EASA Part-66 Module 7 — Maintenance Practices Essay Questions

Aircraft oxygen systems are extremely hazardous because oxygen, while not itself flammable, is a powerful oxidiser that accelerates combustion violently. A material that burns slowly in normal air can burn fiercely or explosively in an oxygen-enriched atmosphere, and oils, greases and other hydrocarbons can ignite spontaneously on contact with high-pressure oxygen. For these reasons the maintenance technician must treat any work on or near an oxygen system with the same discipline applied to any other high-energy hazard, working strictly in accordance with the Aircraft Maintenance Manual and the manufacturer's data.

The first and most important precaution is absolute cleanliness. Oxygen lines, fittings, regulators and components must be kept scrupulously free of oil, grease, fuel and any combustible contamination, including that transferred from the technician's hands, gloves, clothing or tools. Only lubricants and sealants specifically approved for oxygen service may be used, and only in the quantities the maintenance data permits. The technician should ensure hands and tooling are clean and avoid wearing oily or greasy overalls. All naked flames, sparks, smoking and any source of ignition must be prohibited in the working area, and electrical equipment that could arc should be kept clear. Good ventilation is essential so that any leaking oxygen disperses rather than enriching the surrounding atmosphere or saturating clothing, which then becomes a fire risk for a considerable time afterwards. High-pressure cylinders and lines must be depressurised slowly and in the correct sequence to avoid the heat of rapid compression, valves opened gently, and the system purged and pressure-checked using the methods and values given in the AMM. Components must be capped or blanked when disconnected to keep contamination out, and only correctly rated, oxygen-clean replacement parts fitted. Cylinders themselves must be handled, secured and stored correctly, kept within their hydrostatic test period, and protected from impact and heat.

If an oxygen-related fire occurs the situation can escalate very rapidly, so the priority is to raise the alarm, evacuate personnel from the immediate area and summon the fire service. Where it can be done safely and without delay, the supply should be isolated by shutting off the system at the source to remove the oxygen feeding the fire, since cutting off the oxidiser is the most effective way to bring such a fire under control. Personnel should not attempt to fight a large or spreading oxygen-fed fire themselves but should withdraw to a safe distance. Any clothing soaked or saturated with oxygen must be removed and aired well away from ignition sources before the wearer approaches any flame. Throughout, the technician must follow the operator's emergency procedures and the manufacturer's data, and after any fire or incident the system must be inspected and made serviceable strictly in accordance with the approved maintenance documentation before further use.

Fire requires three elements simultaneously — fuel, heat and oxygen — commonly described as the fire triangle, and extinguishing a fire means removing one or more of these elements. In a hangar or workshop the technician will encounter fires of different natures, and selecting the wrong extinguishing agent can be ineffective or actively dangerous, so it is essential to match the agent to the class of fire and to follow the markings on the extinguisher and local safety procedures.

Fires are grouped into classes according to the burning material. Fires involving ordinary solid combustible materials such as wood, paper, cloth and similar carbonaceous materials form one class; flammable liquids such as fuels, oils, paints and solvents form another; flammable gases form a further class; and fires involving combustible metals, such as magnesium, form their own distinct class. Separately, fires involving live electrical equipment are treated as a special case, because the presence of electrical energy dictates that a non-conductive agent must be used to avoid the risk of electric shock to the operator.

The principal agents extinguish fires by different mechanisms. Water works mainly by cooling, removing heat from the fire, and is suitable for ordinary solid combustible material fires. However, water must never be used on flammable-liquid fires, because it can spread the burning liquid, and must never be used on live electrical equipment, because it conducts electricity. Foam works by forming a blanket over the surface of a burning liquid, smothering it by excluding oxygen and also cooling, which makes it well suited to flammable-liquid fires; like water it is conductive and is not used on live electrical equipment. Dry powder agents smother the fire and interrupt the chemical reaction of combustion, and are versatile across solid-material, liquid and gas fires, with special formulations available for combustible-metal fires. Carbon dioxide works by displacing oxygen and smothering the fire; being a non-conductive gas that leaves no residue, it is particularly suitable for fires involving live electrical and electronic equipment, and is also effective on flammable liquids. The technician must be aware that carbon dioxide can asphyxiate in confined spaces and so adequate ventilation and care are required.

Combustible-metal fires demand a specialist agent intended for that purpose, since water reacts violently with burning metals and most ordinary agents are ineffective. In every case the operator should raise the alarm and ensure personnel safety first, attack a small fire only if it is safe to do so and from a position allowing escape, aim at the base of the flames, and isolate electrical or fuel supplies where possible. Selecting and applying the correct agent for the class of fire, as marked on the extinguisher and required by the workshop's safety procedures, is fundamental to safe and effective fire-fighting.

Precision measuring tools allow the technician to determine dimensions far more accurately than a simple rule, and the micrometer and vernier caliper are two of the most widely used in aircraft maintenance. Their value depends entirely on being used correctly and kept in good condition, because an inaccurate measurement can lead to an incorrect assessment of wear, fit or serviceability. The actual readings obtained must always be compared against the limits given in the manufacturer's maintenance data or structural repair manual.

The external micrometer measures the dimension of a component held between a fixed anvil and a moving spindle. The spindle is advanced by rotating the thimble on a finely pitched screw thread; the main scale on the barrel and the scale around the thimble are read together to give the measurement, and instruments with a vernier on the barrel allow even finer resolution. To use it correctly the component and the measuring faces must be clean, the work is placed squarely between anvil and spindle, and the thimble is closed gently until light contact is felt. Critically, the ratchet or friction stop must be used to apply a consistent, light measuring force, because over-tightening springs the frame and gives a false reading. The micrometer should be read at right angles to avoid parallax error. The vernier caliper measures external, internal and depth dimensions using fixed and sliding jaws; the reading is taken by noting where the zero of the sliding vernier scale falls on the main scale and then finding the vernier graduation that aligns exactly with a main-scale graduation. The jaws must be clean and the correct jaws used for external or internal work, and the caliper read square-on to avoid parallax.

