Module 7 (Maintenance Practices) Sub Module 7.18 (Aircraft disassembly, inspection, repair and assembly techniques).pdf

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PIA TRAINING CENTRE (PTC) Module 7 - MAINTENANCE PRACTICES Category A/B1 Sub Module 7.18 - Aircraft disassembly, inspection, repair and assembly techniques

PTC/CM/B1.1 Basic/M7/04 Rev. 00 7.18 Mar 2014

MODULE 7

Sub Module 7.18

DISASSEMBLY, INSPECTION, REPAIR AND ASSEMBLY

TECHNIQUES



PTC/CM/B1.1 Basic/M7/04 Rev. 00 7.18 - i Mar 2014

Contents INTRODUCTION ...................................................................... 1

TYPES OF DEFECTS ............................................................... 1

VISUAL INSPECTION TECHNIQUES .................................... 10

CORROSION REMOVAL, ASSESSMENT AND

REPROTECTION ................................................................... 14

GENERAL REPAIR METHODS .............................................. 19

STRUCTURAL REPAIR MANUAL (SRM) ............................... 23

NON-DESTRUCTIVE TESTING/INSPECTION (NDT/NDI)

TECHNIQUES ........................................................................ 25

REMOTE VIEWING INSTRUMENTS ...................................... 27

PENETRANT FLAW DETECTION (PFD) ................................ 31

ULTRASONIC FLAW DETECTION (UFD) .............................. 32

EDDY CURRENT FLAW DETECTION (ECFD) ...................... 39

RADIOGRAPHIC FLAW DETECTION (RFD) ......................... 42

DISASSEMBLY AND RE-ASSEMBLY TECHNIQUES ............ 44

TROUBLESHOOTING ............................................................ 51



PTC/CM/B1.1 Basic/M7/04 Rev. 00 7.18 - 1 Mar 2014

INTRODUCTION Preventative maintenance is concerned with the early detection of defects (using whatever inspection techniques are specified by the aircraft or component manufacturers) and the repair or modification of the defective parts.

The inspection techniques may call for the disassembly of components (before or after cleaning) so that more detailed inspections can be done. Assessment, of the effect of the defect on the continued integrity of the part, will also be required and, following the repair, modification or rejection of the part, re-assembly techniques will be used to restore the aircraft to the appropriate level of serviceability. Troubleshooting techniques are used in the process of identifying the cause of a fault, eliminating the fault and returning the aircraft to service.

TYPES OF 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 below 100%. The manufacturer should specify the inspection areas and the faults, which are expected to be found. In most instances the inspector is looking for indications of abnormality in the item being inspected. Typical examples are:

Metal Parts: as applicable to all metal parts, bodies or casings of units in systems and in electrical, instrument and radio installations, metal pipes, ducting, tubes, rods and levers. These would be 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.




Rubber, Fabric, Glass Fibre and Plastic Parts: such as coverings, ducting, flexible mountings, seals, insulation of electrical cables, windows. These parts would, typically, be inspected for:

Cleanliness Cracks, cuts, chafing, kinking, twisting,

crushing, contraction sufficient free length Deterioration, crazing, loss of flexibility Overheating Fluid soakage Security of attachment, correct connections

and locking. Control System Components: cables, chains, pulleys, rods

and tubes would be 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.

Electrical Components: actuators, alternators and generators, motors, relays, solenoids and contactors. Such items would be 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 reasons for airframe damage is by being struck by ground equipment or 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 puncturing of 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 check of all inlets and exhausts, during inspections, must be made, to ensure that there is nothing blocking them. A blocked duct can result in the overheating of equipment, or major damage to the internal working parts of the engine. 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 (IDG), and also 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 trouble-shooting. 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. The rectification of an external leak is usually achieved by simply replacing the component, seal or pipe work 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 a slower movement of the services or by the erratic operation of services, due to the return line being pressurised. Some hydraulic oils, especially the phosphate ester based fluids, are very toxic and require personnel protection when working on and replenishing their systems. Some oils used are slightly toxic so care must be taken if there is a large leak. Potable water tanks are often permanently pressurised, so that a leak that starts somewhere between the tank and the services will continue, even if the aircraft is not flying. Once the pressure is removed, the leak can be investigated, cured and the tank re-filled. The physical signs of water inside the aircraft or dripping from the hull should be the signs of a leak that requires investigation. The unpredictable passenger consumption of water means that the tank level is no indication of a leak in the system.




Windscreen de-icers are usually in the form of a pressurised container, which supplies fluid on demand to the spray nozzles. If the fluid leaks onto the flight deck it will give off a distinctive odour in the enclosed space. As the containers are replaced when low, it is more likely that the pipe work will be the likely cause of the leak. Gaseous Systems These include gases such as oxygen, nitrogen and air. If the gas is to be used from a system during flight, a leak will be very hard to confirm unless a physical check is carried out using a leak detector such as Snoop or Sherlock. 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. Nitrogen, used in hydraulic accumulators, can leak into the liquid part of the hydraulic system. This will make the hydraulic system feel spongy and reduce the response of the operating actuators. If the gas leaks into the atmosphere, the system will not function correctly and the efficiency of the system may be reduced. The main cause of accumulators leaking externally is due to faulty seals or gauges. Accumulators assist the hydraulic system as an emergency backup, which only works correctly if it is charged to the correct pressure.

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 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.




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 it may be low on pressure and further checks will be required. These checks are usually only done during line maintenance, with checking of the pressure being required for trouble shooting or hangar maintenance. Combined hydraulic and spring dampers, fitted to some landing gears, often have one or more engraved lines on the sliding portion of the unit. This can indicate whether the hydraulic pre-charge is correct or requires replenishment. Tyres Tyres have their serviceability indicated by the depth of the groove in the tyre tread. The AMM gives information of 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. Most tyres can be re-treaded a number of times after they have reached their wear limits, but the retread can only be completed if the complete tyre has not been damaged badly.

