Mod 7 Maint Pract B1

(1)

1. SAFETY PRECAUTIONS

Aviation engineers frequently work in potentially dangerous environments. Virtually every aspect of aircraft maintenance can be potentially hazardous. It is obvious that engineers must be trained to be aware of these potential dangers so that precautions can be taken to minimise them. Each part of your training will emphasise particular hazards associated with the subject. In this section we will look at the particular care that should be taken when working with compressed gasses, electricity oils and chemicals. We shall also consider the safety precautions and procedures relevant to fire in the workplace.

1.1 COMPRESSED GAS

Compressed gases are in common use in aviation. They are required during normal day to day aircraft maintenance. Nitrogen, Carbon Dioxide and Oxygen are all usually present on the flight line.

1.1.1PURPOSE OF THE GASES

Nitrogen is used for aircraft tyre inflation, aircraft hydraulic system accumulators, fuel tank inhibiting and shock strut inflation.

Carbon Dioxide is used in fire extinguishers and for life jacket and other safety equipment inflation bottles.

Oxygen is used for aircraft emergency breathing for aircrew and passengers. Acetylene is used in gas welding equipment.

1.1.2GAS CYLINDER IDENTIFICATION

It is vital that a gas cylinder must be positively identified to prevent possible disastrous results of charging a system or component with the wrong gas. In the past, the accepted practice was to paint the cylinder in a distinctive colour and also to paint the name of the gas on the cylinder in letters of a contrasting colour. In the UK, gas cylinders are normally supplied by The British Oxygen Company (B.O.C.). The cylinders are colour coded in accordance with British Standard 381 C, but it is no longer compulsory for the suppliers or users of compressed gases to follow it's requirements. The only positive method of identifying the contents of a gas cylinder is to read a label on the neck of the cylinder, showing the cylinder contents, the gas pressure and any special safety requirements. It is compulsory for this label to be attached to the cylinder during transportation of the cylinder. If colour coding is used, the normal convention in the UK is as follows.

Nitrogen Colour Light Grey with Black neck

Lettering - Nitrogen in BlacK

Use – Charging aircraft accumulators, tyres, shock absorbers, Oxygen

Colour - Black with White neck Lettering - Oxygen in White

Use - Aircrew & Passenger breathing Carbon Dioxide Colour – Black

(10)

(11)

1.1.3SAFETY PRECAUTIONS

The storage or “transport” cylinders supplied by BOC are large (approximately 6ft long) and contain gas at a pressure of 4,000 - 6,000 pounds per square inch (p.s.i.). Extreme care must be taken when working with gas at this pressure. If the bottles are dropped or damaged they could explode or propel the cylinder at high velocity like a rocket projectile. Gas at pressure as low as 100 p.s.i. can inject into the skin and cause serious, even fatal injuries. Some gasses support combustion and will make fires burn much more fiercely. Oxygen is particularly dangerous as it is also capable of causing explosions when in contact with oils or greases. Oxygen safety precautions will be dealt with in more detail in module

11.

1.1.4CHARGING RIGS

Aircraft gas cylinders contain gas at a much lower pressure and so the gas is decanted from the larger “transport” cylinders. A charging trolley is often used, this being generally a towed trolley with one, two or even four high-pressure gas cylinders, a flexible supply hose, a supply shut-off valve, and pressure gauges showing supply pressure and storage cylinder pressure. Some rigs are also fitted with a pressure regulator, by means of which the supply pressure can be limited to the maximum required by the component or system. Alternatively a fixed charging rig may be used.

1.1.5CASCADE CHARGING

This is a procedure that should be adopted when gas charging to avoid wastage of gas. If not used, the result could be a set of four gas bottles, each with a substantial amount of gas at slightly lower pressure than the maximum system pressure. In this process fully charged cylinders in a set, are not used for the initial part of a charge. Partially exhausted cylinders are used initially and higher pressure cylinders to complete the process. Example: A large capacity system needs to be charged to 2,000 p.s.i. The current pressure is 500 p.s.i. There are four gas bottles on the charging trolley have pressures of 3,500, 1,800, 1,500 and 1,000 p.s.i.. You might be tempted to connect the bottle with 3,500 p.s.i. to the system and charge it with that one only. Cascade charging saves gas, first charging from the 1,000 p.s.i. gas bottle, then the 1,500 p.s.i. gas bottle and so on until the aircraft system is at 2,000 p.s.i. conserving gas for more charges. 1.1.6BEFORE USE CHECKS

(12)

• Ensure the cylinders contain enough pressure for the charge.

• Make sure the delivery hose is in good condition and clean.

• If Oxygen gas is being charged, there should be no oil or grease around the charging connections or the charging rig.

1.1.7AIRCRAFT COMPRESSED GAS CHARGING

Any system or component containing compressed gas must be handled and serviced carefully, because the sudden release of gas under pressure could have disastrous consequences. Oxygen systems are an additional hazard in that the gas supports combustion and that oil and grease are prone to spontaneous combustion in the presence of undiluted oxygen. The gas pressure in some components varies according to the ambient temperature, and in order to ensure that the correct pressure is maintained, the relationship between temperature and pressure is generally presented in the form of a graph, both in the Maintenance Manual and on a placard adjacent to the charging point. In the case of tyres or shock absorbers on larger aircraft, the required gas pressure may vary according to the aircraft weight. Since rapid compression of a gas results in an increased temperature, gas pressure will also increase. On cooling down, the pressure will drop and may result in an inaccurate reading. This effect can be minimised by charging slowly. A sudden release of gas produces the reverse affect i.e. lowering the temperature. This is particularly important when deflating a tyre, as ice may form and block the valve, giving the impression that the tyre is fully deflated when it may be partially inflated. This may prove disastrous if the next step was to attempt to dismantle the wheel. Prior to work on any unit from which the gas has been exhausted, the charging valve should be completely removed. 1.1.8AIRCRAFT GAS CHARGING VALVES

These may be of two types. One is a needle type valve that opens and closes automatically when pressure is applied or released (Schrader valve). This type of valve is identical to the valves used in car or bicycle wheels. The other type of valve has a nut which must be unscrewed partially before the gas may be

released. In both types, a valve cap should always be fitted to prevent entry of dirt and moisture. The cap should be removed when the system requires charging. The cap may be attached by a chain, thus preventing it from being lost. On no account should the valve body be unscrewed while the system or component is pressurised, since this could result in the valve blowing out, causing damage or injury.