Care of these instruments is essential to preserve their accuracy. They must be kept clean and lightly protected against corrosion, handled gently and never used as clamps, levers or makeshift tools. They should be stored in their cases, kept apart from other tools to prevent damage to the measuring faces, and protected from being dropped, since a knock can distort the frame or jaws. Measuring faces should be wiped clean before use, and the instrument allowed to stabilise so that temperature differences do not distort the reading.

Because measuring tools wear and can drift out of accuracy, they must be calibrated. Calibration is the comparison of the instrument against a known reference standard whose accuracy is traceable to a recognised national standard, carried out at the interval specified by the organisation's procedures. A simple zero check is made before use by closing the micrometer or caliper and confirming it reads zero, and a micrometer may be checked against gauge blocks or its setting standard. Every controlled measuring tool carries a calibration label or identification showing it is in date; an instrument that is out of calibration, damaged or overdue must be withdrawn from service and not used until re-calibrated. Working only with in-calibration, well-maintained instruments is fundamental to reliable measurement.

When two components are assembled together, the relationship between the dimensions of the mating parts determines how they behave in service, and this relationship is described as the fit. Because no part can be manufactured to an exact size, every dimension is given a tolerance, an upper and lower limit between which the finished size must lie. The combination of the tolerances on a shaft and on the hole it enters defines the fit, and the engineer must understand these so that worn or repaired components are assembled to the condition specified in the manufacturer's maintenance data.

Fits are generally grouped into three classes. A clearance fit exists where the shaft is always smaller than the hole, so that the parts assemble freely and there is space, or clearance, between them; this is required wherever one part must rotate or slide relative to the other, such as a shaft running in a plain bearing. An interference, or force, fit exists where the shaft is always larger than the hole, so that the parts must be pressed, or shrunk, together and are held firmly without movement; this is used where a component, such as a bearing outer race or a bush, must be retained permanently in position. A transition fit lies between the two, where depending on the actual sizes within the tolerances the assembly may have a small clearance or a small interference; this gives accurate location while still allowing the parts to be assembled and dismantled. The clearance is the difference between the hole and the smaller shaft, while interference is the amount by which the shaft exceeds the hole, and the permitted values for any particular application are taken from the standard fits and clearance schedule in the maintenance data.

Checking shafts and bearings is essentially the measurement of wear against these limits. A shaft is checked using precision measuring tools such as the micrometer, taking measurements at several points along its length and at different angular positions around its circumference, because wear is rarely uniform. Comparing the measured diameters reveals general wear, ovality, where the shaft has become out of round, and taper, where the diameter varies along the length. Straightness is checked by mounting the shaft between centres or on vee-blocks and rotating it against a dial indicator to reveal any run-out or bend. Bearings are first cleaned and inspected visually for damage, then plain bearings or bushes are measured internally, often with internal micrometers or a bore gauge, and the clearance between the shaft and bearing established and compared with the schedule. Rolling-element bearings are checked by feeling for roughness, looking for pitting, spalling, brinelling, corrosion and discolouration, and by checking for excessive radial and axial play, again using a dial indicator where appropriate. In every case the measured wear and clearances are judged against the limits in the manufacturer's data, and any component outside those limits is rejected or restored to the specified condition.

The electrical wiring interconnection system, or EWIS, depends on enormous numbers of electrical connections, and the crimped joint is the standard method of attaching terminals and contacts to wires. A crimp forms a connection by deforming a metal barrel, or contact, around a stripped wire so tightly that the two are joined by cold metal flow into a gas-tight, mechanically strong and electrically continuous joint. Because the integrity of the whole system depends on each of these joints, the correct tools and process specified in the manufacturer's maintenance data must be used throughout.

Crimping is carried out with purpose-designed crimping tools rather than ordinary pliers, which cannot form a reliable joint. The most common is the hand crimping tool, a controlled-cycle tool fitted with the correct die or positioner for the particular contact and wire size. Such tools incorporate a ratchet that prevents the handles from being released until the crimp cycle is complete, so that every crimp is fully formed to the same degree, and the tools themselves are calibrated and identified to confirm they are serviceable. For larger conductors, where the force required by hand would be excessive, hydraulically operated crimping tools are used, again with the appropriate dies for the size of cable and lug being crimped. Whichever tool is used, it must be the correct one for the contact and wire combination and must be in calibration.

To make a sound joint the technician selects the correct terminal or contact for the wire size and application and the matching tool and die. The wire insulation is stripped to the correct length using a tool that does not nick or cut the strands, since damaged strands weaken the joint, and all the conductor strands are inserted fully into the barrel without any being left out or doubled back. The conductor is positioned so that the stripped length sits within the barrel with the insulation up to the correct point, and the tool is then operated through its full cycle until the ratchet releases, forming the crimp. The result should be a clean, symmetrical crimp with no missing strands, no birdcaging of the strands and the insulation correctly supported.

The completed joint is then tested to confirm its integrity. A visual inspection checks for correct strand fill, the absence of damaged or protruding strands and a properly formed crimp. A mechanical pull, or tensile, test confirms that the joint will withstand the load specified in the maintenance data without the wire pulling out, demonstrating the mechanical strength of the connection. Electrically, a continuity check confirms the joint carries current with low resistance, and a low resistance across the joint indicates a sound connection while a high or unstable resistance indicates a poor crimp. Any joint that fails inspection or test is rejected and remade, and all work is carried out and verified in accordance with the manufacturer's EWIS standards and the approved maintenance data.

