regularly checked and, when found worn, they should be repaired or replaced. Losing a panel in flight is dangerous enough, but may be more so if it is drawn into one of the engines and causes its destruction. 2022-07-26 B2-07d Maintenance Practices Page 17 of 110 CASA Part Part 66 - Training Materials Only  Panels and Doors Panels and doors can be any size and can be faulty for several reasons. They can be damaged by excessive use and their frames can become damaged where items have to be passed through them (such as with baggage hold doors). Check inside other doors and panels when they are removed, including the inside of the door itself. Pay particular attention to the fastener holes, where dissimilar metal corrosion may be present. If corrosion is discovered along seams of doors and panels, they should be removed to determine its extent. Different aircraft have different types of quick-release fasteners and doors which require specialist tools. Emergency System Indication Some systems use protective covers to prevent inadvertent operation of a switch. These covers are usually held closed by some form of frangible device that indicates the system has been operated when it is broken. Thin copper wire is sometimes used to hold the protective cover closed on fire extinguisher switches. A broken wire indicates that the cover has been lifted and the system may have been operated. Any indication like this must be thoroughly investigated. Fire extinguisher protective cover 2022-07-26 B2-07d Maintenance Practices Page 18 of 110 CASA Part Part 66 - Training Materials Only  Pressurised Structure Some of the factors to consider when inspecting a pressurised structure are: Constant cyclic loading or hoop stress (fatigue cracks around doors and other cut-outs) Subtle changes to the skin surface that may indicate hidden defects Serviceability of blow-out panels, doors, windows and seals. When sheet metal repairs are necessary, it is important to check the surrounding area for damage that may not be apparent, such as in the underlying structure. It may be difficult to see deformation; instead, feel for deformation such as wrinkles or buckling. It may be necessary to make measurements to ensure airframe symmetry. When removing skin panels, conduct a thorough investigation for corrosion. It is important to make sure that all door, window and cut-out seals in pressurised aircraft are in good serviceable order. If you suspect that blade seals have become damaged in any way, have them inspected as their condition is critical to aircraft serviceability. Aerodynamic cleanliness is important to the performance of aircraft. Not only can it affect flight characteristics, but extra drag increases fuel consumption. To maintain aerodynamic cleanliness, it is essential that all fillet seals on skin joints are intact. Fillet seal All joints of fitted components must be installed with some form of environmental seal. 2022-07-26 B2-07d Maintenance Practices Page 19 of 110 CASA Part Part 66 - Training Materials Only  Other Inspections Moving parts – proper lubrication, security of attachment, binding, excessive wear, proper safety wiring, proper operation and adjustment, proper installation, correct travel, cracked fittings, security of hinges, defective bearings, cleanliness, corrosion, deformation, and sealing and tension. Fluid lines and hoses – proper hose or rigid tubing material, proper fittings, correct fitting torque, leaks, tears, cracks, dents, kinks, chafing, proper bend radius, security, corrosion, deterioration, obstructions and foreign matter, and proper installation. Wiring – security, chafing, burning, defective insulation, loose or broken terminals, heat deterioration, corroded terminals and proper installation. Bolts – correct torque (tamper proof paste condition), elongation of bearing surfaces, deformation, shear damage, tension damage, proper installation, proper size and type, and corrosion. Rivets – Corrosion discovered around rivets requires them to be drilled out to determine the extent. The maintenance manual should always be followed when reinstalling rivets. If there is a gap under the rivet head, it may have stretched from stress. Filters, screens, and fluids – cleanliness, contamination, replacement times, proper types and proper installation. Powerplant – engine mount security, mount bolt torque, spark plug security, ignition harness security, oil leaks, exhaust leaks, muffler cracks and wear, security of all engine accessories, engine case cracks, oil breather obstructions, firewall condition and proper operation of mechanical controls. Propellers – nicks, dent cracks, cleanliness, lubrication, gouges, proper blade angles, blade tracking, proper dimensions, governor leaks and operation, and control linkages for proper tension and installation. Nicks on the leading edge of the blade are important items to inspect for since they produce stress concentrations that need to be removed immediately upon discovery in order to prevent the blade separating at the nick. Ground runs – engine temperatures and pressures, static rpm, magneto drop, engine response to changes of power, unusual engine noises, ignition switch operation, fuel shut-off/selector valves, idle speed and mixture settings, suction gauge, fuel flow indicator operation. Lifed Items A number of items on the aircraft have a specific length of time in service (known as a ‘life’). These are major airframe and engine components with finite fatigue lives. The company technical department monitors them and they are replaced during major servicing. Components which can become unserviceable due to life expiry may include engine fire bottles, cabin fire extinguishers, first aid kits, portable oxygen bottles and emergency oxygen generators. 2022-07-26 B2-07d Maintenance Practices Page 20 of 110 CASA Part Part 66 - Training Materials Only  Light Bulbs Light bulbs have to be checked regularly to ensure they remain serviceable at all times. Most bulbs with important functions, like fire warning lights and undercarriage indication, are duplicated. 2022-07-26 B2-07d Maintenance Practices Page 21 of 110 CASA Part Part 66 - Training Materials Only  Corrosion Identification, Assessment and Handling Locations of Corrosion in Aircraft Certain locations in aircraft are more prone to corrosion than others. The rate of deterioration varies widely with aircraft design, build, operational use and environment. External surfaces are open to inspection and are usually protected by paint. Magnesium and aluminium alloy surfaces are particularly susceptible to corrosion along rivet lines, lap joints, fasteners, faying surfaces and where protective coatings have been damaged or neglected. Exhaust Areas Fairings located in gas turbine and piston engine exhaust paths are subject to highly corrosive influences. This is particularly so where exhaust deposits may be trapped in fissures, crevices, seams or hinges. Such deposits are difficult to remove by ordinary cleaning methods. During maintenance, the fairings in critical areas should be removed for cleaning and examination. All fairings in other exhaust areas should also be thoroughly cleaned and inspected. In some situations, a chemical barrier can be applied to critical areas to facilitate removal of deposits at a later date and to reduce the corrosive effects of these deposits. Engine Intakes and Cooling Air Vents The protective finish on engine frontal areas is abraded by dust and high-velocity air and eroded by rain. Heat-exchanger cores and cooling fins may also be vulnerable to corrosion. Special attention should be given, particularly in a corrosive environment, to obstructions and crevices in the path of cooling air. These must be treated as soon as is practical. Landing Gear Landing gear bays are exposed to flying debris such as water and gravel and require frequent cleaning and touching-up. Careful inspection should be made of crevices, ribs and lower-skin surfaces where debris can lodge. Landing gear assemblies should be examined, paying particular attention to magnesium alloy wheels, paintwork, bearings, exposed switches and electrical equipment. Frequent cleaning, water-dispersing treatment and re-lubrication are required while ensuring that bearings are not contaminated either with the cleaning water or with the water-dispersing fluids used when re-lubricating. 