Aircraft Defects, Corrosion Inspection, Assessment and Reprotection PDF

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...

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 categorically in the following sections. Aviation Australia - David Kingshott General defect identified during inspection 2023-11-16 B1-07d Maintenance Practices Page 222 of 335 CASA Part Part 66 - Training Materials Only  Metal Part Defects 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 Metal defect - underfloor corrosion 2023-11-16 B1-07d Maintenance Practices Page 223 of 335 CASA Part Part 66 - Training Materials Only  Rubber / Fabric / Glass Fibre / Plastic Part Defects 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. Glass defect example - windscreen crack 2023-11-16 B1-07d Maintenance Practices Page 224 of 335 CASA Part Part 66 - Training Materials Only  Control System Component Defects 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 - fraying 2023-11-16 B1-07d Maintenance Practices Page 225 of 335 CASA Part Part 66 - Training Materials Only  Electrical Component Defects Electrical components includes actuators, alternators, generators, motors, relays, solenoids and contactors. Such items are inspected for: Cleanliness and/or obvious damage. Evidence of overheating. Corrosion and security of attachments and connections. Scoring, worn brushes or adequate spring tension after removal of protective covers. Overheating and fluid ingress. Burning or pitting of contacts. Security of contacts after removal of protective covers. Failed starter generator with broken brushes and burnt out 2023-11-16 B1-07d Maintenance Practices Page 226 of 335 CASA Part Part 66 - Training Materials Only  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. External damage caused by ground equipment 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. 2023-11-16 B1-07d Maintenance Practices Page 227 of 335 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. 2023-11-16 B1-07d Maintenance Practices Page 228 of 335 CASA Part Part 66 - Training Materials Only  Visual Inspection Techniques Landing Gear Dimensions Indicating Serviceability 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. Inspecting 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. Inspecting 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. 2023-11-16 B1-07d Maintenance Practices Page 229 of 335 CASA Part Part 66 - Training Materials Only  Inspecting 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. Inspecting 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. 2023-11-16 B1-07d Maintenance Practices Page 230 of 335 CASA Part Part 66 - Training Materials Only  Cabin and Fuselage Inspecting 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. Inspect for indicator status such as APU fire extinguisher bottle indicators 2023-11-16 B1-07d Maintenance Practices Page 231 of 335 CASA Part Part 66 - Training Materials Only  Inspecting 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 are inspected for physical damage Inspecting 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. 2023-11-16 B1-07d Maintenance Practices Page 232 of 335 CASA Part Part 66 - Training Materials Only  Inspecting 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. Inspecting Emergency Systems / Equipment 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. Inspect that the fire extinguisher protective cover is intact indicating that the extinguisher has not been used 2023-11-16 B1-07d Maintenance Practices Page 233 of 335 CASA Part Part 66 - Training Materials Only  Inspecting a 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. Aircraft Systems / Components Inspecting Moving Parts When inspecting moving parts, the AME must check that defects have not been created during component operation. This includes inspecting for excessive wear or for evidence of moving parts binding or interfering with each other during movement. Inspections should identify any cracked or broken fittings, loose hinges, defective bearings, corrosion, and deformation like dents, kinks or bending. 2023-11-16 B1-07d Maintenance Practices Page 234 of 335 CASA Part Part 66 - Training Materials Only  Moving attachments should be secure but have the required range of motion and any retaining mechanisms like pins, clips or safety wiring are in good condition and correctly installed. Where required, moving parts should be correctly lubricated using a lubricant that is permitted in the maintenance manual. Any seals should be inspected for damage, wear, or signs of leaks. Parts should be clean and free from any dirt, dust, aircraft fluids or old lubricants that may have accumulated over time. Any moving components should be correctly adjusted so that they have the correct tension and travel (full range of movement) and operate in accordance with the maintenance manual. Inspecting Fluid Lines and Hoses When inspecting fluid lines and hoses the components should be checked with the material of the line or hose kept in mind. The AME must inspect for correct hosing or rigid tubing material used in the given area, correct installation of the line or hose, proper fittings are used and correct fitting torque. The lines or hoses must also be checked for leaks, tears, cracks, dents, kinks, chafing, proper bend radius, security (how it is secured to the aircraft or end items), corrosion, deterioration, obstructions, and foreign matter. Inspecting Wiring Aircraft wiring should also be checked to ensure it is securely attached. There should be no evidence of wires or wire looms chafing or rubbing against other components or structures, burning or overheating, defective insulation, loose, corroded, or broken terminals, heat or fluid degradation, and