EASA PART 66 - MODULE 11.03 - Airframe Structures

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JAR 66 CATEGORY B1 MODULE 11.03 AEROPLANE STRUCTURES

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1.1 FUSELAGE ... 1-3 1.1.1 Truss Fuselage Construction ... 1-3 1.1.2 Truss Fuselage - Warren Truss ... 1-3 1.1.3 Stressed Skin Structure... 1-4 1.1.4 Pressurised Structure ... 1-5 1.1.5 Attachments ... 1-6 1.1.6 Passengers and Cargo ... 1-9 1.1.7 Doors ... 1-10 1.1.8 Windows and Windscreens ... 1-12 1.2 WINGS ... 1-14 1.2.1 Construction ... 1-14 1.2.2 Fuel Storage ... 1-16 1.2.3 Landing Gear ... 1-18 1.2.4 Pylons ... 1-19 1.2.5 Control Surface and High Lift/Drag Attachments ... 1-20 1.3 STABILISERS ... 1-21

1.4 FLIGHT CONTROL SURFACES ... 1-22 1.5 NACELLES AND PYLONS ... 1-23

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JAR 66 CATEGORY B1 MODULE 11.03 AIRFRAME STRUCTURES

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1 AIRFRAME STRUCTURES - AEROPLANES

1.1 FUSELAGE

The fuselage of a light aircraft is the body of the aircraft, to which the wings, tail, landing gear and engines may be attached. Larger aircraft can have their main landing gear attached to the wings and, on multiple engined aircraft, a number of the power-plants can be wing mounted also.

The loads produced either on the ground or in flight, will at some time, have to pass through the fuselage. In order to absorb these tremendous loads imposed upon the structure, the fuselage must have maximum strength, but this must be combined with the other constraint, that of minimum weight.

There are two types of construction found in the majority of modern aircraft fuselage design, the truss and the stressed skin type.

1.1.1 TRUSS FUSELAGE CONSTRUCTION

By definition, a truss is a form of construction in which a number of members (or struts), are joined to form a rigid structure normally covered with non-load carrying material such as cloth, fabric or thin sheets of wood.

Very early aircraft used a method of construction referred to as a Pratt Truss, where struts were held in compression, and wires, which ran diagonally between the struts, were in tension.

Truss Fuselage – The Pratt Truss Figure 1

1.1.2 TRUSS FUSELAGE - WARREN TRUSS

When fuselages were subsequently made from welded tubes, the Warren Truss became popular. In this arrangement, shown overleaf, the longerons are separated by diagonal members which carry both compressive and tensile loads.

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Warren Truss Figure 2

1.1.3 STRESSED SKIN STRUCTURE

The neccessity of having to build a non-load-carrying covering over a structural truss led to designers to develop the stressed skin form of construction. In this method, a proportion of the load is carried by the outside skin, which can be also be formed into a much smoother and more efficient shape.

The commonest form of a stressed skin structure is a chicken egg (pure monocoque). The seemingly fragile shell can resist high loads, as long as they are applied in a proper direction.

Pure-Monocoque Structure

This form of stressed skin construction is rarely seen in its purest form, because it is normal to add some form of light internal structure to help support the skin. However, there are some aircraft (normally gliders and sailplanes) made from glass reinforced plastic (GRP), which are constructed as a pure monocoque structure. In this design, the GRP skin is quite thick, often with a core of some other lightweight material such as balsa wood or composite honeycomb, so there is no need for any internal, supporting structure.

Semi-Monocoque Structure

This form of construction has a skin carrying a large amount of the loads, but with an internal structure of frames and stringers to keep the skin to its correct shape, where it can best carry the loads. Some have longerons which are more substantial than stringers and carry most of the longitudinal structural loads, with the frames carrying the radial loads.

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1.1.4 PRESSURISED STRUCTURE

High altitude flight places the occupants in a hostile environment in which life cannot be sustained without oxygen. To avoid the need to wear oxygen masks, the pressure in the cabin is raised higher than it is outside, which provides sufficient oxygen in the air for the passengers to breathe normally.