The Electrical Wiring Interconnection System (EWIS) carries the power and signals on which almost every aircraft system depends, and the way that wiring is protected and routed has a direct bearing on continued airworthiness. Because chafed, overheated or contaminated wiring has been a contributory cause of in-flight smoke and fire events, the maintenance engineer must install and inspect wiring strictly in accordance with the manufacturer's maintenance data and the recognised EWIS standard practices. Routing, support, segregation and protection are therefore not cosmetic matters but safety-critical tasks.

Cable routing is planned so that wiring follows the paths shown in the wiring diagram and the installation drawing, kept clear of moving parts, control runs, hot zones such as bleed-air ducts and engine areas, and away from fluid lines so that a leaking pipe cannot drip onto a harness. Where wiring must cross a hydraulic or fuel line it is routed above it and adequately separated, and drip loops are formed so that any condensation or fluid runs away from connectors rather than into them. Segregation rules keep sensitive signal wiring, such as that for audio or sensor circuits, separated from heavy power and from cables that could induce interference, and redundant or duplicated system wiring is run on physically separated paths so that a single localised event cannot disable both channels.

Individual wires are gathered into looms or bundles. Looming, lacing and the use of self-locking cable ties hold the bundle together neatly and prevent individual wires from working loose and chafing. Lacing cord is tied with the spacing and knots called up in the maintenance data, and ties are tensioned firmly but not so tightly that they cut into the insulation. The bundle is then supported at the intervals specified in the data by clamps, usually cushioned or rubber-lined, so that vibration cannot saw the harness against structure; clamps must grip the bundle without crushing it and must be attached to approved structure, never used to carry the weight of the bundle alone.

Additional mechanical protection is provided where the environment is more severe. Conduit, which may be rigid or flexible, encloses wiring passing through areas exposed to abrasion, fluids or mechanical damage, and grommets protect wires passing through holes in structure. Heat-shrink sleeving is applied over splices, terminations and abrasion points to provide insulation, mechanical reinforcement and, in some forms, environmental sealing. Where electromagnetic interference is a concern, shielded or screened cable is used and the screen is correctly terminated and bonded.

In conclusion, sound wire protection and routing combine careful path selection, secure looming, correctly spaced cushioned clamping, conduit and grommets, and heat-shrink protection, all installed to the manufacturer's data and verified by inspection for chafing, security and cleanliness. Done correctly, these techniques preserve insulation integrity and prevent the chafing, overheating and contamination that threaten the safety of the aircraft.

Solid riveting remains a primary method of joining aircraft sheet-metal structure, and the strength and durability of a riveted joint depend on correct layout, the correct tools and proper inspection. The engineer must work to the dimensions and patterns called up in the structural repair manual or the manufacturer's data, because the spacing of the rivets and the way they are formed determine whether the joint will carry its design load without the sheets pulling apart or the rivets working loose.

In a riveted joint the rivets are arranged in rows. The pitch is the distance between adjacent rivets measured along a single row, and the gauge or transverse pitch is the distance between rows; the edge distance is the distance from the centre of a rivet to the edge of the sheet. These dimensions are specified in the maintenance data and are a compromise: rivets placed too close weaken the sheet by perforating it excessively and risk cracking between holes, while rivets placed too far apart, or too close to the edge, allow the sheets to lift between fasteners or let the material tear out at the edge. Edge distance is kept large enough that the rivet does not pull through the edge yet not so large that the joint becomes inefficient. Rivet diameter and length are selected for the combined thickness of the sheets so that a properly formed shop head is produced.

The basic tools for solid riveting are a pneumatic rivet gun fitted with the correct rivet set, and a bucking bar held against the tail to form the shop head; for production and repair work, squeeze riveters and dimpling equipment are also used. Where flush rivets are required in thin sheet the rivet seat is prepared either by countersinking thicker material or by dimpling thinner material, in which the sheet is pressed into a matching recess using a dimpling die set, often with a coin-press or hot-dimpling method to avoid cracking. Supporting tools include drills of the size specified for the rivet, deburring tools, hole-finders, cleco fasteners to hold the sheets in alignment, and rivet cutters where rivets must be trimmed to length.

Inspection of the completed joint is primarily visual and dimensional. The engineer checks that the shop head is of the correct diameter and height and is concentric with the shank, that the manufactured head sits flush or proud as required, and that flush rivets are not under-driven or over-driven. The surrounding skin is examined for cracks, distortion, smoking marks indicating a loose rivet, raised heads and gaps between the sheets. Rivets that are tipped, clinched, cracked, off-centre or otherwise unsatisfactory are marked for removal and replacement. In conclusion, a sound riveted joint results from correct spacing, pitch and edge distance, the right riveting and dimpling tools, and a careful visual and dimensional inspection against the manufacturer's data.

Rigid pipes and flexible hoses carry fuel, hydraulic fluid, oil, air and other media throughout the aircraft, frequently at high pressure, so the engineer must form, inspect, test and install them with great care. A pipe that is cracked, distorted or poorly supported can fail and cause loss of system function, fire or structural damage, so every stage of work is carried out to the manufacturer's maintenance data using the correct tools and materials.

Rigid metal pipe is bent to follow the run shown on the installation drawing. Bending is done with a pipe-bending tool or bench bender of the correct size for the tube so that the bend radius is not less than the minimum allowed by the data; bends that are too tight wrinkle or flatten the tube and reduce flow and strength. After bending, the tube end is prepared to accept its fitting. Many aircraft fittings require a flare, where the tube end is opened out to a cone and clamped between the fitting and a nut; bell-mouthing or bell-forming similarly expands the tube end to receive an inserted joint or to allow it to be brazed or fitted to a sleeve. The flare or bell must be smooth, free of cracks and splits, concentric and of the correct angle and size for the fitting.