2022-07-26 B2-07d Maintenance Practices Page 22 of 110 CASA Part Part 66 - Training Materials Only  Bilge and Water Entrapment Area Although specifications call for drains wherever water is likely to collect, these drains can become blocked by debris, such as sealant or grease. Inspection of these drains must be frequent. Any areas beneath galleys and toilets/washrooms must be very carefully inspected for corrosion, as these are usually the worst places in the whole airframe for severe corrosion. The protection in these areas must also be carefully inspected and renewed if necessary. Recesses in flaps and hinges Potential corrosion areas are found at flap and speed brake recesses, where water and dirt may collect and go unnoticed because the moveable parts are normally in the ‘closed’ position. If these items are left ‘open’ when the aircraft is parked, they may collect salt from the atmosphere or debris which may be blowing about on the airfield. Thorough inspection of the components and their associated stowage bays is required at regular intervals. The hinges in these areas are also vulnerable to dissimilar metal corrosion between the steel pins and the aluminium tangs. Seizure can occur at the hinges of access doors and panels that are seldom used. Spot-Welded Skins and Sandwich Constructions Corrosive agents may become trapped between the metal layers of spot-welded skins, and moisture entering the seams may set up electrolytic corrosion that eventually corrodes the spot-welds or causes the skin to bulge. Generally, spot-welding is not considered good practice on aircraft structures. Cavities, gaps, punctures or damaged places in honeycomb sandwich panels should be sealed to exclude water or dirt. Water should not be permitted to accumulate in the structure adjacent to sandwich panels. Inspection of honeycomb sandwich panels and box structures is difficult and generally requires that the structure be dismantled. Electrical Equipment Sealing, venting and protective paint cannot wholly obviate the corrosion in battery compartments. Spray from electrolyte spreads to adjacent cavities and rapidly attacks unprotected surfaces. Inspection should also be extended to all vent systems associated with battery bays. Circuit-breakers, contacts and switches are extremely sensitive to the effects of corrosion and need close inspection. 2022-07-26 B2-07d Maintenance Practices Page 23 of 110 CASA Part Part 66 - Training Materials Only  Control Cables Loss of protective coatings on carbon steel control cables can, over time, lead to mechanical problems and system failure. Corrosion-resistant cables can also be affected by corrosive marine environments. Any corrosion found on the outside of a control cable should result in a thorough inspection of the internal strands, and if any damage is found the cable should be rejected. Cables should be carefully inspected in the vicinity of bell-cranks, sheaves and other places where the cables flex as there is a greater chance of corrosion getting inside the cables when the strands are moving around (or being moved by) these items. Prevention of Corrosion Due to the high cost of modern aircraft, operators expect them to last much longer than perhaps even the manufacturer anticipated. As a result, manufacturers have taken more care in the design process to improve aircraft corrosion resistance. This improvement includes the use of new materials and improved surface treatments and protective finishes. Preventative maintenance has also been emphasised more than previously. Preventative maintenance, relative to corrosion control, should include: Adequate and regular cleaning of the aircraft Periodic lubrication (often after cleaning) of moving parts Regular and detailed inspection for corrosion and failure of protective treatments Prompt treatment of corrosion and touch-up of damaged paint Maintenance of clear drain holes Drainage of fuel cell sumps Daily wiping down of most critical areas Sealing of aircraft during foul weather and ventilation on sunny days Use of protective covers and blanks. 2022-07-26 B2-07d Maintenance Practices Page 24 of 110 CASA Part Part 66 - Training Materials Only  Corrosion Removal General treatments for corrosion removal include: Cleaning and stripping the protective coating in the corroded area. Removing as much of the corrosion product as possible as soon as it is discovered Neutralising the remaining residue Checking if damage is within limits and choosing the action to be taken. Restoring protective surface films Applying temporary or permanent coatings or paint finishes. 2022-07-26 B2-07d Maintenance Practices Page 25 of 110 CASA Part Part 66 - Training Materials Only  Paint Removal Corrosion under a film of paint cannot be thoroughly inspected without first removing all of the paint. It is essential that the complete suspect area be cleaned of all grease, dirt or preservatives. This aids in determining the extent of corrosive spread. The selection of cleaning materials depends on the type of matter to be removed. Prior to applying a paint remover, all areas not to be stripped should be masked with heavy aluminium foil to keep the stripper from accidentally coming into contact with them. Water-rinseable paint remover of a syrupy consistency is usually best for aircraft surfaces. This type of paint remover is applied with a brush by dabbing rather than brushing. The surface should be covered with a heavy coating of paint remover and allowed to stand until the paint swells and wrinkles. This breaks the bond between the paint finish and the metal. It may take anywhere from 10 min to several hours. It may be necessary to reapply the remover to areas which have not been stripped. In this case, first scrape away the old paint with a plastic or aluminium scraper, and then apply the second coat of remover. This process allows the active chemicals to reach the lower layer of finish paint. After all of the finish has swelled and broken away from the surface, it should be rinsed off with hot water or steam. A stiff nylon brush may be required to scrub the surface around rivet heads and along seams to get all of the stubborn paint that adheres to these places. Observe caution when using any paint remover. Many solvent products used in paint removers attack rubber tyres and synthetic rubber products. Therefore, tyres, hoses and seals must be protected from contact with paint removers. Most paint removers are highly toxic; therefore, special care must be exercised to avoid contact with the skin and eyes. Rubber gloves and acid-repellent aprons and goggles should be worn by personnel involved with paint removal operations. If paint remover is spilled or splashed on your skin, flush the area with water immediately. If any gets into your eyes, flood them with water and get to a doctor as soon as possible. Note: Strippers can damage composite resins and plastics, so every effort should be made to mask these vulnerable areas. 