Creep is the movement of a cover around the rim, in very small movements, 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. To provide an indicator, small white marks are painted across the wheel rim and the tyre side wall cover so, if creep takes place, the marks will split in half and indicate clearly that the tyre cover has moved in relation to the wheel rim. The installation of tubeless covers has reduced the problem of creep, as the valve is permanently fitted to the wheel. It is still possible for tyres to creep a small amount, but the air remains in the tyre as the seal remains secure. Tyre-inflation devices usually consist of high-pressure bottles fitted with a pressure-reducing valve or a simple air compressor. The pressure a tyre should be inflated to depends on various factors such as the weight of the aircraft. The correct pressure for a specific aircraft is given in the relevant AMM for the aircraft in question. It is possible for a tyre to lose a small amount of pressure overnight. A pressure drop of less than 10% of the recommended pressure is not unusual, but the exact figures are given in the AMM.




If a tyre is completely deflated with the weight of the aircraft on it, or is one of a pair on a single landing gear leg, which has run without pressure, all the tyres concerned must be replaced due to the possible, unseen damage within the cover. Again the AMM will dictate the conditions. Wheels Defects to 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 the impact damage and the shearing of wheel bolts, which hold the two halves of a split wheel together. Wheels are usually inspected thoroughly during tyre replacement and it is very unusual for serious defects to be found during normal inspections of a wheel. Brakes Brake units are normally attached onto the axle of an undercarriage leg, and located inside the well of the main wheels. During braking 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 results in the total thickness of the brake pack being reduced, which means that by measuring either the thickness of the pack, the amount of wear can be monitored. Once the amount of wear reaches a set figure, the brake pack will be 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 will require immediate replacement of the complete brake unit. A rejected take-off at maximum weight will produce the maximum possible amount of heat and wear. It is usual to replace all brake units and main wheels after this has happened, but again the AMM will give the required information on what must be changed and when.




Landing Gear Locks These items are normally fitted to the aircrafts undercarriage as a safety device to prevent them inadvertently collapsing. They are usually fitted when the aircraft is to stay on the ground for some time, and removed before the next flight. The most likely defects will be damage to the locking pin ball bearing device or the loss of the high visibility warning flags. These flags will, hopefully, attract attention to themselves to ensure that they are not left in position when the aircraft next goes flying. Indicators The most common type of indicator is the blow-out disc used in fire extinguishing and oxygen systems. This shows that a high-pressure gas bottle has discharged its contents overboard, blowing the disc from its flush housing in the aircrafts skin. The reason for the ruptured disc (refer Fig. 1) could be due to a fire extinguisher having been operated or the extinguishant having been discharged due to an excessive pressure being reached.

Frangible Disc

Retaining Ring

Gas Bottle and Pressure

Relief Valve

Gas Bottle Bursting Disc

Fig. 1




External Probes There are several different types of probe, projecting into the airflow, to send information to the flight deck. These can include the pitot/static probes and the angle-of attack (AOA) probes. To prevent these from freezing they have electrical heating elements built into them and, occasionally, they can become overheated. Usually this is when they are left switched on on the ground with a faulty weigh-on-wheels (WOW) switch. This switch is designed to reduce or remove power to the probes when on the ground, and to increase or restore it in flight. On smaller aircraft there is no WOW switch and it is up to the pilot to turn them off after landing. If the elements overheat they can burn out and the probes will show this by discoloration. Probes are designed to project out from the aircraft skin, and this makes them vulnerable to physical damage. Probes need to be regularly inspected for signs of physical damage or discoloration. 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 will wear slowly and these panels must be secured in flight.

Most fasteners have a positive form of closing or locking, whilst 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. Panels and Doors These items can be of 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). If the latches are poorly designed or badly adjusted, they may have been operated with incorrect tools during service and may have been damaged.




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 will indicate 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 will indicate that the cover has been lifted and the system may have been operated. Any indication like this must be thoroughly investigated. Lifed Items There are a number of items on the aircraft that have a specific length of time in service (known as a life). They would be major airframe and engine components with finite fatigue lives. The company technical department monitors these and they will be replaced during major servicing. The 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.

Light Bulbs These 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 will be duplicated. This can be achieved either by using two separate bulbs or by a single, twin-filament type. The bulb covers can also be damaged, leading to broken glass or plastic on the flight deck, with its subsequent foreign object damage (FOD) hazard. Permitted Defects All aircraft have a list of permitted defects that do not have to be immediately corrected. These defects can be left outstanding by the operator until a more convenient time can be found to rectify them.




VISUAL INSPECTION TECHNIQUES Often the first stage in the examination of a component is visual inspection. Examination by naked eye will only reveal relatively large defects, which break the surface, but the effectiveness of visual inspection for external surfaces can be improved considerably through use of a hand lens or stereoscopic microscope. Generally, high magnifications are not necessary for this type of inspection. Optical inspection probes, both rigid and flexible, which can be inserted into cavities, ducts and pipes, have been developed for the inspection of internal surfaces. An optical inspection probe comprises an objective lens system at the working end and a viewing eyepiece at the other end, with a fibre optic coherent image guide linking the two. Illuminating light is conveyed to the working end of the probe through an (Figure A) optical fiber light guide, and both the optical and illumination systems are contained within either a stainless steel tube, for rigid probes, or a flexible plastic or braided metal sheathing in the case of flexible probes. Inspection probes are made in many sizes with, for rigid probes, diameters ranging from about 2 mm up to about 20 mm. The minimum diameter for flexible probes is about 4 mm. Probe lengths may vary considerably also, and the maximum working length for a 2 mm probe is about 150 mm. The maximum permissible working length increases as probe diameter increases and may be up to 5 m for a 20 mm diameter probe. Inspection probes can be designed to give either direct viewing ahead of the probe end, or to give a view at some angle to the line of the probe. It is possible to mount a miniature TV camera in place of the normal eyepiece lens system and display an image on a monitor screen.