Inspection of pipes is largely visual: the engineer looks for cracks, dents, scoring, corrosion, chafing, flattening, kinks, and damage at the flares and threads. Hoses are inspected for cracking, perishing and hardening of the outer cover, blistering, leakage, twist, and for security and condition of the end fittings; a hose installed with a twist is rejected because pressure will tend to unscrew the fitting. Identification markings and any cure or life-limit information are checked against the documentation. Testing is carried out after installation or repair by applying a proof or leak pressure as specified in the maintenance data, the assembly being checked for leaks, weeping and distortion; pneumatic systems may be leak-checked with the appropriate medium and method.

Installation requires that pipes and hoses are supported and routed correctly. Rigid pipes are clamped at the intervals given in the data using cushioned clamps that prevent vibration chafing, while leaving the small amount of flexibility the assembly needs and not over-stressing the flares. Hoses are installed with enough slack to allow for flexing and length change under pressure and to avoid sharp bends, and where a hose connects to a moving component it is positioned so that movement does not stretch, twist or rub it. Pipes and hoses are kept clear of, and separated from, electrical wiring and hot areas. In conclusion, correctly formed, thoroughly inspected, properly tested and securely clamped pipes and hoses are essential to safe system operation, and all such work follows the manufacturer's data.

Bearings allow shafts and other components to rotate or move with minimum friction while carrying load, and they are found throughout an aircraft in engines, gearboxes, controls, wheels and accessories. Because a failed bearing can seize a mechanism or shed debris into a system, the engineer must clean, inspect, lubricate and test bearings carefully and in accordance with the manufacturer's maintenance data, replacing any that are worn or damaged beyond the published limits.

Before inspection a bearing is cleaned to remove old grease, dirt and any contamination, using the cleaning fluid approved in the maintenance data and taking care not to spin a dry, ungreased bearing with compressed air, which can over-speed it and damage the elements. After cleaning, the bearing is dried and re-protected so that it does not corrode while awaiting inspection. Testing of a serviceable bearing is partly a feel test: lightly oiled, it is rotated slowly by hand and the engineer feels and listens for roughness, notchiness, catching, excessive play or noise, all of which indicate internal damage. Ball and roller bearings are checked for radial and axial play against the limits in the data, and plain bearings and bushes are checked for wear and ovality.

Inspection is mainly visual, often with magnification. The engineer examines the races, balls or rollers, and the cage for pitting, spalling, flaking, indentation, scoring, discoloration from overheating, corrosion, cracks and damage or distortion of the cage. Any of these is cause for rejection. The fit of the bearing on its shaft and in its housing is also checked, since a spun or fretted seat indicates a loose fit.

Lubrication requirements depend on the type of bearing. Most rolling-element and plain bearings must be lubricated, with the specific oil or grease, the quantity and the re-lubrication interval all stated in the maintenance data; the engineer must use only the approved lubricant, because mixing incompatible greases or using the wrong type can cause the lubricant to break down and the bearing to fail. The correct amount matters: too little gives inadequate film and accelerated wear, while too much causes churning, overheating and seal damage. Some bearings are pre-packed and sealed for life and must not be re-greased, and a few self-lubricating or dry-film bearings must be kept free of grease and solvent.

Common defects and their causes include spalling and fatigue pitting from normal fatigue or overload; brinelling, the indentation of the races, from static overload or shock such as a heavy impact or dropping; false brinelling and fretting from vibration of a stationary bearing; discoloration and softening from overheating due to lack of lubricant or over-tight fit; corrosion and water staining from moisture ingress or loss of protection; and contamination damage from dirt or debris entering past a failed seal. In conclusion, thorough cleaning, careful feel and visual inspection, correct approved lubrication and recognition of the typical defects and their causes are all required to keep aircraft bearings serviceable.

Flight and engine controls in many aircraft are operated by steel control cables, and because a control-cable failure can cause loss of control, the manufacture, inspection and testing of these cables is a critical maintenance task. End fittings such as terminals, eyes and ball ends are commonly attached to the cable by swaging, and the resulting assembly must be made and verified strictly to the manufacturer's data so that the joint develops the full strength of the cable.

Swaging is a cold-forming process in which a hollow terminal fitting is slipped over the prepared cable end and then compressed onto it, either in a rotary or roll swaging machine or in a hydraulic or hand swaging tool with the correct dies, so that the terminal grips the wires permanently. The cable must be cut squarely and inserted fully into the terminal to the depth specified, and the swaging is carried out in the sequence and to the dimensions given in the data. After swaging, the joint is checked: the finished diameter of the swaged shank is measured with a go/no-go gauge or micrometer against the limits in the maintenance data to confirm the correct amount of reduction, the fitting is examined for cracks, splits and a satisfactory surface, and the cable is checked for being fully seated and not bent or distorted at the terminal. A representative test of a swaged terminal may also be proof-loaded or a sample tensile-tested to demonstrate that the joint will not pull off below the required load.

In service, control cables are inspected primarily by visual and tactile means along their length, with particular attention to the areas that pass over pulleys, through fairleads and around quadrants where wear concentrates. The engineer wipes the cable with a cloth to detect broken wires, which snag the cloth, and inspects for individual broken or frayed strands, wear and flat spots, corrosion and rust, kinks, birdcaging where the strands separate, and contamination. The number of broken wires permitted in a given length, and the amount of wear allowed, are limited by the manufacturer's data, and a cable exceeding those limits is rejected. The end fittings, turnbuckles, pulleys, fairleads and guards are also examined for security, correct safetying and condition, and pulleys are checked for free rotation and worn grooves that would accelerate cable wear.