2022-07-26 B2-07d Maintenance Practices Page 26 of 110 CASA Part Part 66 - Training Materials Only  Aluminium and Aluminium Alloys Corrosion attack on aluminium surfaces gives obvious indications since the products are white and voluminous. Even in its early stages, aluminium corrosion is evident as general etching, pitting or roughness. General surface attack penetrates slowly, but accelerates in the presence of dissolved salts. A considerable attack can take place before serious loss of strength occurs. Three forms of attack which are particularly serious are: Penetrating pit-type corrosion through the walls of tubing Stress corrosion cracking under sustained stress Intergranular attack characteristic of certain improperly heat-treated alloys. Treatment involves mechanical or chemical removal of as much of the corrosion products as possible and the inhibition of residual materials by chemical means. This, again, should be followed by restoration of permanent surface coatings. 2022-07-26 B2-07d Maintenance Practices Page 27 of 110 CASA Part Part 66 - Training Materials Only  Alclad Alclad is a trademark of Alcoa but is used as a generic term to describe corrosion-resistant aluminium sheet. A high-purity aluminium surface layer is metallurgically bonded to both sides of high-strength aluminium alloy core material. The cladding thickness on each side of the sheet is approximately 5%. Obviously, great care must be taken not to remove too much of the protective aluminium layer by mechanical methods, as the core alloy metal may be exposed. Therefore, where heavy corrosion is found on clad aluminium alloys, it must be removed by chemical methods wherever possible. Corrosion-free areas must be masked off and the appropriate remover (usually a phosphoric-acid based fluid) applied to the corroded surface, normally with a stiff-bristled brush, until all corrosion products have been removed. Copious amounts of clean water should next be used to flood the area and remove all traces of the acid, and then the surface should be dried thoroughly. Method for checking the aluminium coating is intact Note: A method of checking that the protective aluminium coating remains intact is by applying one drop of diluted caustic soda to the cleaned area. If the Alclad has been removed, the aluminium alloy core will show as a black stain, whereas if the cladding is intact, the caustic soda will cause a white stain. Since the caustic soda is corrosive and promotes corrosion, it must be neutralised with a solution of water and chromic anhydride. 2022-07-26 B2-07d Maintenance Practices Page 28 of 110 CASA Part Part 66 - Training Materials Only  Assessment for Serviceability The maximum depth of metal removed may be determined by using a dial test indicator mounted in a steel block or a depth gauge as illustrated. The Structural Repair Manual (SRM) or Service Bulletin gives the maximum acceptable reduction in thickness. Re-Protection Apply the BRUSH ALOCROM 1200 process as follows: Brush Alocrom is supplied as two liquids: Parts A and B. To make the working solution, mix equal volumes of Part A and Part B in a plastic container. Stir well. Mix only a sufficient amount for use within 24 hours. Any mix remaining after 24 hours must be disposed of in the approved manner. Thoroughly degrease the area to be treated and apply the solution with a nylon brush or cotton cloth until the surface turns to a golden yellow colour. This takes 1–10 min, depending on the temperature. Rinse with clean water, then allow it to dry for a minimum of 2 hr. Finally, apply the specified primer, e.g. epoxy primer, and the appropriate final finish within 48 hr. Note: Etch primers should not be applied over Alocrom 1200. Care must be taken when mixing and applying Alocrom 1200. PVC gloves and eye shields should be worn. Cloths used with Alocrom must be washed before discarding, or they may create a fire hazard. Aluminium Alloy Castings and Forgings, Milled Skin Panels, Etc. After degreasing and removing the paint finish, remove the corrosion. The mechanical methods of removing corrosion are preferred. Nylon scrubbers or Scotch-Brite® pads may be used to remove mild corrosion. Aluminium wool or aluminium wire brush may be used to remove more severe corrosion when the part is not in-situ. Abrasive papers may also be used for mild corrosion. Vacu-Blast abrasive blasting with glass beads smaller than 500 meshes can be used to remove corrosion from pits. After using abrasives or brushing, examine the metal to ensure that all traces of corrosion have been removed. 2022-07-26 B2-07d Maintenance Practices Page 29 of 110 CASA Part Part 66 - Training Materials Only  Under no circumstances should you use a steel wire brush or steel wool since traces of the steel can become embedded in the aluminium and lead to severe corrosion. For more severely corroded aluminium alloys, more drastic treatment must be given to remove all corrosion. Rotary files or power grinders using rubber wheels impregnated with aluminium oxide are used to grind out corrosion damage. However, when using either of these tools, watch carefully to be sure that the minimum amount of material is removed. Maintenance manuals specify the maximum amount of material that can be removed. Finish by sanding the area smooth. Clean the area with solvent or an emulsion cleaner, and neutralise the surface with an inhibitor. After mechanically removing all traces of corrosion, the surface should be treated. A 5% chromic acid solution should be used to neutralise any remaining corrosion salts. After the acid has been on the surface for at least 5 min, it should be washed off with water and allowed to dry. The corrosion pits should be transformed into saucer-shaped depressions which relieve stress concentrations as illustrated. Care must be taken when using power-driven tools to avoid overheating. The SRM or Service Bulletin gives the required proportions for blended areas. Assessment for Serviceability The maximum depth of metal removed must be within the limits specified in the SRM, Service Bulletins, etc. Initial assessment is normally carried out after removal of the loose corrosion to determine whether or not the component can be salvaged. It is usually specified that no pitting is permissible. Where pitting may be blended out, the maximum depth and area will be specified, or the dimensions of the part may be required to remain within the drawing limits. In general, corrosion removal must not weaken a part to such an extent that it endangers the safety of the aircraft. If in doubt, the part must be repaired or replaced. The depth of a blended area may be measured as shown. Ferrous Metals Atmospheric oxidation of iron or steel surfaces causes ferrous oxide rust to be deposited. Some metal oxides protect the underlying base metal, but rust promotes additional attack by attracting moisture and must be removed. Rust shows on bolt heads, nuts or any unprotected hardware. Its presence is not immediately dangerous, but it indicates a need for maintenance and suggests possible further corrosive attacks on more critical areas. The most practical means of controlling corrosion of steel is complete mechanical removal of corrosion products. 