In the 1950’s, piston-engined aircraft, had a pressure differential across the cabin wall about two pounds per square inch (psi) maximum. Modern aircraft cabins can sustain a pressure differential between 8 and 10 psi, so there must not be any part of the structure containing 'stress raisers' which would concentrate stress to an unacceptable level. Much of the structure of modern aircraft has been built to the 'fail safe' philosophy, in which the structure is built with multiple load paths for the major stresses to pass through, to cater for the unlikely failure of a single structural item.

Pressurisation Sealing

All joints in the structure, as well as openings such as doors, panels, emergency exits, etc. must be completely airtight during flight, to prevent the cabin pressure leaking below its required level. Joints are constructed with an interface of sealing compound, whereas windows and doors employ pre-formed rubber seals around their edges. The points where control tubes and cables pass in and out of the pressure hull, utilise some form of flexible bellows which are leak proof but move with the controls.

Pressurisation Sealing Figure 3

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1.1.5 ATTACHMENTS

The fuselage can, as mentioned earlier, carry most of the major loads, both on the ground and in flight. To this end, most of the other airframe components such as the wing, stabilisers, pylon and undercarriage, can be fitted to the fuselage. The wings can be mounted above or below the passenger compartment. As already mentioned, wings are usually attached to the fuselage with multiple attachments, although light aircraft may still have wings attached with as few as two bolts.

Early High Stress Attachment Figure 4

Multiple Fastener Wing Attachment Figure 5

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The horizontal and vertical stabilisers can be fitted to the fuselage in numerous different ways. When the horizontal stabiliser is fitted part-way up or on the top of the vertical stabiliser, there will be only one strong attachment point. Otherwise, there will be separate attachments for the fin and for the left and right tailplane sections.

Where a moving horizontal stabiliser is employed, the attachment will consist of left and right rear pivot fittings and a single forward attachment to a trim actuator. On rare occasions, the rear fuselage is manufactured, together with the stabilisers, as one integral unit. Because the loads generated by the empennage, it is usual to find that the rear fuselage structure has stronger frames around the stabiliser attachment points. These frames transmit the loads along the fuselage and away from the tail.

The same technique is used when the engines are attached to wing or to rear fuselage mounted pylons The Fokker 70/100, for example, has oblique frames to connect the vertical stabiliser to the top mounted tailplane and to the fuselage, plus two heavy frames to transmit all the engine thrust loads into the fuselage.

Strengthened Frames Figure 6

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As previously mentioned, the landing gear can be attached either to the fuselage, the wings, or within wing mounted engine nacelles. Because of the need for cabin space, fuselage mounted landing gear on passenger and freight-carrying aircraft, often have the main landing gears mounted in fairings or nacelles beneath the fuselage as in the ATR-72, detailed below.

Faired ATR 72 landing Gear Figure 7

The landing gear, as for the other attachments, is mounted on to strong fuselage frames which in this case, are also used to mount the wings, attached above the fuselage. The loads that these frames carry, both in flight and on the ground, are transmitted into the fuselage by means of longitudinal stringers and longerons.

Fuselage Strong Points Figure 8

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1.1.6 PASSENGERS AND CARGO

Aircraft that carry passengers as well as crew, all have to have seats that comply with crashworthiness regulations. These regulations dictate that the seats with a person correctly strapped in place, must be able to survive a sudden stop of over 20 times the force of gravity, (20g), without the floor mountings (to which the seat is attached) failing, or the seat itself collapsing.

Although aircraft seats appear to resemble normal domestic seats, the tubular framework and floor attachment 'feet' are very strong, yet are light in weight and can be disconnected from the floor if necessary, by releasing a few quick-release fasteners.