Charging Panel Charging Valves

A typical aircraft gas charging panel will comprise a charging valve and pressure gauge. There is sometimes a temperature graph to show how the pressure varies with temperature.

(13)

1.1.9TYPICAL GAS CHARGING PRECAUTIONS

Charging a component with compressed gas should be carried out carefully observing the following precautions:

• The charging pressure should be checked from the maintenance manual. Also make sure of the pressure units. Most UK engineers are familiar with pounds per square inch (p.s.i.), but some gauges are calibrated in other units such as bars (a bar is approximately 15 p.s.i.). Consideration should also be given to the ambient temperature and that the environmental conditions will not contaminate the system conditions (rain, snow or dust).

• The supply connection (charging hose) should be clean, dry and free from oil or grease; any contamination should be wiped off with a lint-free cloth. This is vitally important when charging oxygen.

• The same care should be taken to ensure the system charging point is clean, after removing the blanking cap.

• Generally the charging hose should be purged, by allowing gas to escape at low pressure from the hose, prior to connection. This ensures there are no foreign bodies or moisture in the hose. Again this is vital in the case of oxygen charging.

• The aircraft system should be charged slowly, so as to minimise the rise in temperature.

• When the required pressure is reached, the shut off valve should be closed and the system pressure allowed to stabilise after cooling down.

• The pressure should be re-checked and adjusted as necessary.

• The supply hose should not be disconnected unless the shut-off valve and the charging valve on the charging rig are closed. On some rigs provision is also made for relieving pressure from the supply hose before disconnection.

• Blanking caps should always be fitted to the charging valve and the supply hose after disconnection.

• When charging oxygen systems, adequate and properly manned fire-fighting equipment should be positioned, and if illumination is required, it should be explosion proof.

1.2 ELECTRIC SHOCK

This is an obvious occupation hazard for both avionic and mechanical aircraft engineers. Much of the systems and maintenance equipment is electrically powered. The main dangers associated with use of electricity are:

• Electric shock which may be fatal.

• Arcing caused by inadequate insulation. This could lead to a fire.

• Overheating which again could lead to a fire.

Most of the personal dangers can be prevented by following a few simple rules:

• Wear the correct clothing. Personal jewellery, especially rings and metal strapped watches should not be worn as they may get caught in machinery or act as a conductor.

(14)

• Ensure that all interlocks and other safety devices are serviceable and not tampered with or over-ridden.

• Do not work on equipment that is switched on. Operate or remove the appropriate circuit protection devices (circuit breakers or fuses).

• Always switch off power before replacing components.

• If using machines that have emergency stop buttons, ensure all personnel know their locations.

• Where possible, ensure a second person is present in case of an accident.

1.3 FIRE PRECAUTIONS

Fire is the product of a chemical reaction in which fuel mixes with oxygen and releases heat and light. Three things are required before a fire can occur:

• There must be a Fuel

• Oxygen must be present (or air, which contains oxygen)

• The temperature must be raised high enough for the fuel and oxygen to combine.

To extinguish a fire, you must either cool it or exclude the oxygen.

Fire is probably the most dangerous of the hazards associated with aircraft maintenance. Aircraft carry large quantities of fuel and other combustible materials. There is also a large amount of electrical equipment on aircraft, so there is a high risk of fire.

1.3.1CLASSIFICATION OF FIRES

Fires are classified into four categories. Extinguishers suited for each classification of fire are marked with the classification letter as shown in the following table:

Fire Classes Letter Designation

Ordinary combustibles - paper, cloth, wood A

Flammable liquids – Fuel, Oil B

Energised electrical equipment C

Combustible metals – Brake units D

1.3.2FIRE EXTINGUISHING AGENTS

Fire extinguishing agents should be selected appropriate to the type of fire on which they are effective.

• Class A - fires with such fuels as paper, wood or cloth (often called solid fuel), can be extinguished with a water spray. This cools the fuel to a temperature below that at which it can burn.

• Class B - fires are best put out with an extinguisher that excludes the oxygen from the burning fuel. Dry powder agents break down in the presence of heat to produce carbon dioxide that displaces the oxygen. Carbon Dioxide

extinguishers displace the oxygen directly. Foam is also used, which blankets the fire and excludes the oxygen. Water should not be used because the burning fuel will float on top of the water.

(15)

• Class C - fires should be treated carefully because of the risk of contact with high voltages. Water should definitely not be used as it will conduct

electricity. Dry powder would be effective, but it is not the best choice as it leaves a sticky residue that makes cleanup difficult. Carbon dioxide is very effective when sprayed via a non-metallic horn. The best extinguishers are halogenated hydrocarbons or halons.

• Class D - fires should never have water sprayed on them as it intensifies the fire and may cause an explosion. Dry powder is the best choice for

extinguishing metal fires.

1.3.3FIRE EXTINGUISHER IDENTIFICATION

The extinguishers should be clearly marked with the appropriate class letter symbol.

Many extinguishers in current use are colour coded to indicate the type of extinguisher. The old colours are as follows:

• Water Gas Red

• Carbon Dioxide (CO2₎ _Black

• Foam Cream

• Dry Powder Blue

Fire extinguishers used in workshops and hangars should now be coloured Red. It is however, unlikely that everyone will be using the new colour cylinders for a long time, so be aware of the old codes. Note the fire extinguishers pictured above use the colour coding.