2022-07-26 B2-07d Maintenance Practices Page 30 of 110 CASA Part Part 66 - Training Materials Only  Abrasive papers, power buffers, wire brushes and steel wool are all acceptable methods of removing rust on lightly stressed areas. Residual rust usually remains in pits and crevices. High-Stressed Steel Components Corrosion on high-stressed steel components may be dangerous and should be removed carefully with mild abrasive papers or fine buffing compounds. Care should be taken not to overheat parts during corrosion removal. Protective finishes should be re-applied immediately. Degrease and remove paint from the affected area using approved paint remover. Any specific instructions for removing corrosion given in the aircraft SRM or Service Bulletins or their equivalents must always be followed. Steels in aircraft structures are normally plated with zinc, cadmium or chrome. The aim is to remove only the minimum of plating during corrosion removal. The most effective and preferred method of removing corrosion products from ferrous surfaces is by mechanical means. On all components, especially highly stressed parts, ensure that the corrosion damage is within acceptable limits. The following methods may be used: Wire brushes Abrasive papers Vacu-Blast equipment using sand, glass beads or aluminium oxide abrasives Great care must be taken when removing corrosion from highly stressed parts so that no scratches are produced or remain on the surface. Remove the corrosion with fine-grade wet and dry abrasive paper, finally restoring the mirror finish. Cadmium compounds, including dust created by disturbing corrosion products on cadmium-plated surfaces, are toxic. They can cause serious illness if ingested or inhaled. All cadmium-contaminated waste materials, such as wet abrasive paper and Vacu-Blast abrasive, are to be treated as industrial waste and disposed of accordingly. Contaminated overalls are to be bagged, marked as such and sent for cleaning. Chemical methods for removing corrosion must not be used in situ unless specified by the manufacturer and must also be of an approved type. Before using a corrosion-removing chemical, remove loose corrosion by mechanical means and mask surrounding areas where applicable by covering with a suitable protective material. Chemical corrosion removers fall into two categories: phosphoric acid- or alkaline-based. After using rust removers, the area must be thoroughly rinsed with clean water and dried. 2022-07-26 B2-07d Maintenance Practices Page 31 of 110 CASA Part Part 66 - Training Materials Only  Assessment for Serviceability The procedure followed during the assessment of steel parts is the same as that described for non- clad aluminium alloys. Re-Protection Where possible, cadmium-plated steel parts should be re-plated in accordance with the aircraft manufacturer’s instructions. Any special instructions given in the SRM or Service Bulletins must be followed. Magnesium Alloys When magnesium corrodes, the corrosion products occupy more space than the metal. Therefore, magnesium corrosion typically raises paint or, if it forms between lap joints, swells the joint. When corrosion is found on a magnesium structure, all traces must be removed and the surface treated to inhibit further corrosion. Degrease and remove paint from the affected area. Since magnesium is anodic to almost all commonly used aircraft structure metals, corrosion should not be removed with metallic tools. The following mechanical methods are usually specified: Stiff non-metallic brush, e.g. nylon Abrasive papers Vacu-Blast using glass beads Scotch-Brite™ pads Care must be taken not to remove the original chromate film unnecessarily. Where there is no danger of trapping the solution, light corrosion can be cleaned off by swabbing with a magnesium approved corrosion preventative solution. Using a clean non-fluffy cloth, rub the solution into the corroded area until all the corrosion has been removed. Rinse thoroughly with clean water and dry the surface. Care should be taken to confine the solution to the corroded area as it can damage the existing chromate film. Assessment for Serviceability The procedure followed during the assessment of magnesium parts is the same as that described for non-clad aluminium alloy castings and forgings. 2022-07-26 B2-07d Maintenance Practices Page 32 of 110 CASA Part Part 66 - Training Materials Only  Blend of a Single Depression Blend of a single depression Blending Ratios Blending ratios 2022-07-26 B2-07d Maintenance Practices Page 33 of 110 CASA Part Part 66 - Training Materials Only  Blend of Multiple Corrosion Areas Blend of multiple corrosion areas Using a Dial-Type Depth Gauge Using a dial-type depth gauge 2022-07-26 B2-07d Maintenance Practices Page 34 of 110 CASA Part Part 66 - Training Materials Only  Acid Spillage An acid spilled on aircraft components can cause severe damage. Acids corrode most metals used in aircraft construction. They also destroy wood and most other fabrics. Correct Health and Safety procedures must be followed when such spillages occur. Aircraft batteries of the lead-acid type give off acidic fumes, and battery bays should be well ventilated, while surfaces in the area should be treated with anti-acid paint. Vigilance is required of everyone working in the vicinity of batteries to detect (as early as possible) the signs of acid spillage. The correct procedure in the event of an acid spillage is as follows: Mop up as much of the spilled acid as possible using wet rags or paper wipes. Try not to spread the acid. If possible, flood the area with large quantities of clean water, taking care that electrical equipment is suitably protected from the water. If flooding is not practical, neutralise the area with a solution of sodium bicarbonate with water. Wash the area using this mixture and rinse with cold water. Test the area using universal indicating paper (or litmus paper) to check if acid has been cleaned up. Dry the area completely and examine it for signs of damaged paint or plated finish and signs of corrosion, especially where the paint may have been damaged. Remove corrosion, repair damage and restore surface protection as appropriate. Alkali Spillage Alkali spillage is most likely to occur from the alternative nickel-cadmium (NICAD) or nickel-iron (Ni- Fe) types of batteries, which contain an electrolyte of potassium hydroxide (or potassium hydrate). The compartments of these batteries should also be painted with anti-corrosive paint, and adequate ventilation is as important as with lead-acid batteries. Proper Health and Safety procedures are, again, imperative. Removal of the alkali spillage and subsequent protective treatment follow the same basic steps as outlined in acid spillage, with the exception that the alkali is neutralised with a solution of chromic acid crystals in water. 2022-07-26 B2-07d Maintenance Practices Page 35 of 110 CASA Part Part 66 - Training Materials Only  Metallic Mercury Corrosion on Aluminium Alloys Spilled mercury on aluminium should be cleaned immediately because mercury causes corrosion attack which is rapid in both pitting and intergranular attack and is very difficult to control. The most devastating effect of mercury spillage on aluminium alloys is the formation of an amalgam which proceeds rapidly along grain boundaries, causing liquid metal embrittlement. If the aluminium alloy part is under tension stress, this embrittlement will result in splitting, with an appearance similar to severe exfoliation. X-ray inspection may be an effective method of locating the small particles of spilled mercury because the dense mercury shows up readily on X-ray film. Mercury Spills/Corrosion Damage The presence of mercury and mercury salts in air cargo is a definite possibility. Loading, unloading and general shifting of such cargo can and does occasionally result in damaged containers, cartons, electronic tubes, etc., with subsequent possible leakage of mercury on the aircraft structure. Spillage of mercury or mercury compounds within an aircraft requires immediate action for its isolation and recovery. These steps prevent possible corrosion damage and embrittlement of aluminium alloy structural components, stainless steels and un-plated brass components such as cable turnbuckle barrels. Mercury, by the amalgamation process, can penetrate any break in the finish (anodic film/Alodine), paint and seal coating of a metal structural component. Bright, polished, shiny or scratched surfaces hasten the process, as does moisture. Mercury or mercury compounds attack the metal grain boundaries and seriously embrittle and reduce the strength of parts. Corrosive attack of freshly scratched aluminium alloy is very rapid, and complete penetration of sheet material is known to occur within 3 or 4 min. 2022-07-26 B2-07d Maintenance Practices Page 36 of 110 CASA Part Part 66 - Training Materials Only  Mercury is highly toxic and spreads very easily from one surface to another. It adheres to hands, shoes, clothes, tools, etc. The following precautions for mercury spills should be strictly followed: 1. Avoid contact with surfaces suspected of being contaminated. Use wood or fibre sheets to support the body while working in the area. 2. Wear wing socks (shoe protectors), protective (disposable) clothing, and latex or vinyl gloves in the contaminated area to avoid scratching metal surfaces. 