Figure A




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 the path of the exhaust gases of gas turbine and piston engines, 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 easier 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 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, paint-work, bearings, exposed switches and electrical equipment. Frequent cleaning, water-dispersing treatment and re-lubrication will be required, whilst ensuring that bearings are not contaminated, either with the cleaning water or with the water-dispersing fluids, used when re-lubricating.




Bilge and Water Entrapment Areas 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 toilet/wash-rooms 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 also occur, at the hinges of access doors and panels that are seldom used.

Magnesium Alloy Skins These, give little trouble, providing the protective surface finishes are undamaged and well maintained. Following maintenance work, such as riveting and drilling, it is impossible to completely protect the skin to the original specification. All magnesium alloy skin areas must be thoroughly and regularly inspected, with special emphasis on edge locations, fasteners and paint finishes. Aluminium Alloy Skins The most vulnerable skins are those which have been integrally machined, usually in main-plane structures. Due to the alloys and to the manufacturing processes used, they can be susceptible to intergranular and exfoliation corrosion. Small bumps or raised areas under the paint sometimes indicate exfoliation of the actual metal. Treatment requires removal of all exfoliated metal followed by blending and restoration of the finish.




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 causes rapid attack on 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. Control Cables Loss of protective coatings, on carbon steel control cables can, over a period of 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 in other places where the cables flex as there is more chance of corrosion getting inside the cables when the strands are moving around (or being moved by) these items.




CORROSION REMOVAL, ASSESSMENT AND REPROTECTION Due to the high cost of modern aircraft, operators are expecting them to last much longer than perhaps even the manufacturer anticipated. As a result, the manufacturers have taken more care in the design of the aircraft, to improve the corrosion-resistance of aircraft. This improvement includes the use of new materials and improved surface treatments and protective finishes. The use of preventative maintenance has also been emphasised more than previously. Preventative maintenance, relative to corrosion control, should include the:

Adequate and regular cleaning of the aircraft Periodic lubrication (often after the 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 Keeping of drain holes clear Draining 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.

General treatments for corrosion removal include:

Cleaning and stripping of the protective coating in the corroded area

Removal of as much of the corrosion products as possible

Neutralisation of the remaining residue Checking if damage is within limits Restoration of protective surface films Application of temporary or permanent coatings or paint

finishes. Cleaning and Paint Removal It is essential that the complete suspect area be cleaned of all grease, dirt or preservatives. This will aid in determining the extent of corrosive spread. The selection of cleaning materials will depend on the type of matter to be removed. Solvents such as trichloroethane (trade name Genklene) may be used for oil, grease or soft compounds, while heavy-duty removal of thick or dried compounds may need solvent/emulsion-type cleaners. General-purpose, water-removable stripper is recommended for most paint stripping. Adequate ventilation should be provided and synthetic rubber surfaces such as tyres, fabrics and acrylics should be protected (remover will also soften sealants). Rubber gloves, acid-repellent aprons and goggles, should be worn by personnel involved with paint removal operations. The following represents a typical paint stripping procedure:




Brush the area with stripper, to a depth of approximately 0.8 mm 1.6 mm (0.03 in 0.06 in). Ensure that the brush is only used for paint stripping

Allow the stripper to remain on the surface long enough for the paint to wrinkle. This may take from 10 minutes to several hours

Re-apply the stripper to those areas which have not stripped. Non-metallic scrapers may be used to assist the stripping action

Remove the loosened paint and residual stripper by washing and scrubbing the surface with water and a broom or brush. Water spray may assist, or the use of steam cleaning equipment may be necessary.

Note: Strippers can damage composite resins and plastics, so every effort should be made to 'mask' these vulnerable areas. 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 will indicate a need for maintenance and will suggest possible further corrosive attack on more critical areas. The most practical means of controlling the corrosion of steel is the complete removal of corrosion products by mechanical means.

Abrasive papers, power buffers, steel wool and wire brushes are all acceptable methods of removing rust on lightly stressed areas. Residual rust usually remains in pits and crevices. Some (dilute) phosphoric acid solutions may be used to neutralise oxidation and to convert active rust to phosphates, but they are not particularly effective on installed 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. 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. Aluminium alloys form a smooth surface oxidation, which provides a hard shell, that, in turn, may form a barrier to corrosive elements. This must not be confused with the more serious forms of corrosion. General surface attack penetrates slowly, but is speeded up in the presence of dissolved salts. 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. Alclad WARNING: USE ONLY APPROVED PAINT STRIPPERS IN THE VICINITY OF REDUX BONDED JOINTS. CERTAIN PAINT STRIPPERS WILL ATTACK AND DEGRADE RESINS. USE ADEQUATE PERSONAL PROTECTIVE EQUIPMENT WHEN WORKING WITH CHEMICALS. USE ONLY THE APPROVED FLUIDS FOR REMOVING CORROSION PRODUCTS. INCORRECT COMPOUNDS WILL CAUSE SERIOUS DAMAGE TO METALS. 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, normally with the use of a stiff (nylon) bristled brush, to the corroded surface, 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, then the surface should be dried thoroughly. Note: A method of checking that the protective aluminium coating remains intact is by the application of 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. The acid must be neutralised and the area thoroughly washed and dried before a protective coating (usually Alocrom 1200 or similar) is applied to the surface. Further surface protection may be given by a coat of suitable primer, followed by the approved top coat of paint. Magnesium Alloys The corrosion products are removed from magnesium alloys by the use of chromic/sulphuric acid solutions (not the phosphoric acid types), brushed well into the affected areas. Clean, cold water is employed to flush the solution away and the dried area can, again, be protected, by the use of Alocrom 1200 or a similar, approved, compound.