Passenger compartment floors of modern aircraft are often panels of the composite material ‘Fibrelam’, which are strong enough to carry most of the general loads created by passengers and galley equipment. The panels are themselves supported by lateral and longitudinal beams, which are primary structure, into which the panels fit. Lateral beams are attached to the lower portion of the (usually) circular fuselage frames and longitudinal beams supported by the lateral beams, are those upon which the seats are fitted.

Seat Track Fittings Figure 9

The top of each longitudinal beam is fitted with location holes which are a standard size and into which all seats are slotted. Additionally, the galleys and bulkhead partitions can also be attached to them. The frequent and equal spacing of the seat track attachment holes, allows the seats to be fitted at a variable increment, or pitch, to cater for different classes of cabin (economy or first class). On some aircraft, such as the Fokker 100, there are five longitudinal seat tracks in the cabin floor which allow a five abreast seating to be installed (3+2 or 2+3), with the off-set aisle on whichever side the customer wishes.

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Cargo Loading Systems

Aircraft which are used for carrying all or part freight loads have to have the floor modified to allow the movement of pallets or containers.

Usually this will consist of substantial reinforcement of the flooring with tracks, guides and rollers fitted, to allow safe and easy motorised movement up and down the freight bay. In the entrance door area, a ‘ball-mat’ is installed to allow the freight to be easily loaded, rotated and man-handled on to the rollers.

1.1.7 DOORS

This topic covers most methods of entry and exit from the fuselage, including those for passengers, crew, refreshments and meals, baggage and major maintenance access. In addition, some doors are dedicated to emergencies only and will therefore remain unused during normal operations.

If the aircraft has a cabin pressurisation system, the doors have to be more substantial than for a non-pressurised type and be fitted with safety devices to prevent accidental opening. One method to prevent this happening is allow the door to open inwards so that the door 'plugs' the aperture when closed and is held in place by the cabin pressure in addition to the door frame locating bolts. Any door on pressurised aircraft that does open outwards, must have additional devices and protection mechanisms fitted to prevent accidental opening and a flight deck warning system to inform the crew if it is not properly closed and secured.

Non-pressurised aircraft doors still have to be safe, with a system of handles and latches that operate in a specific order or after the application of a certain force. Doors on most aircraft are constructed in a similar way to the fuselage with an inner and an outer skin and vertical and horizontal members. The sometimes complex locking and latching mechanisms, plus the indicating and warning electrical wiring systems are all contained within this structure.

Most fuselage doors are operated manually, but much larger freight/cargo doors are either electrically or hydraulically operated. Another requirement on all cabin doors, (normal exit/entry and emergency type) is the need for efficient emergency egress in the event of a mishap on the ground. They must be operable by a single handle whose operation shall be ‘rapid and obvious’. Most doors have decals and large red arrows, to clearly indicate the way in which the handles are to be rotated or moved to open the door.

Dedicated emergency exits are almost always 'plug' type and, therefore, cannot be opened in flight due to the cabin pressure acting on door opening mechanism (usually an over-centre type a cam arrangement) thus preventing handle rotation.

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Door Mechanism Figure 10

Door Structure and Sealing Figure 11

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To prevent leakage of the cabin pressure, all doors have to have a substantial seal around their edges to keep the aperture between door and surrounding fuselage frame airtight. Some seals just compress and fill the space when the door is closed, others use cabin air to inflate and therefore expand the seal to achieve the same result. Fig 11 shows a typical door seal arrangement.

1.1.8 WINDOWS AND WINDSCREENS

All the transparencies on non-pressurised aircraft are normally made from acrylic or some other clear plastic material. On pressurised aircraft, flight deck windscreens have to comply with very strict bird-strike regulations and are made from a toughened sandwich of glass/plastic/glass The passenger cabin windows are manufactured from acrylic, mylar or other plastics.

It must be considered that an aircraft travelling at 400 knots which collides with a bird weighing 3kg, could suffer severe structural damage, engine failure and more importantly, if the bird struck a windscreen and broke through, it could cause serious injury. Furthermore, rapid decompression of the pressure cabin would result. The regulations state that during testing, when a dead bird is fired at it from a large air gun, the screen must be able to survive the impact.