3. Do not wear clothing used in contaminated areas on jobs in uncontaminated areas. Dispose of wing socks and protective clothing in unused metal containers outside of the aircraft. Contact the Waste Management Service or similar authority of the local State Health Department for proper mercury disposal procedures. 4. Have personal clothing cleaned. Wash shoes with soap and water. Clean all tools that have been used in the contaminated area with steam or hot water and soap. Discard any drill bits used on mercury-contaminated areas. Thoroughly clean any vacuum cleaners used in the clean-up process. 5. Always wash hands, face and exposed skin thoroughly with soap and water after contacting mercury. Keep hands away from mouth. Do not eat, smoke or blow your nose without first washing your hands thoroughly. 6. Appreciable amounts of mercury will vaporise at normal temperatures. Stagnant air will become dangerous to personal health. 7. Do not use cleaning products, such as solvents, solids or polishes, on contaminated areas. Such materials may promote corrosion. 8. If your hands become contaminated with mercury while working with cleaning equipment, do not touch any exposed metal in surrounding area, as you may contaminate it. Warning: do not pick up mercury Caution: ventilate mercury contaminated areas 2022-07-26 B2-07d Maintenance Practices Page 37 of 110 CASA Part Part 66 - Training Materials Only  Each Instance of mercury or mercury compound spillage should be considered hazardous, and immediate action should be taken to safeguard the aircraft. Inspect for mercury and corrosion as follows: 1. Determine the point at which mercury was spilled. Remove any mercury on floor coverings, and then remove coverings. 2. Do not remove access/inspection plates, screws, rivets, bolts, etc which would allow mercury to spread. 3. Inspect, floor, seat tracks, cargo rails, and adjacent structure for mercury contamination. 4. Inspect areas of suspected mercury contamination with a magnifying glass. 5. If corrosion is evident and clean-up cannot be completed immediately, coat the contaminated area with corrosion preventative compound or engine oil. This helps to slow down the rate of corrosion. 6. In none accessible areas such as the lower cargo compartment use of portable x-ray equipment maybe used to check for hidden corrosion areas. Removal procedure and precautions necessary when mercury is spilled are: 1. Use a high-capacity vacuum cleaner with a trap-type glass container attached to the large vacuum hose. The size of the pickup hose from the container should be about 1/4 in. in diameter to increase the amount of suction applied to the mercury. Due to the weight of mercury, the container will catch the mercury before it can enter the vacuum cleaner hose. 2. An all-rubber storage battery water syringe or a medicine dropper may be used to remove mercury if the trap-type glass container and vacuum cleaner are not available. Cellulose tape may be used to pick up very tiny particles. 3. General clean-up and inspection (using equipment available) should be accomplished immediately after spillage occurs or is detected. Vacuum removal of mercury 2022-07-26 B2-07d Maintenance Practices Page 38 of 110 CASA Part Part 66 - Training Materials Only  Syringe Mercury removal warning and caution sign 2022-07-26 B2-07d Maintenance Practices Page 39 of 110 CASA Part Part 66 - Training Materials Only  Corrosion Protection Aluminium Anti-Corrosive Surface Treatments Cladding Pure aluminium is considered noncorrosive. However, this is not altogether true because aluminium readily combines with oxygen to form an oxide film. This film is so dense that it excludes air from the metal’s surface, thereby preventing additional corrosion from forming. The disadvantage of using pure aluminium is that it is not strong enough for aircraft structural components and therefore must be alloyed with other metals. Aluminium alloys can be protected from corrosion by coating them with a layer of pure aluminium. This is known as cladding. In manufacturing of clad aluminium, pure aluminium is hot-rolled onto the surface of an aluminium alloy and accounts for 10% of the sheet’s total thickness (5% per side). Aluminium cladding 2022-07-26 B2-07d Maintenance Practices Page 40 of 110 CASA Part Part 66 - Training Materials Only  Surface Conversion Coatings In areas where cladding is not practical, conversion coatings are added. These films also have the benefit of acting as a base for paint finishes to adhere to. For aluminium alloys, there are two types of conversion coats. Applying an oxide film is performed in factories by an electrolytic process known as anodising. Magnesium alloys may also be anodised. The anodising process is an electrolytic treatment in which a part is bathed in a lead vat containing a solution of chromic acid and water to form an oxide film. When small parts are fabricated in the field, or when the protective anodising film has been damaged or removed, the part can have a protective film applied through chemical processing. This process is known as alodising. The part may be submersed in a solution of chromic acid and water, or it may be brushed or swabbed on the part. Alodising leaves a gold-coloured film on aluminium alloys. The process for Alodine is as follows: 1. Chemically clean the area or part to achieve an unbroken water film finish. Any breaks in the water film indicate contamination and require the step to be repeated. 2. While the surface is still wet with rinse water, brush or spray on a liberal coating of the Alodine solution. 3. After the Alodine has stood for the recommended time, the area should be flushed with fresh water and allowed to dry. (Ref Alodine Process) The area is ready for painting when dry. Powder appearing on the surface after the material is dried indicates poor rinsing or a failure to keep the surface wet during processing, and the part must be re-treated. 