Acid Spillage An acid spillage, on aircraft components, can cause severe damage. Acids will corrode most metals used in the construction of aircraft. They will also destroy wood and most other fabrics. Correct Health and Safety procedures must be followed when working with such spillages. 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 to be taken, in the event of an acid spillage, is as follows:

Mop up as much of the spilled acid, 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 10%

(by weight) solution of bicarbonate of soda (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 the area for signs of damaged paint or plated finish and signs of corrosion, especially where the paint may have been damaged.

Remove corrosion, repair the damage and restore the

surface protection as appropriate.

Alkali Spillage This is most likely to occur from the alternative Nickel-Cadmium (Ni-Cd) or Nickel-Iron (Ni-Fe) type of batteries, containing 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 the lead/acid type of batteries. Proper Health and Safety procedures are, again, imperative. Removal of the alkali spillage, and subsequent protective treatment, follows the same basic steps as outlined in acid spillage, with the exception that the alkali is neutralised with a solution of 5% (by weight) of chromic acid crystals in water.




Mercury Spillage WARNING: MERCURY (AND ITS VAPOUR) IS EXTREMELY TOXIC. INSTANCES OF MERCURY POISONING MUST, BY LAW, BE REPORTED TO THE HEALTH AND SAFETY EXECUTIVE. ALL SAFETY PRECAUTIONS RELATING TO THE SAFE HANDLING OF MERCURY MUST BE STRICTLY FOLLOWED. Mercury contamination is far more serious than any of the battery spillages and prompt action is required to ensure the integrity of the aircraft structure. While contamination from mercury is extremely rare on passenger aircraft, sources of mercury spillage result from the breakage of (or leakage from) containers, instruments, switches and certain test equipment. The spilled mercury can, quickly, separate into small globules, which have the capability of flowing (hence its name Quick Silver) into the tiniest of crevices, to create damage. Mercury can rapidly attack bare light alloys (it forms an amalgam with metals), causing intergranular penetration and embrittlement which can start cracks and accelerate powder propagation, resulting in a potentially catastrophic weakening of the aircraft structure. Signs of mercury attack on aluminium alloys are greyish powder, whiskery growths, or fuzzy deposits. If mercury corrosion is found, or suspected, then it must be assumed that intergranular penetration has occurred and the structural strength is impaired. The metal in that area should be removed

and the area repaired in accordance with manufacturers instructions. Ensure that toxic vapour precautions are observed at all times during the following operations:

Do not move aircraft after finding spillage. This may prevent spreading.

Remove spillage carefully by one of the following mechanical methods:

Capillary brush method (using nickel-plated carbon fibre brushes).

Heavy-duty vacuum cleaner with collector trap. Adhesive tape, pressed (carefully) onto globules may pick

them up Foam collector pads (also pressed, carefully, onto

globules). Alternative, chemical methods, of mercury recovery entail

the use of: Calcium polysulphide paste Brushes, made from bare strands of fine copper wire

Neutralise the spillage area, using Flowers of Sulphur Try to remove evidence of corrosion The area should be further checked, using radiography,

to establish that all globules have been removed and to check extent of corrosion damage

Examine area for corrosion using a magnifier. Any parts found contaminated should be removed and replaced.




Note 1: Twist drills (which may be used to separate riveted panels, in an attempt to clean contaminated surfaces) must be discarded after use. Note 2: Further, periodic checks, using radiography, will be necessary on any airframe that has suffered mercury contamination. GENERAL REPAIR METHODS There are two classifications of repairs in this SRM: (1) Repairs that have been evaluated and analyzed for damage tolerance capability and are classified as Category A, B, or C repairs. (2) Repairs that have not been evaluated and analyzed for damage tolerance capability and are classified as Permanent, Interim or Time-Limited Repairs. NOTE: If a repair is not identified as an interim or time-limited repair, it is a permanent repair. The definitions of the different categories of damage tolerant repairs are as follows: (1) Category A Repair: A permanent repair for which the inspections given in the Baseline Zonal Inspection (BZI) are sufficient and no other actions are necessary. (2) Category B Repair: A permanent repair for which supplemental inspections are necessary at the specified threshold and repeat intervals.

(3) Category C Repair: A time-limited repair which must be replaced or reworked within a specified time limit. Also supplemental inspections can be necessary at a specified threshold and repeat interval. The definitions of the different types of repairs that have not been evaluated and analyzed for damage tolerance are as follows: (1) Permanent Repair: A repair where no action is necessary, except the operators normal maintenance. (2) Interim Repair: A repair that has the necessary structural strength and could stay on the airplane indefinitely. The repair must be inspected at specified intervals and replaced if deterioration is detected or damage is found. (3) Time-Limited Repair: A repair that has the necessary structural strength but does not have sufficient durability. This repair must be replaced after a specified time, usually given as a number of flight cycles, flight hours or a calendar time. The definitions of the terms as they apply to the repairs are as follows: (1) Baseline Zonal Inspection (BZI): A set of typical maintenance inspection intervals that are assumed to be performed by most operators, and defined in the Repair Assessment Guidelines document. BZI was the basis for the creation of a list of structural areas or types of repairs that would not require supplemental inspection. The type of inspection associated with the BZI is:




General Visual Inspection of all visible structure in the area of being inspected. Some SRM repairs were chosen to be Category A or B comparing their inspection requirements with the baseline zonal inspection intervals for the areas repaired. If the BZI interval was adequate to maintain damage tolerance, the repair was labeled Category A. If not, the repair was labeled Category B. Operators must be aware that if their current inspection intervals exceed the BZI intervals, the repair categories may not apply. See the Repair Assessment Guidelines document (D6-38669) for complete information. (2) Damage Tolerance: The ability of structure to sustain anticipated loads in the presence of damage, such as fatigue cracks until it is detected through inspection or malfunction, and repaired. (3) Damage Tolerant Repair: A repair that meets the necessary damage tolerance conditions. (4) Repeat Intervals: The period in flight cycles, flight hours or calendar time that occurs between the necessary inspections. (5) Supplemental Inspections: Special inspections of the repaired structure that are done in addition to an operators normal maintenance inspections. (6) Threshold: The period in flight cycles, flight hours or calendar time from the time an airplane is delivered or a repair is made until the first supplemental inspection is necessary.