proper installation of any clamps or devices supporting the wiring. Inspecting Bolts Aircraft fittings like nuts and bolts should be checked to ensure they have the correct torque and that any tamper proof paste is still intact. Any elongation of bearing surfaces, deformation, evidence of shear or tension damage should be identified and recorded. All fittings should be installed correctly and be the correct size and type and corrosion free. Inspecting Rivets Any corrosion discovered around rivets should be identified and recorded. A complete inspection requires them to be drilled out to determine the extent of any corrosion. The maintenance manual should always be followed when reinstalling rivets. An inspection should also identify if there is a gap under the rivet head or it is loose. This can be an indication that the rivet has stretched from stress. 2023-11-16 B1-07d Maintenance Practices Page 235 of 335 CASA Part Part 66 - Training Materials Only  Inspecting Filters, Screens and Fluids Inspection of filters, screens and aircraft fluids includes checking for cleanliness, to ensure there is no contamination. These components should be checked to ensure they have been previously inspected and or replaced in accordance with the maintenance manual service schedule. An inspection ensures that these consumable components are the correct part and installed correctly as documented in the aircraft maintenance manual. Inspecting the Powerplant When inspecting the engine nacelles, the engine mounts should be thoroughly inspected for security and any evidence of movement or wear, cracking or corrosion. The engine assembly mounting bolt torques should be inspected to ensure serviceability. Spark plugs, ignition harness and all pipes, hoses and mufflers should be visually and physically inspected for fitment security. An inspection of the engine nacelle should identify any oil leaks, exhaust leaks, muffler cracks and wear, any loose or poorly mounted engine accessories, engine or engine accessory gearbox case cracks, and any obstruction to oil breathers. The engine firewall should be inspected for any physical defects including impact damage, corrosion, or loose fitment. Any mechanical controls or moving parts should be checked to ensure proper operation with no wear and full range of motion. Inspecting Propellers Propellers are one of the most exposed components of an aircraft. They should be cleaned before being carefully inspected for any nicks, dents, cracks, gouges. 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. Propeller inspections should ensure general cleanliness, lubrication. Inspections for proper blade angles, blade tracking, and proper dimensions should be carried out in accordance with the maintenance manual. Propeller governor leak and operational checks are also required. The control linkages should be visually and physically inspected for proper tension and installation. Inspecting Aircraft During Ground Runs During a ground run, the AME is checking for correct 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 and fuel flow indicator operation. 2023-11-16 B1-07d Maintenance Practices Page 236 of 335 CASA Part Part 66 - Training Materials Only  Inspecting Lifed Items Many aircraft parts and components can only be installed on the aircraft for a specific length of time (known as a ‘life’). These vary from major airframe and engine components to small seals and lubricants all with finite service lives. Technical departments within the manufacturer monitor major airframe and engine components and advise maintenance organisation when to replace these components during a major service. Other components which can become unserviceable due to life expiry include engine fire bottles, cabin fire extinguishers, first aid kits, portable oxygen bottles and emergency oxygen generators. Light bulbs are check/tested frequently to ensure they are serviceable. Most lights that perform important functions, like fire warning lights and undercarriage indicators, have two bulbs. 2023-11-16 B1-07d Maintenance Practices Page 237 of 335 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. 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. 2023-11-16 B1-07d Maintenance Practices Page 238 of 335 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 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. Sandwich Constructions 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. 2023-11-16 B1-07d Maintenance Practices Page 239 of 335 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. 2023-11-16 B1-07d Maintenance Practices Page 240 of 335 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. Paint Removal Due To Corrosion 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. Note: Strippers can damage composite resins and plastics, so every effort should be made to mask these vulnerable areas. The Effect of Corrosion on Metals and Alloys 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. 2023-11-16 B1-07d Maintenance Practices Page 241 of 335 CASA Part Part 66 - Training Materials Only  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. 2023-11-16 B1-07d Maintenance Practices Page 242 of 335 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%. Therefore, where heavy corrosion is found on clad aluminium alloys, it must be removed by chemical methods wherever possible. Copious amounts of clean water should next be used to flood the area and remove all traces of the chemical, and then the surface should be dried thoroughly. Assessment for Serviceability To test for sufficient remaining aluminium cladding after corrosion removal from Alclad, a solution of Caustic soda can be sparingly applied. if the coating is intact, a white stain will appear. Where the pure aluminium has been exposed, a black stain will appear. Following this integrity test of the alclad, a solution of Chromic anhydride is used to neutralise the effects of the solution. 