1.3.3.1 Water Gas Fire Extinguishers

These contain water, anti-freeze and a carbon dioxide bottle. When the carbon dioxide gas is released, the water is ejected through a nozzle so that the

(16)

1.3.3.2 Carbon Dioxide (CO2_{) Extinguishers}

CO2_{is an inert gas that is stored in a cylinder under pressure. When it is}

released it expands and it’s temperature drops. It blankets the fire and excludes oxygen, so the fire is extinguished. It is available in various sizes from small hand held units to larger trolley mounted units. The state of charge is normally determined by weighing the cylinder and comparing it’s weight with the weight stamped on the cylinder neck. This extinguisher is most suitable for use on flight lines for engine starting, fuelling and general use. May be available complete with various length hoses and application nozzles for external use on a/c engines.

1.3.3.3 Dry Powder Extinguishers

Dry powder agents such as bicarbonate of soda, ammonium phosphate and potassium bicarbonate are effective against class B, C and D fires. When the agent is heated by the fire, carbon dioxide is released which excludes oxygen from the fire. The dry powder is propelled from the cylinder by a charge of compressed nitrogen. These extinguishers are particularly effective on brake fires, because they do not cool the brakes as would CO2_{, foam or water gas.}

1.3.3.4 Foam Fire Extinguishers

Foam extinguishers are particularly effective for liquids such as fuel or oil fires. Two chemicals are stored separately within a cylinder. When these chemicals are mixed, a large volume of foam under pressure is produced. This foam, when directed onto the burning liquid, blankets the fire and starves it of oxygen. Should not be used for electrical fires.

1.3.3.5 Fire Blanket

Stored in a RED cylindrical container. Usually asbestos or some other good insulator. As it's name suggests, it may be used to blanket the flames. 1.3.4GENERAL PRECAUTIONS

The following general precautions should be observed to minimise the risk of fires and their affect:

• Smoke only in designated areas.

• Observe and obey No Smoking signs on flight lines.

• Do not carry matches or any other source of combustion.

• Do not wear studded or steel tipped footwear.

• All flammable liquids such as paint, dope, hydraulic fluid etc. should be stored in an approved store outside the hangar.

• Supervisors should ensure that all reasonable fire safety precautions are taken and all fire apparatus is serviceable.

• Personnel engaged in maintenance should be fully conversant with the use and operation of fire protection equipment. They should also know the action to be taken in the event of a fire i.e. escape routes, fire alarms, position of fire appliances and assembly points.

• When fuelling a/c electric's should not be switched on or off.

• Aircraft should always be bonded when being worked on.

• When fuel tanks are empty there is probably a greater risk of fire than when they are full.

(17)

1.3.5PROCEDURE ON DISCOVERING A FIRE IN THE WORK-PLACE • Shout "Fire" Loudly.

• Operate the nearest fire alarm or get someone else to.

• Attempt to extinguish the fire with the nearest suitable fire appliance. Do not attempt this if your actions will endanger your own life or your chance of escape.

• Ensure fire brigade is called. Give location of fire i.e. Building and position in building, also type of fire, Fuel or Electrical etc.

• Close all doors and windows if possible (Reduce fire spreading)

• Proceed to assembly point.

1.3.6ACTION TO BE TAKEN IN THE EVENT OF ENGINE FIRES

• Aircraft engines are mostly susceptible to fires on start-up. The following points will minimise the risk of damage due to an engine fire.

• Always have a fire extinguisher of the correct type available prior to starting the engine. A CO2_{extinguisher should be close to hand for each engine}

start.

• A safety person should be available, conversant with the operation of the fire appliance and aircraft procedures.

• In the event of a fire, the fuel supply and ignition should be turned off before attempting to extinguish the flames.

• If possible see if the fire stops after fuel and ignition is cut. If not, apply extinguisher agent via the fire access panels, do not run engines with cowlings open or removed.

1.3.7ACTION TO BE TAKEN IN THE EVENT OF BRAKE FIRES

Brake Fires occur mainly due to overheating after a heavy landing or excess operation of the brakes. They may also be a result of a hydraulic fluid leak onto a hot brake. A brake unit may not catch fire immediately after an incident. The unit may burst into flames a long time afterward a landing. Care should be taken approaching a wheel or brake unit. Never approach in the direction of the axle, always approach in line with the tyre i.e. from the front or rear of the aircraft. Only attempt to extinguish a brake unit if it is on fire. If it is only overheated it is best left alone to cool. A Dry Powder extinguisher is the most effective as it does not rapidly cool the unit. If a dry powder extinguisher is not available, a CO2_or

Foam extinguisher can be used by application of the extinguisher agent onto the GROUND near to the unit. This will allow the agent to warm up before coming into contact with the brake unit.

1.4 THE NEED FOR SAFETY

It is fairly obvious from the previous comments that a maintenance engineer needs to be both knowledgeable concerning the safety requirements and alert when working around aircraft. Various other factors will also have an effect on the level of safety. Human factors such as noise, lighting, fatigue and work pressures

(18)

1.5 WORKING AROUND AIRCRAFT

Many aspects of working on aircraft will be unsafe if the correct safety

precautions are not observed. Even walking around aircraft will be dangerous if you are not aware of the dangers. Typical dangers will be as follows:

• Sharp objects such as probes, wing-tips, propellers, aerials

• Working around engine intakes and exhausts is particularly dangerous (often fatal) when the engines are running.

• Working around propellers especially when rotating.

• Damage to ears from constant exposure to noise.

• High pressure gases can cause explosions.

• Working with many tools, especially power tools.

• Working around electricity in general.

• Hydraulically operated controls or other systems.

• Dangers due to risk of fire.