Testing of the installed system covers correct cable tension and rigging. Cable tension is set to the figure given in the maintenance data using a tensiometer, with allowance made for the ambient temperature because cable tension varies with temperature; the controls are then checked for full and free movement in the correct sense, for correct range of travel, and for the absence of fouling or excessive friction. In conclusion, sound control cables depend on correctly swaged and gauged end fittings, careful inspection for broken wires, wear and corrosion, and proper tensioning and rigging, all performed to the manufacturer's data.

The production of a sheet metal part begins with accurate marking out, in which the flat developed shape (the blank) is laid out on the material using the dimensions taken from the relevant drawing or the manufacturer's maintenance data. Reference datum edges and centre lines are established first, and all hole centres, bend lines and trim lines are scribed or marked relative to these datums so that errors are not compounded. The grain direction of the material is noted, because bends should where possible be made across the grain rather than along it to reduce the risk of cracking. Marking is carried out with care so as not to scratch or score the surface in areas that will remain in the finished part, since such marks act as stress raisers.

A flat blank cannot simply be folded to the outside dimensions of the finished part because the metal stretches on the outside of a bend and compresses on the inside. The neutral axis, which lies within the material and does not change length, is therefore used to calculate the bend allowance — the length of material consumed within the bent radius. The bend allowance depends on the bend radius, the bend angle and the material thickness, and is found from the formula or tables given in the manufacturer's data. Adding the flat lengths to the bend allowance gives the true developed length of the blank, ensuring the formed part finishes to the correct dimensions. A minimum bend radius is specified for each material and thickness; bending tighter than this radius risks cracking the outer fibres.

Forming is then carried out, commonly on a folding machine, press brake or by hand over a former, working the metal progressively to the required angle. The bend is made about the marked sight line so the radius forms in the correct position. Relief holes are drilled at the intersection of bends where required to prevent the metal tearing. Throughout, the part is kept clean and free of contamination, and the radius tooling is selected to suit the specified bend radius.

Inspection of the finished work confirms both dimensional accuracy and the integrity of the material. The flange dimensions, overall length and bend angle are checked against the drawing using a rule, protractor or bend gauge, and the bend radius is verified with a radius gauge. The bent area is examined visually, and if necessary with a magnifier or dye penetrant, for cracks, splits or crazing on the outer surface, and for wrinkling or buckling on the inner surface. The surface protective treatment is inspected to confirm it has not been broken. Any part that is cracked or outside the tolerances given in the data is rejected. In conclusion, sound sheet metal work depends on accurate marking out, correct calculation of bend allowance to the neutral axis, forming within the minimum bend radius, and thorough inspection of the result.

Adhesive bonding and composite repair are highly process-dependent, and the quality of the finished joint is determined as much by the conditions under which the work is done as by the materials themselves. The bond depends on the resin or adhesive wetting and chemically curing against a properly prepared surface, so the working environment must be controlled to the limits stated in the manufacturer's maintenance data or structural repair manual. The most important environmental factors are cleanliness, temperature, humidity and time. Contamination is the principal enemy of a sound bond; oil, grease, release agents, fingerprints, dust and moisture all prevent proper adhesion, so surfaces are cleaned and prepared, and the work area is kept clean and free of airborne contaminants.

Temperature must be held within the range specified for the adhesive, because it controls the working life of the mixed resin, its viscosity and the rate and completeness of cure. Humidity is equally critical: composite materials and many adhesives readily absorb moisture, and excessive moisture trapped at the bond line or within the laminate can cause poor adhesion and porosity, and can flash to steam during a heated cure causing delamination. Materials are therefore stored and handled within their controlled limits, and pre-preg materials in particular are kept frozen until use and have a limited out-time once removed. The cure itself is carried out under the specified time, temperature and pressure schedule, pressure being applied by vacuum bag, press or autoclave as required, and the cure is monitored to confirm the schedule is met. Working with these resins and their solvents also demands attention to health and safety, with appropriate ventilation and personal protection.

Inspection of bonded and composite structures must detect flaws that are frequently hidden beneath the surface. Visual inspection is the first method, looking for surface damage, cracks, dents, resin starvation, discoloration from overheating or lightning, and evidence of fluid ingress. A simple but valuable technique is the coin-tap or tap test, in which the structure is lightly tapped and the change in sound between a dull and a sharp note indicates a disbond or delamination. Beyond this, non-destructive methods are used: ultrasonic inspection is widely used to detect delaminations, disbonds and voids within the laminate and at the bond line, and other techniques such as thermography, radiography and resonance or bond-testing equipment may be specified. The repaired area may also be tested for moisture ingress where the data calls for it. In conclusion, a reliable composite or bonded repair depends on strict control of cleanliness, temperature, humidity and the cure schedule during the work, followed by visual and tap inspection backed by ultrasonic or other non-destructive methods to confirm the structure is free of hidden defects.

Aircraft are weighed to establish their actual empty weight and the corresponding centre-of-gravity position, which form the basis of all subsequent loading calculations. Because the result must be accurate and repeatable, both the preparation of the aircraft and the weighing procedure are carried out strictly in accordance with the manufacturer's weighing data. Preparation begins by establishing the correct configuration in which the aircraft is to be weighed, as defined in that data. The aircraft is cleaned inside and out so that no dirt, water or accumulated debris adds spurious weight. Loose equipment, tools and any items not part of the standard configuration are removed, while items that must be installed are confirmed present so that the result reflects the defined empty-weight condition.

