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JAR 66 CATEGORY B1
Adhesive Bonding is the technique of joining materials using special adhesives.
In the past a common type of adhesive widely used in metal to metal joints was the ‘Redux’ epoxy resin system. ‘Redux’ is the trade name for a range of adhesives produced by the Ciba-Geigy company and the epoxy bonding procedure in general, refers to a hot-melt, hot-cure adhesive, which is available in partly cured strips or sheets.
Note: This type of epoxy resin is also used to provide the reinforcement for fibre composite construction and has already been covered as a separate topic in Module 6.
In metal to metal bonding, the sheets of partly cured adhesive, which at this stage resemble strips of chewing gum, are cut to exact size. With the backing paper peeled away, they are carefully placed between each of the components being joined together and the joint securely clamped. The complete assembly, which for example might consist of a wing skin with all its stringers and ribs in place, is then loaded into an autoclave (pressure cooker) to complete the curing process. The adhesive melts and flows evenly into the narrow gaps between the component parts and cures to produce a very strong bond.
In the autoclave the temperature limits are strictly controlled, (typically not above 100-150C, depending on type of adhesive used), and subjected to a constant clamping force (usually by a vacuum process), resulting in perfect bonded joints which are as strong as, or stronger than, equivalent riveted joints. For composite repairs, figure 45, a portable Autoclave process is employed.
There are a number of aircraft, in which the majority of the primary metal structure is joined together entirely with adhesive bonding, with very few rivets being used. The Fokker 50/70/100 and BAe 146/RJ are good examples of aircraft employing this technique extensively. In fact British Aerospace claims that by using adhesive bonding techniques on the BAe 146/RJ airframe, over 10,000 rivets are not required. This means the weight of the rivets, the work that would be expended in closing them and the risk of subsequent in-service cracks (see Figure 44) emanating from rivet holes, are all saved on each airframe.
A further important advantage of using adhesively bonded structures, is improved sealing of integral fuel tanks, eliminating the leakage problems that are typical of riveted assemblies.
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Comparison between Machined and Bonded Structure Failure Rates Figure 44
Autoclave Curing Process During Composite Repair Figure 45
JAR 66 CATEGORY B1
2.6.17 METHODS OF SURFACE PROTECTION
As mentioned in an earlier chapter, there are many different types of surface protection added to the basic structural materials and hardware.
Anodising
A method of protecting aluminium based alloys from corrosion, especially when cladding is impractical, is by a process called Anodising. This is an electrolytic treatment which coats the host metal with a film of oxide. This film is hard, waterproof, air-tight and to aid in identification of some parts, will permanently accept a coloured dye. The film also acts as an insulator, so when bonding leads are to be attached to an anodised part, the surface treatment must be carefully removed before the bonding lead is attached. Finally, anodising a part also provides an excellent base for the addition of an organic finish and bonding adhesives.
There are a number of different organic finishes applied to aircraft to protect the surfaces:
Synthetic Enamel.- An older finish which cures by the process of oxidation It has a good surface finish, but is poor when it comes to its resistance to chemicals or wear.
Acrylic Lacquer.- A popular finish in the mass production market, easy to apply and has a fairly good resistance to chemical attack and weather.
Polyurethane.- One of the most durable finishes which has high resistance to wear, fading and chemicals. It also has a 'wet look'.
Chromating
Chromate coatings are used to protect Magnesium-based alloys, as well as zinc and its alloys. Components are immersed in a bath containing potassium bichromate and results in a yellowish coating on magnesium alloys. The coating can be restored locally with Alocrom 1200 treatment.
Cladding face of each base alloy sheet, effectively sandwiching the alloy.
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Surface Cleaning
Most aircraft will be cleaned before starting on large inspections, but it is common sense to keep an aircraft clean all of the time. Dirt can cover up cracked or damaged components as well as trap moisture and solvents which can lead to corrosion.
Note: Materials mentioned in this chapter are only used as an example, each aircraft type will have a list of suitable and prohibited materials in its maintenance manuals (AMM).
Exterior Cleaning
Exterior cleaning is an important facet of corrosion control, but there are a number of points which must first be protected from cleaning materials and high pressure water sprays. The pitot tubes and static vents must be properly blanked off to prevent water ingress and the wheels, tyres and brake assemblies need to be covered to keep them free of aggressive cleaning agents.
Only cleaning agents and chemicals recommended by the manufacturer are to used. for the job in hand or the risk of serious contamination may result. One of the unseen effects of using non-approved cleaning agents is hydrogen embrittlement. This is caused by hydrogen from the agent being absorbed into the metal, causing minute cracks and will lead to stress corrosion failure.