This list could be extended considerably. The safety aspect of working around aircraft should be emphasised at all times. Engineers tend to become over-confident as experience increases. They should be alert at all times to the possible dangers. Anyone who has been in the aviation maintenance business for a reasonable time will be able to recount at least one instance of a serious injury or fatality due to a safety related incident. Ask your lecturer!

(19)

(20)

2. WORKSHOP PRACTICES

2.1 CARE & USE OF TOOLS

In order to perform his duties competently and speedily, the Licensed Aircraft Maintenance Engineer needs to provide himself with an adequate tool kit,

maintain it properly and add to it as he progresses from one aircraft to another in the pursuance of his career. It is obvious, therefore, that knowledge of tools is an essential part of his overall field of learning. In this topic we shall consider some aspects of the provision and safe keeping of both personal tools and some special tools. The provision of special tools is usually undertaken by the

organisation for whom the engineer works, but their proper use and safe keeping is very much the responsibility of those who use them.

The care of tools, their correct usage and safe keeping is an aspect of the

engineers work which must be approached with the same degree of responsibility as all other facets of his work. Worn tools, e.g. spanners with spread jaws, screwdrivers with incorrectly ground blades etc. will damage the equipment on which they are being used, as well as risking injury to the user. To minimise the risk of loose articles being left on aircraft, many engineering organisations now use 'Shadow Boards' for tool storage. A black wooden board carries painted silhouettes of all the tools attached by spring clips to that particular board. At the end of a particular period, a brief glance will show which tools are still in use of have not been returned to their storage. This method has contributed very effectively to a reduction in the number of accidents due to loose tools left in aircraft.

Despite some organisations using shadow boards, many only use them for specialist tools therefore in many companies the mechanic / technician will be expected to supply and control his own personal tool kit.

2.2 USE OF MATERIALS

Many different materials are used on aircraft and most of them need to be approved for aircraft use. A few examples of the different materials are:

• Sheet metal, rivets and fasteners for repairs

• Adhesives, sealants and jointing componds.

• Cleaning materials, these may be water based or solvent based.

• Painting materials – etch primers, thinners, paint and paint removers.

• Fuels, engine oil and hydraulic fluid.

• Fluids for a variety of purposes including acids, alkaline fluids.

These and many more will be discussed during the rest of the course. It is most important for you to realise that many of the materials need special care to avoid both damage and injury. The maintenance or repair manuals will always specify the recommended material for a specific task. Sometimes an alternative will be identified, but if not so identified the recommended material must be used. Each of the materials will normally be identified by a part number or identification code. This code number may be a manufacturers code or an internationally standard code. For example many aircraft sheet metal skins are made from an aluminium alloy called durallumin. This may be coded 2017, 2117 or 2024, each being a slightly different specification.

(21)

2.3 DIMENSIONS

One of the main tasks an engineer has to perform is to identify if the aircraft conforms to its design specifications. Much of the maintenance work involves carrying out some form of inspection. This will often involve measuring to check if dimensions are correct.

An engineer will be required to take measurements in a variety of different circumstances, using a variety of measuring devices. The following list gives some of the situations where a measurement may be made:

• Measuring tyre tread depth to ascertain if tread wear is excessive

• Checking the up and down movement of a control surface – this may involve measurement of an angle or a dimension

• Measurement of thickness of brake pads

• Determining the dimensions of damage to aircraft structures

• Measurement of the overall length of an electrical actuator

• Measurement of the volume of fuel during a fuel flow check

• Accurate measurement of the dimensions of a hydraulic cylinder

In each of the previous cases a different method of measurement may be used. In the first example, a tyre depth gauge might be used. In the second the measurement might be carried out with a steel rule or a special tool supplied by the aircraft manufacturer.

Accuracy of Dimensions

As well as using different types of measuring device, the measurements may need to be carried out to a greater degree of accuracy. In all cases it is true to say the dimension cannot be measured exactly. It is only possible to measure to the accuracy of the measuring device used. As well as this, the measuring device will not be totally accurate.

The scale of the rule shown is in millimetres, with the smallest sub-division representing 5mm. The line A is between 30mm and 35mm. You should not estimate the value of A as 33mm (or 34mm). Its value can only be accurately stated as 30mm. If you need to measure more accurately, you need to use a more accurate measuring device such as a vernier caliper.

Another way of giving a false indication of the accuracy of a measurement or dimension is to specify too many decimal places in your measurement. For example, if you measure a dimension of 4inches with a rule calibrated in eight’s of an inch, you might be tempted to state the dimension as 4.125” as this is the

(22)

2.4 ALLOWANCES & TOLERANCE

When components are manufactured, it is impossible for them to be

manufactured to exact dimensions. Part of the reason for this is much the same as we have already stated. The best accuracy we can achieve is dictated by the accuracy of our measuring devices. The ability of a machine to produce identical parts also comes into play. A cutting tool will wear and so will produce slightly different parts each time. If a part is rolled or extruded, the rollers or die will not produce the same results each time. It is essential that components are

interchangeable so that they may fit together. The parts are therefore made to a specified limit so that each may be slightly smaller or larger than the stated “nominal” size. A tolerance is the permitted variation tolerated and is a measure of the accuracy or standard of workmanship. If for example a part should be 25mm in diameter (nominal size), it may be considered acceptable if it is within the limits 25.02mm (high limit) and 24.98mm (low limit). The difference between the two limits is the tolerance, in this case 0.04mm. It is more difficult (and more expensive) to produce items with very small tolerances. We often use the term close tolerance in this case. Aircraft components are usually manufactured to closer tolerances than in other engineering applications.

The allowance is considered when we have two mating parts such as a shaft and a hole. The shaft is obviously designed to fit into a hole. Each will have a high and a low limit. The allowance is the difference between the high limit of the shaft and the low limit of the hole.