Fluids are dealt with as the data directs. Fuel is normally drained, or set to a defined residual or full state, because its weight and changing arm have a large effect on the result; oil, hydraulic fluid and other operating fluids are likewise set to the specified condition. Items such as movable controls and any retractable equipment are positioned as required. Crucially, the weighing is carried out indoors in a hangar that is clean, level and free from draughts, because wind acting on the aerodynamic surfaces would falsify the readings. The weighing equipment — either load cells or platform scales, or jacks fitted with load cells — is confirmed to be in current calibration before use.

The weighing procedure itself involves supporting the whole weight of the aircraft on the scales or load cells at the defined weighing points, which are the jacking points or the wheels depending on the method. The aircraft is then levelled in both the longitudinal and lateral planes using the levelling means and reference points specified in the data, since the centre-of-gravity calculation assumes a known attitude. With the aircraft level and steady, the reaction at each weighing point is read and recorded; the sum of these reactions gives the gross weighed weight. The empty weight is then obtained by applying the tare corrections and any adjustments for the residual fluids or removed items, as listed on the weighing form.

To find the centre of gravity, the horizontal distance of each weighing point from the reference datum is measured or taken from the data. Each recorded reaction is multiplied by its arm to give a moment, the moments are summed, and the total moment is divided by the total weight to give the centre-of-gravity position as an arm from the datum. The figures are recorded on the official weighing report and the aircraft's weight and balance documentation is amended accordingly. In conclusion, accurate weighing depends on a clean, correctly configured aircraft weighed level and indoors on calibrated equipment, followed by a moment calculation about the datum to establish the empty weight and centre of gravity.

Jacking is used to raise the whole aircraft or part of it clear of the ground for maintenance such as undercarriage operation, wheel changes or weighing. Because a raised aircraft is in an inherently unstable condition, the operation must be planned and carried out carefully in accordance with the manufacturer's maintenance data, and the safety precautions are as important as the procedure itself. Before any lifting begins, the correct jacks for the task and the aircraft weight are selected and confirmed to be serviceable, in date for inspection and undamaged, and the correct jacking adapters or pads are obtained. The jacking points specified in the maintenance data are identified and used; lifting at any other point can damage the structure.

The area is prepared so that the work can be done safely. The ground must be firm and level so the jacks do not sink or topple, and the operation should be carried out indoors where possible, since wind acting on a jacked aircraft can move or destabilise it. The aircraft must be at or below its permitted jacking weight, which usually means fuel and other heavy loads are reduced as required by the data. Before lifting, the aircraft is positioned correctly, the wheels are chocked until the moment of lifting, and the people involved are briefed so the lift is coordinated. Where the data requires, tail or nose stands or a steadying weight are fitted to prevent the aircraft tipping as the centre of gravity shifts.

The procedure is to position a jack squarely under each jacking point with its adapter correctly engaged and the jack vertical. The aircraft is then raised slowly and evenly, all jacks being operated together so the aircraft stays level and no single point is overloaded; it is not lifted higher than necessary for the task. As lifting proceeds, the safety locking collars or locking nuts on the jack rams are wound down to follow the load, so that the aircraft is mechanically supported and cannot drop if a jack loses pressure. Once at the required height the locks are fully set and the stability of the assembly is checked before anyone works under the aircraft.

While the aircraft is raised, loads must not be added or removed in a way that would shift the centre of gravity unexpectedly, and personnel should not climb on or off without authorisation. Lowering reverses the process: the locks are released, the jacks are operated together, and the aircraft is brought down slowly and evenly until its weight is back on the wheels, after which the jacks are removed and the wheels are chocked. In conclusion, safe jacking depends on serviceable jacks at the correct points, a firm level draught-free area, the aircraft within weight limits, and a slow, even, coordinated lift with the jack locks kept wound down throughout.

Refuelling and defuelling involve the transfer of large quantities of flammable fuel and its vapour, so the operation is dominated by the need to control fire and explosion hazards while still delivering the correct grade and quantity of fuel cleanly. The work is carried out in accordance with the aircraft maintenance data and the airfield fuelling procedures. The principal danger is ignition of fuel vapour, so the overriding precaution is the elimination of sources of ignition and the safe dissipation of static electricity, which is generated naturally as fuel flows through hoses and into tanks.

Before fuelling, the area is prepared and a fuelling safety zone is established around the aircraft and the fuel vehicle within which no naked flames, smoking, or unauthorised electrical or radio equipment that could spark are permitted. Fire extinguishers of the correct type are positioned and ready, and an exit path for the fuel vehicle is kept clear in case it must be driven away quickly. The aircraft is normally not occupied by passengers during fuelling unless special precautions are in force, and any work likely to cause sparks is suspended. The grade of fuel is confirmed to be correct for the aircraft, as filling with the wrong grade is a serious hazard, and the fuel is confirmed to be clean and free of water by the prescribed checks.

Bonding and earthing are central to the procedure. The fuel vehicle, the aircraft and the fuelling nozzle or hose are electrically bonded together, and earthed where required, before the fuel cap or coupling is opened, so that all parts are at the same electrical potential and static cannot build up and discharge as a spark. These bonding connections are kept in place throughout and removed only after fuelling is complete. The connection is then made to the pressure-fuelling coupling or, for over-wing fuelling, the nozzle is introduced into the filler. Fuel is transferred at the controlled rate, the quantity and tank distribution being monitored against the figures required, and over-filling and spillage are avoided. Any fuel spill is dealt with immediately and the operation stopped until it is made safe.