2022-07-26 B2-07d Maintenance Practices Page 41 of 110 CASA Part Part 66 - Training Materials Only  Ferrous Metal Anti-Corrosive Surface Treatments Nickel or Chrome Plating One method of protecting metals from corrosion is chrome plating. This plating process produces an airtight coating over the surface that excludes moisture from the base metal. Two types of chrome processes are used in aircraft construction: decorative and hard chrome. Decorative chrome is used for its appearance and surface protection. Hard chrome is used to form a wear-resistant surface, e.g. for piston rods and cylinder barrels and crankshaft journals. Parts to be plated with hard chrome are usually ground undersize and plated to their proper dimension. Cadmium Plating The vast majority of all steel aircraft hardware is cadmium plated. Cadmium is silvery-grey and is electroplated onto the steel to a minimum thickness of 0.005 in. Cadmium oxides are similar to aluminium oxides as they form a protective layer and are dense, airtight and watertight. When cadmium plating on a part is scratched, galvanic action takes place and it corrodes. This type of protection is known as sacrificial corrosion as no further corrosion can take place after the initial oxide film is formed. Cadmium plating 2022-07-26 B2-07d Maintenance Practices Page 42 of 110 CASA Part Part 66 - Training Materials Only  Galvanising Galvanising is a process in which steel is coated with a protective layer of zinc. The protection afforded by this process is similar to that provided by cadmium in that when penetrated, the zinc corrodes to create a protective oxide film. Zinc is deposited onto metal by dipping the metal into molten zinc or by electroplating. Galvanising by dipping into molten zinc 2022-07-26 B2-07d Maintenance Practices Page 43 of 110 CASA Part Part 66 - Training Materials Only  Metal Spraying Metal spraying is the process of spraying molten metal onto the steel surface by feeding wire through an acetylene flame, then blowing it onto the surface with compressed air. Aircraft engine cylinders are sometimes protected from corrosion by spraying molten aluminium on their surface. Corrosion protection afforded by this treatment is sacrificial corrosion, similar to that provided by cadmium or zinc coating. Metal spraying Paint and Primer Paint Finishing One of the most universally used corrosion control devices for metal surfaces is a coat of paint. The quality of materials used to cover the substrate should match the desired durability, the type of material to be covered and the desired look. Primer After surface pre-treatment has been completed, primer is applied to provide a good bond between the surface to be painted and finish coats. For years, zinc chromate has been the standard primer for aircraft because of its good corrosion resistance. But its use has been decreasing since it does not provide as good a bond to the surface as some of the new primers, and it is also a hazardous material. Two-component epoxy primer is recommended. 2022-07-26 B2-07d Maintenance Practices Page 44 of 110 CASA Part Part 66 - Training Materials Only  Wash Primer High-volume production of all-metal aircraft has brought about the development of a wash primer, which provides a good bond between the metal and the finish and cures after half an hour. This primer consists of two parts: a resin and an alcohol-phosphoric acid catalyst. The materials are mixed and allowed to stand for a short time. The primer is then sprayed onto the surface as a very light tack coat, followed by a full-bodied wet coat. Wash Primer require moisture to properly convert the acid into the protective film. These primers can be used on aluminium, magnesium, steel or fibreglass surfaces. Epoxy Primer Epoxy primer is the most popular primer as it provides maximum corrosion protection. A typical epoxy primer consists of two component materials which produce a tough, dope-proof primer coat. Epoxy primer can be used on aluminium, magnesium or steel. It can be applied over the top of wash primer to provide maximum protection. It is commonly coloured grey or white. Epoxy primer 2022-07-26 B2-07d Maintenance Practices Page 45 of 110 CASA Part Part 66 - Training Materials Only  Zinc Chromate Zinc chromate is an olive-green inhibiting primer. It is slightly porous, allowing water to be absorbed, which causes chromate ions to be released and held on the surface. This surface is ionised, which prevents electrolytic action and inhibits the formation of corrosion. Prior to applying zinc chromate, the surface to be painted must be cleaned of all fingerprints and traces of oil. After cleaning, a thin, wet coat of zinc chromate is applied using a spray gun. Precautions must be taken when handling chromates because zinc chromate is a hazardous material and a carcinogen. The synthetic resin base of a zinc chromate primer provides a good bond between the finish and the metal. Zinc chromate primer 2022-07-26 B2-07d Maintenance Practices Page 46 of 110 CASA Part Part 66 - Training Materials Only  Protective Finish Coating Four basic types of finish coating are used on aircraft: synthetic enamel, acrylic lacquer, polyurethane and acrylic urethanes. Synthetic Enamel Enamel paint is one of the older finishes for metal aircraft and has been commonly used for automobiles. The finish can be applied over zinc chromate primer and cured by the process of oxidation. These finishes have a good gloss and do not require rubbing or buffing. Their chemical resistance is nominal and not as resistant to abrasion compared to some of the more modern finishing systems. Acrylic Lacquer Many aircraft produced on high-volume production lines are finished with acrylic lacquer because of the speed at which paint can be applied. They are primed with a two-part wash primer, and as soon as the primer is entirely dry, they are sprayed with the acrylic lacquer. These lacquers are easy to apply. They have a lower solids content than enamels, but they produce a good gloss, especially if they are polished. They are fairly resistant to chemical attack and quite weather resistant. Polyurethane One of the most durable and attractive finishes on high-speed, high-altitude aircraft is produced by the polyurethane system. This hard, chemically resistant finish not only provides a beautiful ‘wet’ look, but is the most durable finish for agricultural aircraft, seaplanes and other aircraft that operate in hostile environments. In fact, polyurethane paints are even resistant to Skydrol® hydraulic fluid, which is highly corrosive. The primer used under a polyurethane topcoat is a critical part of the system. Wash primers may be used, but if they do not cure properly, they can cause filiform corrosion. Acrylic Urethanes Acrylic urethane finishes have the advantages of both acrylic lacquer and polyurethane. They are easy to apply like acrylic and nearly as durable as polyurethane. They are mostly used as automotive finishes, although they are becoming popular on General Aviation (GA) aircraft and as a clear topcoat. 2022-07-26 B2-07d Maintenance Practices Page 47 of 110 CASA Part Part 66 - Training Materials Only  Finishing Procedures Surface Finishing Requirements Painting processes vary greatly and often depend on the type of material, the painting surface and the equipment used. Paint and equipment manufacturers often provide helpful information with their products to ensure the appropriate settings for the equipment, which are based on conditions and the type of finishing material being applied. The manufacturer’s directions should be followed closely to create a finish that is smooth and pleasing to the eye. The smooth surface of a quality finish helps reduce drag as well as protect the base material from corrosion and abrasion. A good, intact, professional paint finish is the most effective way to keep metals from being exposed to elements that may cause corrosion. A purpose-built spray booth 2022-07-26 B2-07d Maintenance Practices Page 48 of 110 CASA Part Part 66 - Training Materials Only  Spray Gun Operation There are many different spray gun manufacturers, and each gun is designed, built and operated in a different manner. To obtain the required finish, the gun needs to be correctly adjusted. Prior to painting, test the gun to ensure the desired pressure and spray pattern are set. Adjust the air pressure and fluid adjustment valves to obtain the required flow and spray pattern. A correctly adjusted gun should produce a uniform fan-shaped spray, with the fan perpendicular to the wing ports. Correctly adjusted spray pattern 2022-07-26 B2-07d Maintenance Practices Page 49 of 110 CASA Part Part 66 - Training Materials Only  Applying the Finish Begin spraying the surfaces by first painting the edged or corners. Hold the gun about a hand span or 20 cm (8 in.) from the surface. Move the gun using a steady stroke parallel to the surface. Begin the stroke before reaching the surface you want to paint and continue until passing the other end. The fan shaped spray should be perpendicular to the direction of the stroke. Hold the spray gun about one hand span or 20cm (8") from the surface The spray gun should be moved parallel to the surface 2022-07-26 B2-07d Maintenance Practices Page 50 of 110 CASA Part Part 66 - Training Materials Only  When the edges have been finished, spray the flat portion of the surface with straight passes. Each pass should overlap the previous pass by roughly two thirds. Correct overlapping will provide uniform coverage. 