2.5 CALIBRATION OF TOOLS & EQUIPMENT

Gauges and precision measuring instruments need to be checked against a Standard Value on a periodic basis to ensure accuracy within a given range. If a particular measuring device is designed to be accurate to say 0.001”, it will not give the required accuracy if care is not taken when it is used. It is also common practice to check it every time it is used to confirm it’s accuracy. A micrometer would, for example be checked for its zero ready every time it is used. It is not always essential for the device to give the exact value as long as it is known how inaccurate the device is. Precision gauges should normally be checked and re-calibrated at least every six months.

Torque Wrench Calibration Gauge

(23)

• Micrometers – both external and internal

• Vernier measuring tools

• Tyre pressure gauges

• Torque wrenches

• Cable tensiometers

• Electrical measuring gauges such as multi-meters

• Specialised Non-Destructive equipment

• Avionic Test equipment

When calibrated, it is necessary to keep a record to ensure that it is known when the equipment will need re-calibration. Where necessary it should be identified how accurate the equipment is over the complete measuring range. Sometimes a chart will indicate how much the instrument varies from the stated value over the complete measuring range.

(24)

3. TOOLS

3.1 COMMON HAND TOOLS

A good aircraft engineer will most probably have a very extensive (and

expensive) tool kit. Initially the toolkit will be small and the engineer will need to be selective about the number of tools bought and their quality. The engineer will need to be familiar with many different types of tools. Other than a basic

knowledge pf the different types of tool and their use, it is necessary to describe or “classify” tools. By this we mean how to identify the different types of a tool. For example there are many different types of screwdriver. They differ both in the type of screws they are used on and in the size of the screwdriver. Most tools are available in a variety of sizes and types. At the very least the engineer will need to be able to describe the tools when it comes to buying them.

(25)

Screwdrivers. Classified by length and type of blade e.g. 10" common, 8" Phillips, the blade being made of alloy steel with a wooden or plastic handle. In a good quality tool the blade will be cold rolled to produce great strength and resistance to twist, and the tip drop forged and finally ground to the correct profile. Variations of the common or 'standard' screwdriver include Phillips, Posidrive and Reed & Prince, these being the type with a cruciform configuration blade (commonly termed 'Cross Point'). It is important to select the correct type of cross point driver for the particular screw in use, for although they may look alike at the first glance, the angles and shape of the cruciform slot are different. In the case of the common screwdriver, for use on normal slotted screws, the working tip of the blade should be ground flat to prevent slipping in the slot and the tip should bottom in the slot. Further variations of screwdriver include Ratchet, Pump-action, Changeable-tip (Snap-On) and stubby, this latter type being used in the restricted spaces frequently found in aircraft maintenance work.

(26)

Pliers. Classified by type of jaw and overall length e.g. 6" Fine Nose, 8" Slide Cutting, etc. Made of steel, forged to impart strength to their relatively light and slender form, with the jaws and side cutting section hardened. Care should be taken to use only a pair of pliers capable of coping with the job in hand, since the jaws can easily be twisted or damaged by mishandling. Specialised pliers include those for wire stripping, removal and fitting of circlips and wire locking.

(27)

Hammers. Classified by weight and type of head. The head is made of medium carbon steel with the working faces hardened and tempered, whilst the eye for attachment of the handle is left soft. After long service a hammer may tend to become unsafe due to small jagged pieces breaking off the edge of the striking faces. When this happens, the head should be discarded and a new one fitted, ensuring that the steel retaining wedge is secured in position. The head normally has one flat striking face and one of a variety of shapes. The non flat face is called a “pein”. Hence when we classify a hammer we call it a ball pein, cross pein or straight pein hammer. The flat surface is normally used for normal striking or hitting work such as bending a bar of metal or using a drift, whilst the peins are used for specialised forming operations. When the use of a hammer is necessary on finished surfaces, a soft hammer is used, the head consisting of a detachable plug of rawhide, nylon or similar material. Lead or copper heads are in use for similar reasons.

(28)

Files. Probably the most frequently used tool in the fitting trade, files are classified according to their length, section, type and cut of teeth. The length does not include the tang. Files are made of forged high carbon steel, the tang on which the handle is fitted being reduced in hardness so that it is less brittle than the working part. The teeth of the file may be single or double cut, whilst the grade or tooth spacing may be classed as rough, bastard, second-cut, smooth or dead smooth. These terms describe the number of teeth or 'cuts' to the inch and this will vary with the length of the file. Representative figures for a 12" flat file will be:

Bastard 21 cuts / inch Second Cut 26 cuts / inch

Smooth 40 cuts / inch

(29)

Commonly used files include:

• Flat. Parallel for most of it's length, tapering in both width and thickness at the end. Double cut on both faces, single cut on both edges.

• Hand. Parallel in width throughout it's length, but tapers in thickness at the end. Double cut on both faces, single cut on one edge, the other edge is left un-cut and is known as the 'Safe Edge'. This is used for filing in corners where one side is left untouched.

• Half Round. Double cut on flat face, single cut on curved face. N.B. Curved face is not a full half circle in section. Used in the formation of filed radii.

• Triangle or Three Square. May be single or doubled cut on all faces. Used for work on awkward corners.

(30)

Precautions using Files

• Never use a file without a handle.

• Never use a file as a lever, since due to it's brittle nature it may break with jagged pieces flying off (into eyes!).

• When filing soft metal (Aluminium, Copper), the teeth end to clog. The file should be frequently cleaned by using a file card consisting of short wire bristles on a fabric backing.

(31)

Chisels. The engineers chisel is called a 'Cold Chisel' because they are specially hardened and tempered for cutting cold metals. Consider the

requirements of a chisel. Firstly it must be harder than the metal it is cutting, and yet it must be tough and not brittle if it is to withstand repeated hammer blows. For these reasons they are made from high carbon steels or alloy steels heat treated to induce the properties that give them a satisfactory working life. Classified by length and section of working blade. The most common types are flat, cross-cut, round nose and diamond-point. The angle of the cutting edge varies with the properties of the metal to be cut, e.g. a larger angle for tough and hard materials, say 65 - 70º for steels, while for cutting softer materials like aluminium a fairly sharp angle is needed, say 30º.