Consequently, the glass/plastic/glass sandwich is fitted with a heating element between the interface of the front glass panel and the plastic core. Not only does the heater provide anti-icing protection, but helps absorb impact since it makes the plastic core more pliable and shock absorbent. The section through a typical windscreen below shows how the lamination of glass and plastic layers is arranged.

Windscreen Construction Figure 12

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Passenger cabin windows are almost always made from acrylic plastic. This saves quite a lot of weight as well as cost. For added safety, the acrylic cabin windows are actually two layers with a space in between, so that if one fails the other will carry the pressurisation loads, a typical case of fail safe. In addition, some cabin window assemblies have a third, pane of acrylic fitted to help reduce the engine noise in the cabin from the power-plants outside.

Passenger Cabin Window Figure 13

Most aircraft require one or more flight deck windows that can be opened for signalling to the ground-crew, for fresh air ventilation if the air conditioning is 'off' on the ground and to be able to see out in emergency situations, for example, the windscreen becoming obliterated. To achieve this, aircraft are usually fitted with a pair of opening front corner or side windows, sometimes called Direct Vision windows. If the cabin is pressurised, they will be unable to be opened due to the provision of a similar ‘pressure on’ safety lock system as the cabin doors.

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1.2 WINGS 1.2.1 CONSTRUCTION

The methods by which the wings produce lift were covered in Module 8, so this module will concentrate on wing construction and their attachments.

To classify the many types of wing it is best to break them down into different groups. The first sub-division is either those that are externally braced or those that are of cantilever construction. (no external bracing). In the early days the majority of aircraft were constructed with the whole aircraft, including the wings, being braced by wires and struts. These produced very high drag, although the overall structural weight could be kept down.

As materials and the wing construction became stronger, the number of wires were progressively reduced, until in the mid-1930's the first genuine fully cantilever wings with no external bracing, were put into production. This does not mean the bracing has been eliminated, it just means that all ‘bracing’ is included within the wing structure and made much stronger. Fig 14 below, shows how the external bracing of a biplane has been replaced with more efficient internal bracing on a cantilever wing.

Biplane and Cantilever Wing Bracing Figure 14

To illustrate how complex the inside of even a small aircraft wing can be, the following two pictures show the internal structure of both a wood and a metal wing.

Internal Wing Structures – Wood and Metal Figure 15

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The heart of a wing is the spar (or spars), to which are attached the ribs stringers and other structural items. The number of spars is decided by the designer or design team, but modern airliners normally have two. It is usual to attach landing gears, primary flying controls, leading and trailing edge devices, to one or other of the spars within the wing on larger aircraft.

Simpler wings on, for example, a light aircraft, will have only one main spar but some aircraft can have up to five, which has a measure of 'fail safe' philosophy. If military aircraft are considered, some modern fighters can have more than 15 spars as part of the ‘damage tolerant’ design application.

Wing planforms can show an infinite number of different shapes, that are purpose built and satisfactory for providing lift. These could be generally grouped into straight, swept, delta and combination wings. Straight wings include those with a slightly swept leading edge, trailing edge or both.

Swept wings are usually categorised as those with both leading and trailing edges swept back, at a variety of different angles, whilst the delta-winged shape (from the Greek for triangle) is self-explanatory.

Under the cover-all title of 'Combination', the selection of silhouettes below should give an idea of the wide range of wings that can be found on modern day aircraft, in addition to the more conventional planforms mentioned above.

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1.2.2 FUEL STORAGE Rigid Tanks

Because of their shape, wings are often designed to be used for fuel storage. They can either contain separate fuel tanks within the wing structure, or use the wing structure itself, suitably sealed, to make integral tanks.

Separate internal tanks are usually manufactured from either light alloy or from flexible, rubberised fabric. Rigid light alloy tanks are first riveted, then welded to make them fuel tight and are securely clamped into the wing structure by straps or tie bars. They will often have baffles inside, to prevent fuel surge from one end of the tank to the other.

