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Contents
1 AIRFRAME STRUCTURES - AEROPLANES
.............................. 1-3
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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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 1950s, 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.
Wing Planforms
Figure16
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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 doesnt 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
Turbo-Propeller Mounting Figure 22
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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 such as titanium or stainless steel.
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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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AEROPLANE
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Cowling and Pylon Fairing Installation
Figure 28
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AIRFRAME
STRUCTURES
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INTENTIONALLY BLANK
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Issue 1 04 Sept 2001 Page 1-1
JAR 66 CATEGORY B1
MODULE 11.03
AEROPLANE
STRUCTURES
engineering
uk