Typical uses for various shapes of chisels are:

• Flat. General fitting work, chipping away large areas prior to filing, removal of rivet heads during repairs.

• Cross cut. For cutting grooves, key-ways on shafts and to divide up flat surfaces into strips prior to cutting with flat chisel.

• Half Round. For cutting an oil groove in a bearing.

• Diamond Point. For cutting a hole in a plate, forming sharp corners, or for moving the centre of a drilled hole which has started to run off-centre. Scrapers. Used for final surfacing work to correct slight warping and distortion and for blending out damage due to corrosion etc., common types can be flat and half round. These can be locally produced by grinding a flat file with a slightly curved cutting edge and finished to a high degree of sharpness with an oil stone. Used in conjunction with marking fluid (e.g. engineers blue) and bearing in mind that the surface to be worked on must be very nearly true initially, a scraper can be a most useful addition to the aircraft engineers tool box. For instance, the high spot of a bearing can be removed and the correct fit of the shaft can be obtained by scraping first the lower half, testing the fit with marking fluid with the shaft in position, then repeating the operation on the top half.

Hacksaws. Classified by frame size and type (fixed, adjustable, tubular etc.). The blade is tensioned by either tightening a wing nut or the handle itself. Lengths vary from approximately 8" - 14", frequently 10" and the blade will be made from carbon or alloy steel. Hacksaws may also be fitted with a round blade for cutting in all directions (useful for cutting out damaged structure in sheet metal). Usually the blade teeth only will be hardened, but the blade may be hardened throughout. Number of teeth vary, 18 T.P.I. (teeth per inch) being satisfactory for general cutting use, while 30 T.P.I would be preferable for cutting thin sheet or tubing and 14 T.P.I. is suitable for cutting solid brass or copper. The main cause of accidents to operators using hacksaws is blade breakage,

resulting in hands coming sharply into contact with the work. Breakage is usually due to either insufficient tightening of the blade, excessive downward pressure or excessive twisting of the blade on the forward stroke. Special care is necessary when cutting thin sheet or tube, only a slightly downward pressure is required. Note: The blade is designed to cut only on the forward stroke, with the blade installed correctly, i.e. teeth forward.

(32)

(33)

(34)

Spanners. These are available in a wide range of shapes and sizes and are intended for tightening or slackening a nut on a screw thread. Their length is related to the size of the nut for which they are designed and any misuse (e.g. extending the length with a tube) will certainly result in damage to both thread and spanner. Similarly, a hammer blow imparted to the end of the spanner to move a stubborn nut will also reduce the working life of the spanner. Properly maintained and used, with a light smear of oil to protect their surface finish, spanners will last for many years, and the practical engineer can never have too many of them. Generally made from Vanadium Steel, heat treated to provide hard, long lasting jaws combined with an extremely tough, resilient handle, the traditional double ended (i.e. open jaw) type of spanner is the most common. The jaws are usually set at 15º, 30º or 60º to the shank, so that for a relatively small handle movement a useful turning moment is attained at the nut simply by turning the spanner even when working space is limited.

The size of the spanner is clearly marked at or near the jaw and will be expressed as a B.A. number or a Whitworth, A.F. or Metric size. Spanners intended for Unified threads have their size marked on the jaw expressed as a figure correct to two decimal places, but the decimal point is omitted e.g. 50 would be 1/2" across the flats, 25 would represent 1/4" etc.

• Ring Spanners. These would be used in preference to open jaw spanners since they apply the load equally to all faces of the hexagon. In practice, most modern ring spanners have a 12 point configuration to the head and are referred to as bi-hexagonal. This makes for greater versatility where movement is restricted, permitting a nut to be turned when only 30º of movement is possible.

• Combination Spanners. These combine the best features of both open spanners and ring spanners as they have one head of each type, both being the same size. The heads may be off-set to the handle and to each other, and in some cases the ring spanner may be deeply off-set to allow the head to be fitted to a nut in a shallow countersink.

• Socket Spanners. These are produced in two parts, i.e. the socket, placed over the nut or bolt head and the handle which is attached to the socket, usually by a square driving shaft. A wide variety of handles are available, such as 'T' handle, ratchet, screwdriver grip and speed-handle (rather like a car wheel brace). The square drive, usually 1/4", or 3/8" or 1/2" square incorporates a spring loaded ball which engages in a groove in the socket. This should ensure that the socket lifts off the nut when the operator wishes to reposition the socket on the nut, and prevents the socket becoming detached, possible in an awkward position. Refinements to the basic socket and handle include extension rods to fit between the socket and handle, universal drive joints, flexible rods, posidrive bit adapters, crows foot

attachments and converter adapters enabling one to use handles with small square drives to connect to sockets with large drives or vice-versa. Note: Care should be taken not to over torque a socket when using a handle with a large square drive with a socket with a small square drive. Socket sets are available in all current size ranges and the practical engineer will be well advised to equip himself with the best quality, most comprehensive set he can afford. Cheap tools of inferior material have very limited life and may damage the component on which it is being used.

• Allen Keys. Certain screws or bolts have a hexagonal recess in their heads. An 'Allen Key' is used to tighten or slacken the screws. The basic tool is of hexagonal cross section (to suit the recess) and is cranked through 90º to form an 'L' shape. They are made of hardened and tempered steel, tough enough to withstand fracture and abrasion / wear. Allen keys are also made in straight lengths to fit into socket bits. Allen keys are classified by their dimension across their hexagon flats.

(35)

Special Spanners. Included in this category are 'C' spanners, Torque Spanners, Peg Spanners etc. 'C' spanners are used on round nuts, pipe connections etc. where the nut has a series of notches around it's periphery. The spanner usually has a curved articulated arm with a hook on the end. This hook is intended to engage into one of the notches on the nut. Peg spanners are similar except that a peg (or two) engages on a hole in the edge or face of the nut.