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 de-grease 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. 2023-11-16 B1-07d Maintenance Practices Page 243 of 335 CASA Part Part 66 - Training Materials Only  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. 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. The corrosion pits should be transformed into saucer-shaped depressions which relieve stress concentrations. 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. 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. 2023-11-16 B1-07d Maintenance Practices Page 244 of 335 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. 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. 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. 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. 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. 2023-11-16 B1-07d Maintenance Practices Page 245 of 335 CASA Part Part 66 - Training Materials Only  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. 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. 2023-11-16 B1-07d Maintenance Practices Page 246 of 335 CASA Part Part 66 - Training Materials Only  Corrosion Removal Techniques Blend of a Single Depression Blend of a single depression 2023-11-16 B1-07d Maintenance Practices Page 247 of 335 CASA Part Part 66 - Training Materials Only  Blending Ratios Blending ratios Blend of Multiple Corrosion Areas Blend of multiple corrosion areas 2023-11-16 B1-07d Maintenance Practices Page 248 of 335 CASA Part Part 66 - Training Materials Only  Using a Dial-Type Depth Gauge Using a dial-type depth gauge 2023-11-16 B1-07d Maintenance Practices Page 249 of 335 CASA Part Part 66 - Training Materials Only  Corrosion Due To Acids and Akali 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. 2023-11-16 B1-07d Maintenance Practices Page 250 of 335 CASA Part Part 66 - Training Materials Only  Corrosion Due To Mercury 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. 2023-11-16 B1-07d Maintenance Practices Page 251 of 335 CASA Part Part 66 - Training Materials Only  Corrosion Due To Mercury Spills 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 Spill Precautions The following precautions for mercury spills should be strictly followed: Avoid contact with surfaces suspected of being contaminated. Use wood or fibre sheets to support the body while working in the area. Wear wing socks (shoe protectors), protective (disposable) clothing, and latex or vinyl gloves in the contaminated area to avoid scratching metal surfaces. 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. 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. 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. Appreciable amounts of mercury will vaporise at normal temperatures. Stagnant air will become dangerous to personal health. Do not use cleaning products, such as solvents, solids or polishes, on contaminated areas. Such materials may promote corrosion. 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. 2023-11-16 B1-07d Maintenance Practices Page 252 of 335 CASA Part Part 66 - Training Materials Only  Mercury Removal Procedure 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. An all-rubber storage battery water syringe or a medicine dropper may be used to remove mercury 2023-11-16 B1-07d Maintenance Practices Page 253 of 335 CASA Part Part 66 - Training Materials Only 2023-11-16 B1-07d Maintenance Practices Page 254 of 335 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 2023-11-16 B1-07d Maintenance Practices Page 255 of 335 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. 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. 2023-11-16 B1-07d Maintenance Practices Page 256 of 335 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 under size 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 2023-11-16 B1-07d Maintenance Practices Page 257 of 335 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 2023-11-16 B1-07d Maintenance Practices Page 258 of 335 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. 2023-11-16 B1-07d Maintenance Practices Page 259 of 335 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 2023-11-16 B1-07d Maintenance Practices Page 260 of 335 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 2023-11-16 B1-07d Maintenance Practices Page 261 of 335 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. 2023-11-16 B1-07d Maintenance Practices Page 262 of 335 CASA Part Part 66 - Training Materials Only  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 2023-11-16 B1-07d Maintenance Practices Page 263 of 335 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 2023-11-16 B1-07d Maintenance Practices Page 264 of 335 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 2023-11-16 B1-07d Maintenance Practices Page 265 of 335 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. 2023-11-16 B1-07d Maintenance Practices Page 266 of 335 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. 2023-11-16 B1-07d Maintenance Practices Page 267 of 335 CASA Part Part 66 - Training Materials Only  Aircraft Cleaning to Prevent Corrosion 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. It is important to use only approved chemical cleaners and to follow the product manufacturer’s recommendations for use. 2023-11-16 B1-07d Maintenance Practices Page 268 of 335 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. 2023-11-16 B1-07d Maintenance Practices Page 269 of 335 CASA Part Part 66 - Training Materials Only