Rigid Fuel tank Figure 17

Flexible Fuel tank Figure 18

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Flexible Tanks

Flexible tanks, (Fig 18), also referred to as 'bladder' tanks, have to be located snugly into the tank bay within the wing, because the sides of the bay provide support to the relatively weak tank skin. Older types of flexible tanks were made from rubber- covered fabric. These days the fabric is replaced by man-made fibres, impregnated with neoprene or some similar fuel tight material.

Integral Tanks

Integral fuel tanks are found on most, if not all, modern commercial aircraft. During manufacture, practically the entire wing structure becomes a box, comprising front and rear spars, top and bottom wing skins, inboard and outboard sealed ribs, into which are installed pumps, drains, filler caps and vents.

The main advantage of the integral tank, is that it provides maximum fuel capacity for the minimum amount of weight and the only sealing required, is that applied to the seams after construction is completed.

Boeing 737 Integral Fuel Tank capacities Figure19

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1.2.3 LANDING GEAR

As mentioned earlier, the attachments for major components can often be strong points on the wing spars, or even a separate spar built specifically for that purpose.. One such component that falls into this category is the main landing gear, otherwise known as the undercarriage. On some very large aircraft, like the Boeing 747 or Airbus A340, additional body gears, as well as conventional wing gears are to be found. These have to have reinforcements built into the lower fuselage structure to absorb the extreme loads at touch down.

Landing Gear Attachments Figure 20

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1.2.4 PYLONS

Many aircraft have engines mounted on pylons attached to the wing. With this so called ‘podded engine’ configuration, the pylons have to take very large thrust forces from the engines and transfer it to the airframe. This is normally achieved by attaching the engine to strong points on the pylon and attaching the pylon to the wing spars. Thrust links are then fixed to the engine frame and the wing spars to transfer the engine thrust efficiently. Pylons must be positioned low enough so that the engine exhaust doesn’t strike the wing structure, but not too close to the ground to risk a runway scrape. The Boeing 737-600 is a fine example of this compromise.

Pylon Engine mounting Figure 21

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Boeing 737-600 Engine Pylon Mountings Figure 23

1.2.5 CONTROL SURFACE AND HIGH LIFT/DRAG ATTACHMENTS

All of the flying controls on the wing will be attached to strong points on either the front or rear spars. This includes high and low speed ailerons, leading and trailing edge flaps, slats, roll spoilers, speed brakes and lift dumpers. The wing structure must therefore be made strong enough not only to carry the lift forces in flight but the additional loads of pilot control inputs, additional drag devices, etc. Consequently, the spars, are always the strongest part of the wing structure.

Control Surface mountings - Wings Figure24

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1.3 STABILISERS

The vertical stabiliser (fin) produces directional or lateral stability, whilst the horizontal stabiliser (tailplane) produces longitudinal stability. As was mentioned in the aerodynamics section, these surfaces are of similar construction to the wings with spars, ribs, stringers etc,. They have to resist the twisting forces from the control surfaces mounted on the trailing edges. In many cases, the fin is similar to one half of the tailplane and on a number of light aircraft, it is actually constructed in this way, thereby simplifying production and component parts. Light aircraft have stabilisers manufactured from welded tube or fabricated from thin aluminium sheet of simple construction. As the aircraft size and weight increases, the surfaces will be made from stronger milled or machined skins and forged spars. Below can be seen examples of the empennage of light aircraft, Piper Cub and Cherokee and Cessna 150, showing their simple construction.

Empennage Construction Figure 25

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1.4 FLIGHT CONTROL SURFACES

The construction of most flight control surfaces is critical, since the designer wants to make them as light as possible. The control surfaces in the early years of aviation were a light, tubular frame covered with fabric and in later years when light alloy was adopted the quest for lightness continued. Today, metallic structures with honeycomb cores or epoxy reinforced composite construction are utilised for most control surfaces. The control surfaces are attached to the wing, fin or stabiliser by hinges, the spars being reinforced where these attachments are located.