2022-07-26 B2-07d Maintenance Practices Page 51 of 110 CASA Part Part 66 - Training Materials Only  Corrosion Prevention and Removal Corrosion Prevention The best method to prevent corrosion is to eliminate one of its basic requirements. Corrosion requires an anode, a cathode, an electrolyte (water) and an electrical contact, which is usually provided through the electrolyte. Corrosion requirements It is generally impossible to eliminate the anode and cathode from the system. The amount of electrolyte can be controlled through the drains, but moisture cannot be completely eliminated. Corrosion is typically prevented by removing the electrical contact through the application of protective coatings. 2022-07-26 B2-07d Maintenance Practices Page 52 of 110 CASA Part Part 66 - Training Materials Only  Aircraft Surface Cleaning Regular cleaning not only protects the surface finish and structure by removing corrosive agents, dirt and grime, but makes thorough visual inspections for corrosion, cracks and other surface defects possible. If dirt and grime are allowed to accumulate, they can cover surface defects, trap moisture and even affect the overall weight of the aircraft. Prior to aircraft washing, pitot tubes and static ports should be covered, and wheel and brake assemblies should be covered to keep out cleaning agents. Surface cleaning warning and caution It is important to use only approved chemical cleaners and to follow the product manufacturer’s recommendations for use. Surface cleaning note 2022-07-26 B2-07d Maintenance Practices Page 53 of 110 CASA Part Part 66 - Training Materials Only  Non-Metal Cleaning Non-metallic components sometimes require different cleaning techniques than metallic components. Plastic should be rinsed with water before drying with a soft cloth. This prevents surface scratching that occurs when it is not rinsed beforehand. Oil and hydraulic fluid rapidly attack rubber aircraft tyres. Whenever these materials are spilt onto a tyre, they should be immediately wiped off with a dry towel. The tyre should then be washed with soap and water. Rubber de-ice boots have a conductive coating to dissipate static charges, and some composite structures are painted with special materials that are transparent to radio signals. These areas should be cleaned gently and never subjected to stiff brushes or abrasive materials. Post-Wash Procedure During washing operations, water can remove lubricants from bearings and fittings. Landing gear, flight controls, hinges, etc. should be re-lubricated after washing. Pressure greasing forces out any water that may have entered the component and prevents corrosion. 2022-07-26 B2-07d Maintenance Practices Page 54 of 110 CASA Part Part 66 - Training Materials Only  Non-Destructive Testing (7.18.3) Learning Objectives 7.18.3 Describe non-destructive inspection techniques including: penetrant, radiographic, eddy current, ultrasonic and borescope methods (Level 2). 2022-07-26 B2-07d Maintenance Practices Page 55 of 110 CASA Part Part 66 - Training Materials Only  Non-Destructive Inspections and Testing Non-Destructive Inspection Techniques Non-destructive testing methods are techniques used in the production and in-service environments to test the serviceability of a component without damaging or destroying it. Personnel involved in Non-destructive Inspection (NDI) have a Maintenance Authority (MA) issued to them directly from the National Airworthiness Authority (NAA), such as CASA or EASA. For this reason, the training and qualifications for approved NDI personnel are highly specialised. NDI personnel are responsible for confirming the aircraft’s structural integrity. An NDI may be called out either through a scheduled interval or due to a suspected defect identified during an initial visual inspection. Additional Information about NDI will be taken from AC 43.3-1B Chapter 5. NDI techniques include the following inspection techniques: Visual inspection Borescope Penetrants Magnetic particle Acoustic Eddy current Ultrasonic Radiography Thermography Moisture detector Laser holography Shearography. 2022-07-26 B2-07d Maintenance Practices Page 56 of 110 CASA Part Part 66 - Training Materials Only  Non-destructive inspection methods 2022-07-26 B2-07d Maintenance Practices Page 57 of 110 CASA Part Part 66 - Training Materials Only  Selecting a NDI Technique The NDI method and procedure to be used is generally specified by the component manufacturer’s documentation. Factors affecting the inspection include: The critical nature of the component The material, size, shape and weight of the part The type of defect sought The maximum acceptable defect size and distribution Possible locations and orientations of defects Part accessibility or portability The number of parts to be inspected. An important factor in selecting an NDI technique is the degree of inspection sensitivity required. A more sensitive method is required where a small defect in a critical part could cause a catastrophic failure. © Aviation Australia Engine component inspection 2022-07-26 B2-07d Maintenance Practices Page 58 of 110 CASA Part Part 66 - Training Materials Only  Visual Inspection Basic Inspection The most common form of NDI is the visual inspection. This procedure may be enhanced by using appropriate combinations of light sources, magnifying instruments and mirrors. Although there is no formalised qualification for this technique, it is the first line of defence when assessing the integrity of an aeronautical product. Borescopes and video scanners are additional tools used to assist with visual inspections, but may require some specialist training in their use. When using this technique correctly, cracks can be very quickly determined by viewing ‘shine’ or ‘shadow’. By using the technique shown below, the maintainer is able to see shine as the light is reflected from the newly cracked material. Holding the torch and viewing the same crack from the opposite direction generates a shadow due to the changes in surface height. Visual inspection technique 2022-07-26 B2-07d Maintenance Practices Page 59 of 110 CASA Part Part 66 - Training Materials Only  Borescopes Borescopes are used to inspect inaccessible areas, especially within engines. A lens and light source are mounted on the end of a shaft, similar to a submarine’s periscope. Variations of a borescope include a fibre optic scope in which a bundle of optical fibres conveys the light source and image, allowing increased flexibility when accessing difficult areas. Current electronic developments have resulted in a video borescope in which a video chip records images for display on a monitor. Optical fibre borescope 2022-07-26 B2-07d Maintenance Practices Page 60 of 110 CASA Part Part 66 - Training Materials Only  Liquid Penetrant The liquid penetrant method of inspection is suitable for ferrous and non-ferrous materials, including non-porous plastics. It is able to locate cracks, arrears of porosity and other types of faults, provided they are open to the surface. NDT technician carrying out liquid penetrant in a lab The basic principle relies on the capillary attraction of a fluid that has a very low viscosity and very low surface tension. The area under test is flooded with the penetrant fluid long enough for capillary action to draw the penetrant into any fault that extends to the surface. The penetrant is then washed off and the surface is covered with a developer. The developer blots the penetrant out of the cracks or fault and forms a visible line indicating the crack or fault. 