For Category B repairs, the threshold starts from the time the repair was installed if the repair fasteners in the critical rows have been installed in new fastener holes or existing fastener holes that have been zero-timed. If the repair fasteners are installed in existing fastener holes that have not been zero-timed, the inspection threshold will start from the time the airplane was delivered. (7) Time-Limit: The maximum period in flight cycles, flight hours or calendar time that is permitted until it is necessary to replace or rework a time-limited repair. (8) Zero-Timing: The process used to improve the repair durability in order to make the inspection threshold start from the time the repair is installed. This involves the removal of small cracks and fatigue damaged material by over sizing the existing fastener holes before the repair is installed as given in GENERAL. Zero-timing must only be used where specifically permitted in an SRM chapter-section-repair. Also zero-timing must not cause short edge margin and fastener spacing, and knife-edging on the repair fasteners. (9) Critical Fastener Row: Fastener row to be inspected to meet damage tolerance requirements.




Damage Tolerance Assessment of Repaired Structure (1) The damage tolerance assessment of a repair is done to determine the effect that the repair has on the damage tolerance capability, and inspect ability, of the initial structure. This assessment is also used to identify the inspections that are necessary to keep the repaired structure in an airworthy condition. The SRM will provide the inspection requirements for fuselage pressure boundary repairs that are published in the SRM. However, fuselage pressure-boundary repairs developed by the operators will need to be assessed using the Repair Assessment Guidelines document. Damage tolerance assessment of the repaired structure can be completed after an airplane is returned to service. Types of inspections that are used to detect damage in structure are as follows:

(1) General Visual (Surveillance) Inspection (GVI): A visual examination of an interior or exterior area, installation or assembly to detect obvious damage, failure or irregularity. This level of inspection is made from within touching distance unless otherwise specified. A mirror may be necessary to enhance visual access to all exposed surfaces in the inspection area. This level of inspection is made under normally available lighting conditions such as daylight, hangar lighting, flashlight or drop-light and may require removal or opening of access panels or doors. Stands, ladders or platforms may be required to gain proximity to the area being checked.

(2) Detailed Inspection (DET): An intensive examination of a specific item, installation or assembly to detect damage, failure or irregularity. Available lighting is normally supplemented with a direct source of good lighting at an intensity deemed appropriate by the inspector. Inspection aids such as mirrors, magnifying lenses, etc., may be used. Surface cleaning and elaborate access procedures may be required. (3) Special Detailed (Non-Destructive Testing) Inspection (SDI): An intensive examination of a specific item(s), installation, or an assembly to detect damage, failure or irregularity. The examination is likely to make extensive use of specialized inspection techniques and/or equipment. Intricate cleaning and substantial access or disassembly procedure may be required. Non-Destructive Testing (NDT) inspections are used to examine all subsurface damage and most small cracks. NDT is also used in areas where a visual inspection is not sufficient to find the dimensions of damage. NDT procedures recommended for use in the SRM are as follows:

(a) Eddy Current: An NDT procedure that uses eddy currents to find damage in metals that have good conductivity properties. The Eddy Current inspection is the preferred NDT procedure used to find most damage on metal parts.




1) The three types of Eddy Current inspections used in the SRM are as follows: i) High Frequency Eddy Current (HFEC) Inspection: Used to find surface cracks, porosity, and corrosion. ii) Medium Frequency Eddy Current (MFEC): Used to find subsurface cracks in the first layer that start and grow along the faying surface. It also will detect surface cracks. iii) Low Frequency Eddy Current (LFEC) Inspection: Used to find subsurface cracks and corrosion. (b) Ultrasonic: An NDT procedure that uses sound waves to find surface and subsurface damage; for example, cracks, porosity, delamination, or disbonds, on metal and composite materials that have good permeability properties. (c) Resonance Frequency: A tap test NDT procedure that can be used to find delaminations and interply disbonds in composite, honeycomb or bonded structures that have thin skin. (d) X-Ray: An NDT procedure that uses radiography to find cracks and damage; for example, disbonds, in metallic and composite structures which cannot be accessed for visual inspection. X-Rays can identify if fluids are inside honeycomb parts and can be used to identify the dimensions of the damage. Refer to NDT Part 2, for the X-Ray inspection procedures.

(e) Magnetic Particle: An NDT procedure that applies a magnetic field to a ferro-magnetic part that has fine magnetic particles on the surface. The magnetic field causes the magnetic particles to group together in areas that have cracks on or near the surface. (f) Penetrant: Penetrant examination uses the property of a liquid to go into a defect that is open at the surface of the part. The liquid is applied to the surface and permitted to soak in. A developer is applied to pull the liquid out of the defect so it can be seen. Visible penetrants are examined under white light. Fluorescent penetrants are examined under ultraviolet light.




STRUCTURAL REPAIR MANUAL (SRM) The structural repair manual is developed by the manufacturers engineering department to be used as a guideline to assist in the repair of common damage to a specific aircraft structure. It provides information for acceptable repairs of specific sections of the aircraft.

CORROSION CONTROL PROGRAMES These are intended to remain intact throughout the life of the component, as distinct from coatings, which may be renewed as a routine servicing operation. They give better adhesion for paint and most resist corrosive attack better than the metal to which they are applied. Electro-Plating There are two categories of electro-plating, which consist of:

Coatings less noble than the basic metal. Here the coating is anodic and so, if base metal is exposed, the coating will corrode in preference to the base metal. Commonly called sacrificial protection, an example is found in the cadmium (or zinc) plating of steel.