(36)

3.1.1MARKING OUT TOOLS

In the absence of special jigs or fixtures which locate the work and provide some means of guiding the cutting tool, most work necessitating removal of metal involves the scribing of guidance lines to indicate the positions of finished surfaces or the centre lines of holes. Some of the tools used are as follows: Rules. Engineering workshop rules are used for general measuring and are made from high carbon steel suitably hardened and tempered. They are usually graduated in Imperial and Metric systems of measurement and classified by length. Rules should be kept free from rust and never subjected to rough usage or careless handling. The end of the rule in particular should be carefully treated since it generally forms the basis of one end of the measurement being taken. One common malpractice if the use of steel rules to de-burr sheet metal. This may not only damage the rule, but it removes good metal from the sheet metal as well as the burr.

Scribers. Scribers are used for marking guidance lines on the surface of work; they are made of high carbon steel, suitably hardened and tempered and are classified by length. Scriber points like those of dividers, must be kept keen and fine, and they should be fully protected when not in use.

Dividers Fitters Square

Dividers. Dividers are used to set out distances and to scribe arcs and circles. Their legs are made of high carbon steel, hardened and tempered, with a spring steel spring. Dividers are classified by the length of the legs. The points should be kept keen and of equal length, by stoning on the outside. Grinding, unless done very carefully will change the temper of the points and render them soft. When the dividers are not in use, the points should be protected by sticking them into a cork.

Fitters Squares. Fitters Squares are used for setting out lines at right angles to an edge or surface, and for checking right angular work for truth. Squares are made of high carbon steel, hardened and tempered and are classified by the length of the blade. The square is made to very fine limits and this initial accuracy must be preserved by careful handling and keeping it in the box provided when not in use.

(37)

The blade and stock have their opposing edges ground truly parallel with the limbs set at exactly 90º to each other. This accuracy must be checked from time to time. This can be done by checking the square for truth against a master square or against a V - block. An alternate test (see diagram to the right) is to place the stock against a flat surface, using the outside edge of the blade as a guide. The square is then turned over and the outside of the blade checked against the line. The test should be repeated using the inside edge of the blade. Combination Set. A combination set (see diagram below) is virtually three tools in one, consisting of a blade or rule and three 'heads'; the blade is made from high carbon steel, hardened and tempered, while the heads are of close-grained cast iron. The blade is graduated in inch and metric scales, and a central groove along it's entire length accommodates a clamping screw fitted to each of the heads, thus enabling a head to be secured at any desired position along the blade.

The details of the three heads are as follows:

• Square Head. This head is provided with two working faces, one at 90º and the other at 45º to the blade, thus enabling the tool to be used both as a square and as a mitre. A spirit level is incorporated into the head and a scriber is provided.

• Centre Head. This is used in conjunction with the blade to locate the centre of a round bar or the centre line of a tube.

• Protractor Head. Used in conjunction with the blade for checking or setting up any angle up to 180º. A spirit level is often incorporated.

(38)

• Inside & Outside. These are used in conjunction with a rule or other measuring instrument for measuring distances between or over surfaces, or for comparing dimensions. Inside calipers are used for measuring inside dimensions and outside for external dimensions. To set the calipers, set nearly to size by hand and then tap one leg (not at the point) to make the final adjustment. When calipers are used for comparison purposes, the results obtained largely depend on the sense of feel of the user.

• Odd Leg Calipers. This tool is really half caliper and half dividers. It may be used for scribing lines parallel to an edge or for scribing arcs on cylindrical bars to aid in finding the centre. These tools are often referred to as 'jenny calipers'.

Marking Off (Surface) Table. Used to support work for marking out and to form a base for measurements. Made from close grained cast iron, strongly ribbed for rigidity. The working surface is accurately machined to give a true, flat surface and square edges. After use, the working surface should be protected with oil and the protective cover replaced. No work other than marking or measurement should be carried out on the table.

Surface Plate This may be used in place of the marking out table for relatively small work. It is much smaller than the table and the finish is at least equal to that found on a good table. Surface plates are usually portable and used on a work-bench. To test a flat surface for accuracy, the plate is smeared with engineers blue and the surface to be tested rubbed on the plate. The amount of marking transferred will indicate its flatness.

Vee Blocks These are used on a marking table or surface plate to support round work. They are made of cast iron or case hardened mild steel, are supplied in identical pairs, each unit of a pair being stamped with the same identification number. All surfaces are accurately machines and the Vee angle is exactly 90º. Vee blocks are classified by the maximum diameter of the work which can be held. The clearance slot at the base of the Vee allows objects to be set firmly. Scribing Blocks (see diagram below). A scribing block is used to mark out lines parallel to a true surface, such as the working surface of a marking off table or a surface plate. The accurately machines base is made of cast iron, or case-hardened mild steel, the scriber is of high carbon steel, case-hardened and tempered and the pillar angle, scriber height and angle are all adjustable. A fine

adjustment is provided for the pillar and dowels in the base can be pushed down so that lines can be scribed parallel to the edge of the surface table or plate. Scribing blocks are classified by the height of the pillar.

(39)

Key-Seat Rules These are sometimes termed 'box squares', and are used for marking lines parallel to the axis on the surface of tubes and round bars. These rules are usually graduated and are classified by their length.

Key Seat Rule Use of Feeler Gauges

Feeler Gauges 'Feelers' are used to measure small clearances or gaps; they consist of a series of thin flexible steel blades in graduated thickness varying in most cases from 1.5 to 15 or 25 thousandths of an inch. The blades are secured in a protective metal scabbard by a fulcrum pin and all blades not in actual use should be withdrawn into the scabbard to prevent accidental distortion. Feeler gauges are classified by the length of the blades. When not in use, the blades should be lightly smeared with oil to prevent rusting.