Defuelling follows the same hazard control: it is carried out in a defined safety zone with the same bonding, earthing, fire precautions and control of ignition sources, the difference being that fuel is drawn from the tanks into a receiving vehicle, with care taken not to create low pressure that could collapse a tank or draw in air. On completion, the couplings are disconnected, the caps refitted and secured, the bonding leads removed, and the quantity transferred is recorded. In conclusion, safe refuelling and defuelling depend on a controlled safety zone free of ignition sources, correct bonding and earthing against static, confirmation of the correct clean fuel grade, controlled transfer without spillage, and fire-fighting equipment kept ready throughout.

Ground de-icing and anti-icing protect an aircraft from the hazards of frozen contamination before flight. Frost, snow or ice adhering to the wings, tailplane and control surfaces disrupts the airflow, reduces lift, increases drag and weight, and can jam control surfaces or be ingested into engines. Because of this the aircraft must satisfy the clean-aircraft concept before take-off, meaning all critical surfaces must be free of contamination. De-icing is the operation that removes existing ice, snow or frost, whereas anti-icing is the subsequent operation that provides a period of protection against further accumulation while the aircraft taxis and awaits departure. The two operations are often carried out together as a single two-step or combined procedure using approved de-icing or anti-icing fluids applied in accordance with the AMM and the fluid manufacturer's data.

The fluids used are classified into different types with different properties, and they are applied diluted and heated as specified in the approved data. The engineer must use the correct fluid type, dilution and application temperature for the prevailing conditions, and must observe the holdover time, which is the estimated period the anti-icing fluid will remain effective. The holdover time depends on the fluid, the outside air temperature and the type and rate of precipitation, and the relevant holdover tables in the manufacturer's or operator's data must be consulted; the engineer must never guess these values.

Several precautions apply during the operation. Fluid must not be sprayed directly into engine and APU intakes, exhausts, static ports, pitot probes, angle-of-attack sensors, control surface gaps or onto windscreens, since this can cause sensor errors, contamination or damage. Brakes, undercarriage micro-switches and similar areas should be protected as required by the AMM. Bleed air, air conditioning and pressurisation systems may need to be configured to prevent fumes entering the cabin, and the crew must be advised. Personnel must wear the appropriate protective equipment because the fluids are hazardous to skin and eyes, the surfaces are slippery, and the area must be controlled to keep people clear of the spray and of moving equipment. After treatment a representative surface check is carried out, and the type of fluid, mixture, and the start time of the final application must be recorded and passed to the flight crew so they can monitor the holdover time.

In conclusion, de-icing removes contamination and anti-icing prevents its return, and a clean aircraft must be confirmed before flight. By selecting the correct fluid and procedure from approved data, respecting holdover times, protecting sensitive areas and observing personnel and environmental safety, the engineer ensures the aircraft departs free of the frozen contamination that would otherwise seriously degrade its airworthiness.

Corrosion is the chemical or electrochemical deterioration of a metal by reaction with its environment, and it is one of the most common defects an engineer must manage on an aircraft structure. It is encouraged by moisture, dissimilar metals in contact, trapped contaminants, salt-laden or industrial atmospheres, and damaged protective coatings. Corrosion appears in several forms, including general surface corrosion, pitting, intergranular and the more serious exfoliation corrosion, filiform under paint, galvanic corrosion between dissimilar metals, crevice corrosion in joints, and stress corrosion. Because corrosion weakens structure, can act as the origin of fatigue cracking, and may be hidden in lap joints, fastener holes and faying surfaces, it must be found and dealt with promptly.

Visual inspection is the primary and most widely used technique for detecting defects, and corrosion is often first revealed by it. The engineer looks for blistering, lifting or flaking of paint, powdery or coloured deposits, staining, pitting and surface roughness, using good lighting and, where helpful, magnification, a mirror and a borescope for hidden areas. Visual inspection also reveals other defect types such as cracks, dents, scores, distortion and fretting, and it commonly precedes more sensitive non-destructive methods that confirm the extent of a suspected defect.

Once corrosion is found it must be removed, assessed and re-protected strictly in accordance with the structural repair manual and the AMM. Removal is carried out by approved means such as abrasion or careful blending, taking care not to remove sound material or to introduce new stress raisers, and the removed area is cleaned of all corrosion products. Assessment then determines whether the remaining material is within limits: the depth and area of material removed are measured and compared against the negligible-damage and allowable-damage limits in the approved data. If the damage exceeds those limits the part must be repaired or replaced rather than simply blended out, and the decision is recorded.

Re-protection restores the corrosion-protective scheme so the area does not deteriorate again. This generally involves restoring any surface treatment, applying primer and the specified paint or coating system, and reinstating sealant, corrosion-inhibiting compounds and bonding as called for in the approved data. The correct materials, surface preparation, cure conditions and dissimilar-metal precautions must be observed.

In conclusion, corrosion control is a cycle of detection, removal within limits, assessment against approved data, and re-protection. Visual inspection underpins the process as the first line of detection, supported by other inspection methods, and at every stage the engineer works to the SRM and AMM, records the findings and the action taken, and ensures the structure is returned to a properly protected and airworthy condition.

Non-destructive inspection allows defects to be found without damaging or dismantling the component, and several methods are available, each suited to particular materials and defect types. Dye penetrant, eddy current and ultrasonic inspection are three commonly used methods, and selecting the right one depends on the material, the type and location of the suspected defect, and the access available. All require trained and qualified personnel and must be carried out in accordance with the AMM and the relevant NDI procedure.

Dye penetrant inspection detects defects that are open to the surface. A penetrant liquid is applied and drawn into surface-breaking flaws by capillary action; after a dwell time the surplus is removed and a developer is applied that draws the penetrant back out to show the defect as an indication, often viewed under ultraviolet light when a fluorescent penetrant is used. Its advantages are that it is simple, inexpensive, can cover large areas and works on a wide range of non-porous materials including non-magnetic metals and many non-metals. Its main limitation is that it only finds defects that break the surface; it cannot detect subsurface flaws, the surface must be clean and free of paint, porous materials cannot be inspected reliably, and careful cleaning and process control are needed.