2022-07-26 B2-07d Maintenance Practices Page 61 of 110 CASA Part Part 66 - Training Materials Only  Liquid penetrant process Three types of penetrant are used, and all of them require the surface of the component to be perfectly clean and free of oil, grease and other contaminants. Water-Soluble Penetrant Water-soluble penetrants are the easiest to remove. Water is sprayed at 30–40 psi and at an angle of 45° to prevent the dye from being flushed from the defect. Post-emulsifying Penetrant Post-emulsifying penetrants are not water soluble. They must be treated with an emulsifier before they can be washed from the surface. This allows some control over the amount of penetrant removed. Solvent-Removable Penetrant A solvent that is sprayed onto an absorbent towel is used to remove a solvent-removable penetrant. The part should not be sprayed with solvent or submerged as this will flush all the penetrant away. 2022-07-26 B2-07d Maintenance Practices Page 62 of 110 CASA Part Part 66 - Training Materials Only  Developers Two types of developer are used: fluorescent and coloured. The fluorescent dye is visible in the presence of ultraviolet light, while the coloured dye shows up as a red line on the white developer background. Exposed cracks in a bolt A wet developer is white powder mixed with water which is flowed over or floods the component. The component must be air dried before inspection. A non-aqueous developer uses a solvent instead of water. The benefit is rapid drying of the developer, shortening the process. 2022-07-26 B2-07d Maintenance Practices Page 63 of 110 CASA Part Part 66 - Training Materials Only  Developer 2022-07-26 B2-07d Maintenance Practices Page 64 of 110 CASA Part Part 66 - Training Materials Only  Magnetic Particle Inspection Magnetic particle inspection is usable only on parts that are made using a ferrous material. The principle of operation is based on magnetic theory, in which a single piece of magnetised material comprises a north and south pole. A crack in the material causes the creation of an additional pair of magnetic poles. Each pole attracts magnetic particles, indicating the location of the break. Magnetic particle inspection is useful for detecting cracks, splits, seams and voids that form when metal ruptures under stress, as well as during the manufacturing process for detecting cold shuts and foreign matter. The part being tested is magnetised and then an oxide containing magnetic particles is poured or sprayed over the surface. Any discontinuities on or near the surface create disruptions in the magnetic field of the part. The magnetic particles align with these disruptions, indicating the presence of a fault in the area. Magnetic particle inspection To be effective, the orientation of the magnetic field must be perpendicular to that of the crack. This is achieved by magnetising the material both longitudinally and circularly. 2022-07-26 B2-07d Maintenance Practices Page 65 of 110 CASA Part Part 66 - Training Materials Only  Achieving correct magnetic orientation Magnetising Force An electric current is used to magnetise the part under test. 2022-07-26 B2-07d Maintenance Practices Page 66 of 110 CASA Part Part 66 - Training Materials Only  Testing Medium The testing medium consists of finely divided ferromagnetic particles, which have high permeability and low retentivity. They may be dyed to improve visibility and applied dry or mixed in kerosene. Testing medium application Test Sensitivity and Standards Some factors affecting sensitivity include the method of magnetisation, the magnetising amperage, the current type (AC or DC), the type of particles used and the method of particle application. Once these settings have been determined for testing a particular component, a test piece with known flaws is processed and the results should equate with the standard laid down. Magnetic particle inspection test pieces 2022-07-26 B2-07d Maintenance Practices Page 67 of 110 CASA Part Part 66 - Training Materials Only  Once the magnetic particle inspection has been completed, the part needs to be demagnetised so that aircraft operation is not compromised. This is achieved by placing the part in an AC magnetic field and gradually reducing the magnetising force to zero. In cases where this process is inadequate, the part needs to be placed in a DC field and the field direction needs to be manually reversed and reduced in intensity until no residual magnetic field can be detected. Acoustic Inspection Tap Testing Sometimes referred to as audio, sonic or coin tap. A surprisingly accurate method in the hands of experienced personnel, tap testing is perhaps the most common technique used to detect delamination and/or dis-bonds in composite materials. The method is accomplished by tapping the inspection area with a solid round disc or lightweight hammer-like device and listening to the response of the structure to the hammer. A clear, sharp, ringing sound indicates a well-bonded solid structure, while a dull or thud-like sound indicates an area at fault. Tap testing of composites 2022-07-26 B2-07d Maintenance Practices Page 68 of 110 CASA Part Part 66 - Training Materials Only  Electronic Inspection Eddy Current Inspection Eddy current inspection is an NDI method that can detect surface and sub-surface defects in most metals. When compared to a reference, the technique is able to differentiate among metals and alloys and a metal’s heat treatment condition. It requires little preparation. Eddy current inspection Eddy currents are electrical currents that flow through electrically conductive material under the influence of an induced electromagnetic field. Eddy current inspection is based on the principle of AC current acceptance, i.e. it determines the ease with which a material accepts an induced current. There are two methods of eddy current inspection: absolute comparison. 2022-07-26 B2-07d Maintenance Practices Page 69 of 110 CASA Part Part 66 - Training Materials Only  Absolute Method The absolute method measures the permeability of a material, i.e. its ability to accept magnetic lines of flux. A bridge-type eddy current tester is used for this method. With the tester’s probe in contact with the reference material, its centre reading meter is set to 0. The probe is then positioned on the part being tested. Differences between the current induced in the test piece and the current induced in the part being inspected cause the meter to deflect from 0, indicating a defect in the part. Absolute method of eddy current test 2022-07-26 B2-07d Maintenance Practices Page 70 of 110 CASA Part Part 66 - Training Materials Only  Comparison Method The comparison method measures the conductivity of a metal. The conductivity is the ease with which a current flows through a conductor and varies depending on alloy type, grain size, degree of heat treatment and tensile strength. The comparison method uses two probes: Test probe Reference probe. Each probe contains two coils. One induces an electromagnetic field into the material it is in contact with. The second is connected to a meter-reading circuit. With the test probe and the reference probe in contact with the reference material, the meter is adjusted to a null indication. The test probe is then placed on the part being tested and the meter deflects from the null position if the conductivity of the material is different from that of the reference m