Coatings more noble (e.g. nickel or chromium on steel) than the base metal. The nobler metals do not corrode easily in air or water and are resistant to acid attack. If, however, the basic metal is exposed, it will corrode locally through electrolytic action. The attack may result in pitting corrosion of the base metal or the corrosion may spread beneath the coating.




Sprayed Metal Coatings Most metal coatings can be applied by spraying, but only aluminium and zinc are used on aircraft. Aluminium, sprayed on steel, is frequently used for high-temperature areas. The process (aluminising), produces a film about 0.1 mm (0.004 in) thick, which prevents oxidation of the underlying metal. Cladding The hot rolling of pure aluminium onto aluminium alloy (Alclad) has already been discussed, as has the problem associated with the cladding becoming damaged, exposing the core, and the resulting corrosion of the core alloy. Surface Conversion Coatings These are produced by chemical action. The treatment changes the immediate surface layer into a film of metal oxide, which has better corrosion resistance than the metal. Among those widely used on aircraft are: Anodising of aluminium alloys, by an electrolytic process,

which thickens the natural, oxide film on the aluminium. The film is hard and inert

Chromating of magnesium alloys, to produce a brown to black surface film of chromates, which form a protective layer

Passivation of zinc and cadmium by immersion in a chromate solution.

Other surface conversion coatings are produced for special purposes, notably the phosphating of steel. There are numerous proprietary processes, each known by its trade name (e.g. Bonderising, Parkerising, or Walterising).




NON-DESTRUCTIVE TESTING/INSPECTION (NDT/NDI) TECHNIQUES Among the many inspection tasks, done by aircraft serving technicians, are those involving Scheduled Maintenance Inspections (SMIs). SMI's are special inspections, detailed by the manufacturer, to be done at a specified time period. When doing these inspections the ultimate aim is to ensure that the aircraft (or part) being inspected, remains in a safe condition or that it complies with the original design specification. The common factor, in all the inspection/test procedures is that they entail techniques that do not affect the continued serviceability of the components under inspection. They are, in fact, non-destructive testing/inspection techniques. Non-destructive testing (NDT) or, in America, Non-destructive inspection (NDI) techniques, involve the use of such methods as:

Visual and Assisted Visual Inspections Remote Viewing Instruments Penetrant Flaw Detection (PFD) Magnetic Particle Flaw Detection (MPFD) Eddy Current Flaw Detection (ECFD) Ultrasonic Flaw Detection (UFD) Radiographic Flaw Detection (RFD).

It is incumbent on all aircraft servicing technicians, regardless of trade or level of certification, to be constantly vigilant and to use their eyes to detect the slightest imperfection in and around the areas of aircraft or component parts on which they are working. When approaching an aircraft, a perfunctory glance may reveal the fact that one wing is lower than the other, which could indicate a difference in the fluid levels of the respective landing gear struts, different tyre pressures or, perhaps, a deflated tyre. Missing or badly secured panels have often been discovered by such alert observations, as have potentially catastrophic structural failures, and the student is urged to adopt this vigilant attitude as quickly as possible to ensure the safety of all aircraft and the people that fly in them. While all aircraft servicing technicians can, therefore, do visual and assisted visual inspections, only those who have received appropriate training will be authorised to do certain PFD techniques. The more sophisticated MPFD, ECFD, UFD, and RFD techniques will be done by specially trained and approved NDT (NDI) technicians.




Visual/Assisted Visual Inspections The appropriate visual or assisted visual inspection techniques will be detailed in the relevant servicing manuals but, generally, they will depend on such factors as:

The nature of the item being inspected (i.e. the material from which it is made): It may be metallic, plastic, rubber or any other type of material

The purpose of the inspection: It may be to establish whether the item is suffering from a known fault or to confirm the integrity of a previous repair

The location of the item to be inspected: It may be installed in an aircraft or removed from an aircraft. In most cases the maintenance schedule will specify that an item is always inspected without removal from the aircraft. The term in-situ has previously been used to describe this instance

The inspection surface: Whether it is an internal or an external surface. The normal convention is that inspections are external unless otherwise stated

The time available for the inspection: This is often dictated by circumstances, in that, if a tyre needs to be inspected for wear, it should be able to be checked in a few minutes. A major aircraft inspection, on a large aircraft, is however, normally planned to take many days

The degree or depth of the inspection: Depending on the criticality of the component, or its adjacent structure, to the safety of the aircraft.

It should be stressed here that, whenever a visual inspection is being done, there must be adequate illumination of the inspection site, to ensure that small defects are able to be detected. Some visual inspections may dictate that a specific amount of illumination (in a stated number of lux) be available during the inspection. To assist in visual inspections, use is frequently made of such aids as:

Inspection Mirrors Magnifying Glasses.

Inspection mirrors enable the technician to see the remote surface of components and into places that normal vision is restricted. Selections of inspection mirrors are available, mounted on the end of a handle or rod. Such mirrors should be mounted by means of a universal joint so that they can be positioned at various angles. A development of this device has the ability to change the angle of the mirror by remote control. A rack and pinion mechanism passes through the stem and is controlled by a knob on the handle.




This permits a range of angles to be obtained, after insertion of the instrument into the structure. Some instruments come equipped with integral non-dazzle illumination. Magnifying glasses are most useful instruments, to assist with the close inspection of an airframe. They are capable of clarifying details, when normal visual inspection only produces a suspicion of a crack or corrosion. Magnifying glasses vary in design from the pocket type, with a magnification factor of times two (x2), to the stereoscopic type with a magnification of up to x32. The magnification factor relates to the size of an object, seen through the magnifying lens, compared with the size of the object, viewed with the naked eye, at a distance of 250 mm (10 in). For day-to-day inspection of structures, a hand instrument with a x8 magnification and integral illumination could be used. Magnification above this value should not be used unless specified, because the limited area of observation does not reveal the surrounding area. A higher magnification lens can be used, once the lower powered lens has identified a problem. Note: Magnifying glasses and similar inspection instruments will provide the best results only when the area under inspection is well illuminated.