Eddy current inspection uses an alternating-current coil to induce eddy currents in a conductive material; a crack or change in the material disturbs these currents and alters the coil's impedance, which the instrument detects. It is excellent for finding surface and slightly subsurface cracks, for inspecting fastener holes, and for sorting materials or measuring coating and conductivity. Its advantages are speed, sensitivity to small surface cracks, the ability to inspect through thin coatings without paint removal, and no consumables. Its limitations are that it works only on electrically conductive materials, penetration is shallow, it is sensitive to standoff, edges and geometry, and interpreting the signal requires skill and reference standards.

Ultrasonic inspection passes high-frequency sound into the material through a probe and a couplant; reflections from the back wall and from internal defects are timed and displayed, allowing the depth and size of a flaw to be estimated. Its advantages are that it can detect deep subsurface defects, gives good depth information, needs access to only one side, and can also measure thickness. Its limitations are that it requires a couplant and skilled operators, calibration against reference blocks is essential, complex shapes and coarse-grained materials are difficult, and very near-surface defects can be masked.

In conclusion, dye penetrant is the simple choice for surface-breaking defects, eddy current excels at surface cracks in conductors and around fastener holes, and ultrasonic reaches subsurface flaws and gives depth information. The engineer chooses the method to match the material and defect, and always works to qualified procedures and approved data.

A lightning strike passes a very large electrical current through the airframe, which normally enters at one point and exits at another while the aircraft is in flight. High-intensity radiated fields, or HIRF, are strong external electromagnetic fields from sources such as radar and transmitters. Both can damage structure and, more subtly, affect electrical and electronic systems, so a reported encounter is treated as an abnormal event requiring a special inspection before the aircraft is released to service. The inspection is necessary because a strike can burn, pit or melt structure at the attachment points, damage bonding and lightning-protection provisions, and induce transients that may have degraded avionics, leaving hazards that are not obvious to the crew.

Following any report of a strike or HIRF penetration the engineer must consult the AMM, which contains the conditional or special inspection requirements for the event; the inspection is carried out in accordance with that approved data and the findings are recorded. The starting point is to establish what happened from the crew report, including any systems anomalies noted in flight and any indications recorded by the aircraft.

The structural inspection concentrates first on the likely entry and exit points, which are typically the extremities such as the nose, radome, wing tips, tail, elevator and stabiliser tips, trailing edges, and protruding items like static dischargers and antennas. The engineer looks for burn marks, pitting, melting, small holes, discolouration and arcing damage, paying particular attention to composite areas, the radome and other non-metallic structure where the embedded lightning-protection mesh or foil and the bonding can be damaged. Control surface hinges, bearings and bonding jumpers are checked because current can arc across them. Static dischargers and antennas are inspected and the bonding and earth-continuity of affected areas is checked against the AMM.

Because lightning and HIRF can also affect systems, the electrical and avionic systems indicated by the data and by the crew report are checked, including operational checks of navigation, communication and other affected equipment, and the magnetic compass may need to be checked for any change. Fuel system components and tank areas near possible attachment points warrant particular care because of the ignition hazard, and these are inspected as specified.

In conclusion, a lightning strike or HIRF penetration demands a structured special inspection driven by the AMM, covering the probable entry and exit points, bonding and lightning-protection provisions, control surfaces and protruding items, and the electrical and avionic systems. By following the approved data, inspecting thoroughly, recording the findings and rectifying any damage before release, the engineer ensures that no hidden structural or system damage remains to compromise airworthiness.

Documentation and communication are central to safe maintenance because work is frequently shared between several engineers and across shifts, and a clear record is the means by which one person reliably passes accurate information to another. A well written maintenance report records what was found and what was done so that the work can be understood, certified, traced and, if necessary, investigated later. Poor reporting and poor handover have been factors in maintenance errors, so both must be carried out to a high standard.

A written maintenance report should be factual, accurate, legible, unambiguous and complete. It should clearly identify the aircraft and the component concerned, the date and the person making the entry. It should describe the task or the defect that was reported, the findings of any inspection or troubleshooting, the cause where this has been established, and the action taken to rectify the defect, including reference to the approved data used such as the AMM, the SRM or the relevant manual section. Where parts have been replaced these should be identified, and any tests or functional checks carried out, together with their results, should be recorded. The report should make clear the status of the task, that is whether it is complete or outstanding, and any work that remains to be done, any deferred defects and any special conditions. Only standard, recognised terminology and approved abbreviations should be used, and the entry should ultimately support the certification of the work by an authorised person.

Effective shift and task handover communication ensures that information is not lost when responsibility passes from one person or shift to another. The principles are that the handover should be timely, structured and two-way, so that the incoming engineer not only receives the information but also confirms understanding and can ask questions. It should rely on the written record rather than memory alone, with verbal explanation supporting the documentation. The handover should make absolutely clear the state of every open task: what has been done, what remains, the condition the aircraft is in, whether systems are safe or disturbed, fasteners left out, panels open, jacks or safety devices fitted, and any defects deferred or outstanding. Anything that affects safety, such as work left incomplete or systems made inoperative, must be highlighted so it cannot be overlooked. Clear language, a shared understanding of the aircraft's configuration, and a positive confirmation of acceptance all reduce the risk of error.

In conclusion, a maintenance report must be a complete and accurate record of the task, the findings and the action taken, supporting certification and traceability, while shift and task handover must be a structured, documented and confirmed exchange that leaves the incoming engineer in no doubt about the state of the work. Together they guard against the loss or distortion of information that can lead to maintenance error.