Aircraft should ideally be washed on a proper platform with suitable drains. It is better if the outside air temperature is not too high, so the cleaning agent does not evaporate. Typically, a mix of water and an emulsion-type cleaner, to a ratio of between 3:1 and 5:1 is applied, allowed to soak for a few minutes and then rinsed off with a high pressure stream of water.
Engine cowlings and wheel well areas usually have grease, oil or brake dust deposits that require special treatment. These require stronger mixtures ratios and scrubbing with a soft bristle brush to loosen the dirt before rinsing off with a high pressure water jet. It must be borne in mind however, that oil and grease could be accidentally removed from places where they are meant to be, for example in wheel bearings etc. These will often require re-lubrication after washing has been completed.
Exhaust residue from both piston and jet engines is very corrosive and must be removed on a regular basis. These deposits usually require a special proprietary solvent to mix with the water. Sometimes a simple emulsified mix of kerosene and water may be approved. Dry-cleaning solvent or naptha is sometimes used for oil and grease removal. Some naptha compounds are harmless to rubber or acrylic items, whilst others will attack these same materials, so only approved specifications are to be used.
JAR 66 CATEGORY B1 manufacturer of that aircraft to ensure no abrasives or solvents are applied where they can do damage.
Non-Metallic Cleaning
Non-metallic components sometimes require different cleaning techniques from metal parts. For example, the slightest amount of dust on plastic or acrylic panels will scratch and severely reduce the optical quality if rubbed with a dry cloth. This can also build up a static charge and attract more dust so the correct procedure in this situation is to wash down, rinse with water without rubbing with a cloth. Oil and hydraulic fluid also attack rubber components such as tyres, so any spillages must be cleaned up immediately. Neoprene rubber leading-edge de-icer boots and composite structures are other examples of parts that need special cleaning procedures, all of which will be detailed in the AMM.
Engine Cleaning
Apart from external cleaning carried out on the engine cowlings, with the associated protection of electrical components; gas turbine engines are regularly washed internally to remove the deposits of dust, sand and salt, that tend to accumulate on internal parts of the engine.
This coating if not removed, can have a serious effect on the engine's performance. Indeed, the output of the engine could fall below the manufacturers minimum figures, resulting in an unscheduled and expensive engine change
Alignment and Symmetry
Aircraft can have abnormal occurrences during their life, when for example, a very heavy landing could occur, some accidental external damage or the need to replace a major component, etc. All of these instances will require special checks to be carried out to guarantee that the aircraft is perfectly symmetrical and aligned before its next flight.
The checks consist of measuring very accurately from a number of datum points on the airframe, such as from wing tips, the nose, the horizontal stabiliser and the top of the vertical stabiliser. The checks vary, depending on the aircraft manufacturers requirements, but all ensure that measurements taken on the left-hand side of the aircraft are within a minimum tolerance of the measurements from the right-hand side. These checks are usually taken with the aircraft on jacks and in the rigging position, ie: a nominally level ‘in flight’ attitude.
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On light aircraft, these measurements are usually taken using a surveyors tape measure. (It is a check of comparison, not of outright measurement). As the aircraft get larger, optical theodolite style methods are used. These can be a microscopic level with the use of sighting rods or even a laser ranging alignment device.
Deeper checks that are carried out after any of the above mentioned situations, as well as on a routine basis, include checks on the wing, tail and control surfaces to ensure that they are set at the correct angles. These checks are usually known as 'rigging checks' and are carried out using purpose built levelling boards and an accurate measuring device known as a Clinometer.
Rigging Checks - Older Aircraft Figure 46
Symmetry Checks – Modern Aircraft Figure 47
JAR 66 CATEGORY B1 MODULE 11.02
AIRFRAME STRUCTURES
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Issue 1 – 04 Sept 2001 Page 3-1
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Contents
3 AIRFRAME STRUCTURES - AEROPLANES ... 3-3 3.1 FUSELAGE ... 3-3
3.1.1 Truss Fuselage Construction ... 3-3 3.1.2 Truss Fuselage - Warren Truss ... 3-3 3.1.3 Stressed Skin Structure... 3-4 3.1.4 Pressurised Structure ... 3-5 3.1.5 Attachments ... 3-6 3.1.6 Passengers and Cargo ... 3-9 3.1.7 Doors ... 3-10 3.1.8 Windows and Windscreens ... 3-12 3.2 WINGS ... 3-14 3.2.1 Construction ... 3-14 3.2.2 Fuel Storage ... 3-16 3.2.3 Landing Gear ... 3-18 3.2.4 Pylons ... 3-19 3.2.5 Control Surface and High Lift/Drag Attachments ... 3-20 3.3 STABILISERS ... 3-21 3.4 FLIGHT CONTROL SURFACES ... 3-22 3.5 NACELLES AND PYLONS ... 3-23
JAR 66 CATEGORY B1