REMOTE VIEWING INSTRUMENTS These instruments have a variety of different names, although they all, basically, operate on similar principles. Whether they are called borescopes or fibrescopes, (or, collectively, introscopes), they are optical instruments used for the inspection of the remote areas of structures, components or engines, which would be, otherwise, not directly viewable. Note: A detailed knowledge of the internal structure of the component under inspection is essential, and proper training in their use should be obtained, before inspections involving remote viewing instruments are attempted. Borescopes consist of ostensibly rigid tubes of nickel-plated brass or of stainless steel. The outer diameters of the tubes may range from approximately 5.5 mm (0.22 in) to 11 mm (0.43 in) with lengths from 230 mm (9 in) to 1 750 mm (69 in). While they do possess a degree of rigidity, they can be very easily bent if too much sideways force is applied to them, so great care must be taken in their use. Inside the thin metal tube is a complex series of precision optical lenses and mirrors, surrounded by a bundle of very fine glass fibre filaments, which guide light to the viewing end of the tube.




The light is provided by a box, containing an electrical transformer, a high-intensity, light bulb of quartz-iodine, Xenon or something similar (which is mounted in front of a reflector), and a cooling fan. The light source box is usually connected to a mains outlet and the powerful light is transmitted to the borescope by means of a connecting flexible cable which also contains a guide bundle of glass fibres. In this way cold yet brilliant light is provided at the viewing area, to give the necessary high quality illumination without the hazards associated with heat and any flammable fluids which may be present in the viewing area. Rigid borescopes are provided with several versions of viewing ends, which allow either a forward view, a lateral view (normal to the longitudinal axis of the tube), a forward oblique or a retrograde (reverse) view of the inspection area. With the exception of those with a forward view end, all the other borescopes may also have the capability of rotating the tube around the longitudinal axis, so that a full 360 internal view of the area is possible. They also have adjustable focus of the eyepiece, to minimise eye strain on the viewer and to accommodate the various levels of acuity of the inspectors eyesight. Fibrescopes are flexible and, probably because of this, they are extremely prone to abuse and damage. As the name implies, they rely on fibre optic cables rather than a rigid tube and lenses/mirrors to provide the image of the inspection area.

The image is viewed through a bundle of fibre optic strands, while the object is illuminated by light transmitted through another surrounding bundle of fibre optic strands. Diameters and lengths of fibrescopes are similar to those of rigid borescopes and they are also provided with the various viewing ends and focussing arrangements. Some fibrescopes have a controllable distal viewing end, to allow articulation through almost 360 on both an X and Y lateral axis. These (refer to Fig. 2) are most often used (in addition to borescopes) to inspect the inside of gas turbine engines, but can also be used for many other inspections such as; loose article checks, fuel leaks etc. The images, presented by borescopes and fibrescopes, may be viewed directly through an eyepiece, as stated, or they may be displayed on a TV screen via a video camera, which can be attached to the eyepiece. The results of the inspection can also be recorded, by means of a video tape, and retained, for future comparisons of possible deterioration of the inspection area.




Borescopes and Fibrescopes may be used for the inspection of gas turbine engine:

Compressors: for damage to Fans, FOD, Interference between Rotors and Stators, Surge damage, and Bearing Oil Leakage

Combustion Sections: for signs of Burning, Cracking, Distortion, and Carbon Build-up

Turbine Sections: for signs of Burning, Cracks, Dents, Deposits of Melted Metals and Nicks.

Note: When using remote viewing instruments for engine inspections it must be ensured that:

The engine must be allowed to cool down before inserting the scopes

Windmilling (or inadvertent Starting) of the engine must be prevented by gagging or removing the appropriate fuses/circuit breakers and placing warning placards on the flight deck

Contamination of the instruments, by Fuel, Grease and Oil, must be avoided

Borescopes do not get bent and Fibrescopes do not get kinked nor crushed.

Remote viewing instruments may also be used to inspect many other areas of an aircraft. Typical areas would include:

Electrical Components Electrical Looms Enclosed Structural Parts Fuel System Components Hydraulic System Components.

Wherever they are used, there are certain difficulties involved with the interpretation of what is seen through the instruments. When using remote viewing instruments, it is recommended that the inspecting technician should:

Be fully trained in the use (and care) of the instruments

being used Be familiar with the layout of the structure or component

under inspection If possible, have a spare or an example of the part near

at hand with which to compare the images from the inspection area

Use the experience of other inspectors where doubt exists (or consult previous video recordings etc.)

Refer to the appropriate servicing manual for guidance whenever necessary.




PENETRANT FLAW DETECTION (PFD) Before discussing the application of PFD techniques it is necessary to highlight the health hazards associated with working with PFD materials and to consider the recommended First Aid treatments and the Safety Precautions, which need to be observed, during their use. The hazards include:

Contact with the eyes: to prevent the possibility, chemical proof goggles should be worn. If, despite this, eye contamination occurs, then the eyes must initially be irrigated with copious amounts of water and proper medical assistance sought

Contact with the skin: due to the de-fatting action of the chemicals, barrier cream should be applied to the hands before work commences and, where prolonged contact is probable, protective PVC-type gloves should be worn. Contaminated skin should be thoroughly washed with warm soap and water and, after drying, a lanolin-based cream applied. If irritation persists then medical attention is needed

Ingestion: food must not be consumed while doing PFD procedures and hands should be carefully washed before eating. If chemicals are ingested then medical help must be sought. VOMITING SHOULD NOT BE INDUCED

Inhalation: face masks should be worn where concentrations of fumes or particles are high and there must always be adequate ventilation. Victims who become nauseous, dizzy or drowsy should be moved to fresh air and medical advice sought. Resuscitation methods should be used where asphyxiation occurs and breathing has stopped and the Emergency Services summoned.