Centre Punches A centre punch is used to make a small indentation for locating the cutting edge of a drill at the start of a drilling operation. Centre punches are made of high carbon steel, the point being hardened and tempered. A sharp point should be maintained by careful grinding and should have an angle of 90º for general work or 60º for light work, such as marking out. Automatic centre

(40)

Use of a Centre Punch Examples of Marking-Out Work

There will be many instances where it is necessary to fabricate aircraft parts. Some of the skills required in measuring out prior to fabrication of parts are described below.

Marking-off Rectangular Work (Blocks or sheet metal)

File one face of the metal true (check with steel rule or straight-edge) and square one edge to the true face; the work will then stand firmly on the surface table (or plate). Parallel lines can then be scribed across it's face using a scribing block. If marking sheet metal, the sheet can be placed against a V-block. Height marking can also be carried out using a vernier height gauge.

Squaring up End of Round Bar or Tube.

The diagram below shows a simple method of marking-off for squaring the end of a bar. The bar or tube is supported in a pair of V-blocks which set it up parallel to the table and a third V-block laid on its side prevents axial movement. The scriber is firmly clamped in the scribing block at a height and angle which brings the point in a suitable scribing position. The cutting line is then marked by rotating the bar against the scriber point.

(41)

Open out the legs of 'Odd Leg' calipers until they are set at rather less than the radius of the bar. Scribe four short arcs on the end of the bar shown in the diagram (see diagram to the right). The centre of the bar is then in the centre of the small figure. The position may be estimated by eye and centre popped.

Marking-Out - Summary

• Only boundary lines and cutting lines should be scribed on Light Alloy sheet. Scribed lines on this type of material may give rise to cracks. Any lines other than cutting lines should be marked with a soft graphite pencil (all traces should be removed afterwards) or a wax crayon (not black - it may contain graphite).

• The points of scribers and dividers must be kept clean to produce very fine scribing lines. Thick lines lead to inaccuracies.

• Scribing lines must be clear and distinct; prior to marking out, it may be advantageous to apply chalk or white wash to the surface. Bright steel surfaces should be coated with copper sulphate or engineers blue.

• When the scriber is used in a scribing block, it must be clamped rigidly and scribing should be done firmly so that there is no necessity to retrace lines. The scriber point should be set as close as possible to the pillar, thus reducing the tendency of the point to whip.

• Always trail the point when using the scriber so that it does not dig in to the material.

An accuracy of 0.010" is often accepted for marking out although more accuracy may be obtained using a vernier height gauge.

(42)

3.2 COMMON POWER TOOLS

Sometimes hand tools are not practical for reasons of speed and accuracy. A variety of power tools are used during aircraft maintenance. Cutting tools used in an aircraft environment are generally pneumatically operated. Electrically driven cutting tools would be dangerous as they produce sparks which may ignite fuel vapours. The power for the pneumatic tools is supplied via a compressor that supplies air at around 80 p.s.i. The compressor normally incorporates a water trap so that the air is as dry as possible. The air supply is normally supplied via metal pipelines to a quick release coupling. The engineer will normally connect the power tools to the coupling via a plastic or rubber flexible hose. Many different types of pneumatic power tools are used, mainly by the airframe and engine engineers. The most common tools used are pneumatic (windy) drills, rivetting hammers for solid rivets, blind rivetting tools, pneumatic shears, pneumatic sanders, rivet croppers and millers.

3.2.1ELECTRIC HAND DRILLS

(See diagram below). These may be dangerous to use unless they are kept in good condition and handled carefully.

1. Always check the condition of the lead and plug. Do not use the drill if it is damaged in any way.

2. Make sure the job is firmly secured in a vice or on the drill platform.

3. Use a lubricant to keep the point of the drill cool; kerosene is suitable for most metals.

4. Do not force feed or the drill may break.

5. If swarf builds up at the drill point, stop the machine before attempting to clear it away.

6. Always wear goggles to protect your eyes.

Electric Drill Drill Stand

3.2.2PNEUMATIC TOOLS

These are used mainly in structural repair work.

Air Operated (Windy) Drills (see diagrams below). These are available in either straight or pistol grip form. They will, depending on size, accept drills up to 8mm diameter. Angled and off-set drills are provisioned for drilling holes in restricted positions. These drills require a separate collet for each size of drill.

(43)

(44)

Rotary Saw. Used primarily for cutting sheets of metal both on and off aircraft. It may also be used for cutting plywood and plastic. The tool illustrated can cut steel and alloy of thickness 0.8mm and 2mm respectively.

Pneumatic Shears This tool is designed to cut sheet material up to 14 SWG in mild steel or 12 SWG in light alloy. Cutting is achieved by the action of a

reciprocating shear blade against a stationary anvil blade. Stellite tipped blades are available for cutting stainless steel or titanium alloy.

Pneumatic Riveting Hammers. Many types are available to suit a variety of solid rivet sizes. They all operate on a similar principle as shown in the diagram below. The air pressure supply controlled by the throttle button or lever, causes the piston to oscillate rapidly backwards and forwards in the barrel. The piston delivers blows to the rivet via the interchangeable snap. An adjustable air

regulating screw varies the maximum rate and power of the gun. A typical rate is 1,500 blows per minute.

Pneumatic Blind Riveters. These are designed for easy forming of various types of blind rivets. There is usually a special riveter for each type of rivet. Sometimes the riveter is air operated, but many incorporate a hydraulic intensifier. Many types exist, so only a selection is shown below.

(45)

Mandrel Cropping Tool. The air operated cropping tool is used to cut off the protruding mandrel stems of Avdel rivets after they have been set. The tool incorporates two cutting jaws which sever the rivet mandrel when the control button is pressed. The cut mandrel will still need to be milled down with the milling tool, to give a clean finish.