The cutaway below shows an elevator from a Fokker 100 and it can be seen that the construction is very similar to other main surfaces. The only difference is that the rear half of the surface has no internal framework but instead, a core of shaped aluminium honeycomb with the skin adhesively-bonded to it.

Elevator Structure Figure 26

To prevent the risk of flutter, as previously described, the ailerons, elevator and rudder, are all constructed so that the part of the surface behind the hinge line, is as light as possible and a number of calibrated weights are added to the leading edge of the surface. These weights are known as mass balance weights, (see cutaway above) and the procedure is known as mass balancing.

In addition to mass balancing, surfaces that do not have the benefit of hydraulic power assistance, (see later) and are difficult to move when the aircraft is at high speed, have the benefit of aerodynamic balancing. To achieve this simply and as previously discussed, the hinge of the control is inset, so that part of the surface in front of the hinge line projects into the airstream, when the control is deflected from neutral.

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1.5 NACELLES AND PYLONS

It has been mentioned previously, how the nacelles and pylons are attached to the wings, generally and to other parts of the airframe on selected aircraft. The main purpose of all these engine fairings is to keep the engines outside of the airframe itself. There are several reasons for this, but the major reasons are that it is safer, in the event of a fire or explosion, if it isolated from the fuselage or the wings by firewalls. Also, it is much easier for routine maintenance and engine changes, if the engine is externally mounted.

Most nacelles are simply fairings which cover the power-plant in a streamlined manner, although, they usually also serve as the intake for jet and turbo-propeller engines. Most are covered by large, easy-to-open doors and panels, which allow quick and easy access. On some designs there can be smaller, quick release panels fitted into the larger ones, which allow access for maintenance, such as oil level quantity indicators, which need to be checked every time the engines are shut down.

On light aircraft, engine nacelles are usually fairly simple GRP fairings which are split into two parts and removed by releasing a few screws or quick release fasteners. These also contain a small intake for the air to reach the carburettor of the piston engine.

On many larger aircraft, particularly those with fan bypass engines, are fitted with thrust reversers as part of the cowlings. These are usually doors which translate rearwards and open up panels containing cascade vanes, which re-direct the exhaust thrust in a forward direction, when reverse thrust is selected after landing. These will be covered later in the power-plants chapter.

Although they are much more efficient that the older designs, modern jet engines produce harmful high frequency noise. One way that the noise may be kept below the safe and legal minimum, is by making the cowlings out of honeycomb sandwich, which as well as being very light in weight is excellent at absorbing sound. The honeycomb can be manufactured from glass or carbon fibre and covered with composite or light alloy skin facing panels.

The pylons which support the engines fitted on to the wings or the rear fuselage all have one main purpose, which is to transmit the full thrust of the engines into the airframe. They must be extremely strong and yet flexible, as the wing mounts especially have to move with the flexing of the wings.

On many large aircraft, the space within the pylons is utilised to fit such components as heat exchangers, (radiators); air valves; fuel valves; pipes containing air, oil and fuel and electric cabling.

All engines must be isolated from the rest of the aircraft, so that a fire can be completely contained within the nacelle and extinguished if the aircraft is equipped a fire extinguishing system. To this end, there will be a sealed bulkhead or divider between the engine and the airframe made of a fire resistant material

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All engines are subject to vibration that can be sensed inside the aircraft. To reduce this, the engine mounts are designed not only to hold the engine securely and to transmit the thrust, but the mounts themselves are fabricated with a shock absorbing material. This is usually an elastomeric or metallic woven block and will absorb a large proportion of the vibration providing the passengers and crew with a smooth flight.

Typical Fan Engine Cowlings Figure 27

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Cowling and Pylon Fairing Installation Figure 28

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INTENTIONALLY BLANK

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EASA PART 66 - MODULE 11.03 - Airframe Structures

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Contents

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1

AIRFRAME STRUCTURES - AEROPLANES

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