Fire: all the necessary fire precautions must be observed (CO2 , Foam and Dry Powder extinguishers are the recommended types) and, in the event of a fire, any ventilation should be switched off first

Storage: PFD chemicals should be stored in a dry area, away from heat and direct sunlight

Spillage: any spillages should be soaked up with absorbent materials

Transport: appropriate precautions, depending on the flash point of the particular chemicals should be observed

Disposal: materials should be treated as oily waste and, where large quantities are involved, must not be discharged into public sewers or waterways.

Penetrant flaw detection may be used to detect surface-breaking discontinuities in any non-porous materials, including ceramics metals, and plastics. It may also be used to detect porosity in those materials that should not be porous, leaks in tanks and cracking of internal bores.




ULTRASONIC FLAW DETECTION (UFD) This form of Non-destructive Testing is done by specially trained, and approved, technicians, so only brief details of the background and the procedures are given in this course. The student is, however, required to have a basic knowledge of the principles of the techniques involved in Ultrasonic Flaw Detection (UFD). UFD methods may be used to detect sub-surface defects in the majority of solid materials. Ultrasonics can also be used to: Measure the thickness of materials when it is only possible

to get access to one side of the component Test for the delamination (de-bonding) of composite

structures Monitor real time cracking in spars and struts via Acoustic

Emission methods. The term, ultrasonic, describes sound oscillations at frequencies too high to be detected by the human ear. Normal, healthy adults are, usually, able to detect sound frequencies in the range between 20 Hz 20 kHz. For example, the lowest note of a typical, full-size, piano vibrates at approximately 27.5 Hz, while the highest note is in the region of 3.52 kHz. UFD procedures use sound frequencies ranging from as much as 500 kHz to 25 MHz (and, sometimes, more).

Sound is caused by the sinusoidal oscillations of the particles in a medium and the speed of sound is fixed in different materials, depending on their elasticity and density. Table 1 shows the speed of sound through some common materials. Table 1 SOUND VELOCITIES IN COMMON MATERIALS

Material m/sec ft/sec Air (at 20C) Water (at 20C) Perspex Pyrex Glass Steel Aluminium

343 1,480 2,680 5,640 5,900 6,350

1,125 4,854 8,793 18,500 19,351 20,827

Low-frequency sound travels outwards, from its source, and goes in all directions, whereas the higher the frequency, the more the sound becomes unidirectional until, at the extremely high frequencies employed in UFD, the sound can be considered to be similar to a very narrow beam of light. The principle of UFD is that a narrow beam of sound is introduced into a material and the effects on that beam can indicate the structural state of the material.




The sound beams, used in UFD, are produced (and detected) by means of a piezoelectric transducer (i.e. a device which converts electrical energy to mechanical energy and vice versa). A piezoelectric crystal (formerly quartz but, more commonly, man-made ceramics such as barium titanate or lead zirconate titanate) is made to vibrate when stimulated by electrical energy from the pulse generator of a cathode ray tube (CRT) oscilloscope.

At the same time a pulse is generated across the time base of the oscilloscope. The pulse repetition frequency (PRF) is set so that the time base of the oscilloscope appears as a straight line. When the transducer, mounted in a device known as the probe (refer to Fig. 4), is applied to a material, the vibrations cause a narrow beam of ultrasonic waves to be transmitted through the material.

Simplified UFD System Fig. 4

Probe

Controller

Time Base Controller

Pulse

Generator

Initial Pulse

X-plate

Y-plate

Back Wall

Component under

Inspection Sound Beam

and Echo

Amplifier

Probe

Back Wall Echo

Couplant between Probe and

Inspection Surface




In a similar manner to radar waves in air (and sonar waves in water) the sound waves travel through the material until they meet an interface with a medium which has a different acoustic impedance. The acoustic impedance of a material is a function of the density of, and the velocity of sound in, the material. At the interface of different acoustic impedances the sound will be reflected (as with the radar and sonar echoes) in proportion to their differences. It is usual for the majority of sound to be reflected from an interface and the interface can be caused by:

The far face (also called the back wall), of the component under inspection, with the air on the other side

A crack or a void within the material (which will contain air or another gas)

An inclusion of a foreign body within the material (such

as occurs in welds). The reflected sound (or echo) returns to the transducer probe, where the energy is converted into an electrical pulse, which is fed (via an amplifier) to the oscilloscope. The amplified pulse causes a peak on the time-base, which is calibrated so that the position of the peak represents the distance the reflected sound has travelled in the material under inspection.

Because the transducer crystal is vibrating against the casing of the probe, a great deal of sound is initially reflected within the probe. This is referred to as the initial pulse (Americans refer to it as the main bang) and it is usually placed at the extreme left of the time base, to act as the surface reference, and is not considered as part of the search beam. The face of the probe also creates an interface with the surface of the material under test, due to the microscopic particles of air between them. Because of the vast difference in the acoustic impedance of air compared to other materials, most of the sound would not enter the material, unless a medium, with a closer acoustic impedance to the probe and the material under test, is interposed between them to act as a couplant. Typical couplants used are fluids in the form of glycerine, silicon grease, petroleum jelly or medium-viscosity oils. With this pulse/echo method, the location of a discontinuity in a component can be quite accurately calculated. Unlike the PFD method, it is not only able to detect subsurface flaws but also tight surface flaws which may be filled with oil, grease, paint, rubber o

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Module 7 (Maintenance Practices) Sub Module 7.18 (Aircraft disassembly, inspection, repair and assembly techniques).pdf