Aircraft Structurses - Part II
Aircraft Structurses - Part II
Aircraft Structurses - Part II
The integrity of an aircraft joint depends on the way the parts are attached together. The most common
method of attachment is by the use of rivets or more sophisticated types of rivets, known as fasteners.
However, where high strength is required, nuts and bolts are used whilst other structural assembly is
achieved by the use of adhesive bonding techniques.
Solid Shank Rivets
Special and Blind Fasteners.
Bolts and Nuts
Adhesive Bonded Structures
Methods of Surface Protection
SOLID SHANK RIVETS
The vast majority of aircraft structure is held together with solid rivets. As will be explained later, many
of the more modern designs use special fasteners and some bonded construction, but the majority are still
solid rivets.
In internal locations where a flat head rivet can be driven more easily than either a round or universal
head rivet, the AN442 Flat Head rivet may be used.
Where a smooth skin is important, flush rivets such as AN426 or MS20426, with a 1000 countersink head
are used.
Additionally, rivets with a different countersink angle, such as 900 and 1200 degrees can be found.
Nb; i. Universal and the 100° countersunk head are the most commonly used in aircraft structures.
Designed as a replacement for both the round and brazier head rivets. These rivets replaced all
protruding head rivets and are used primarily where the protruding head has no aerodynamic
significant. They have a flat area on the head, a head diameter twice the shank diameter, and a
head height approximately 42.5 %of the shank diameter.
ii. The countersunk rivet is primarily intended for use when aerodynamics smoothness is critical,
such as on the external surface of a high-speed aircraft.
1
Solid Rivet Identification
The identification marks on rivet heads serve two important functions.
Firstly, the marks are used to identify the rivet alloy required for a special installation area and,
secondly, the head markings are necessary when trying to identify which kind of rivets are being
removed from an aircraft during disassembly or repair.
The alloy identifying marks are made on rivet heads at the time they are being stamped out during
manufacture.
For non-structural applications, rivets made from pure aluminium, sometimes known as 'A' rivets, may be
used.
A very popular rivet is the 'AD' rivet, which has copper and magnesium added to the aluminium base
metal. This rivet is heat treated during manufacture to make it strong, whilst still being soft enough to be
formed easily.
When much more strength than the 'AD' rivets is required, there are two stronger rivets available. These
are 'D' and 'DD' rivets but they must be heat treated to make them softer before they can be formed. The
'D' types are of 2017 alloy and the 'DD' types are manufactured from 2024 alloy. Both of these rivet types,
after heat treatment, must be formed within a specific period of time (one hour for 'D' and ten minutes for
'DD' types) or they may be put into a refrigerator to maintain the softening effect. Once refrigerated these
will remain useable for about 10 days.
When riveting magnesium alloy sheets, there must be no copper in the rivet alloy, or dissimilar metal
corrosion will set in. Therefore, a 'B' rivet, manufactured from 5056 alloy is used. This contains a large
amount of magnesium with a little manganese and chromium but no copper.
2
Dimensions
Aircraft rivet dimensions are categorised by the diameter of the shank, ‘D’, and the length, ‘L’, measured
from the end of the shank to the portion of the head that will be flush with the surface of the metal. This
means that a countersink rivet is measured from the top of its head, whilst the remainder are measured
from under the head.
Rivet Dimensioning
Identification
The complete identification of a rivet includes its
head style,
its material,
its diameter and
its length.
The identification code shows the diameter as a number of 1/32ths of an inch and the length as a number
of 1/16ths of an inch.
For example,i. An MS20470AD4-4 has a universal head (MS20470), is made from alloy 2117 (AD), is
1/8" diameter (4 x 1/32”) and 1/4" long (4 x 1/16”).
ii.MS20426AD5-8
Identification codes
Identification codes used are derived from a combination of the Military Standard (MS) and National
Aerospace Standard (NAS) systems, as well as an older classification system known as AN for
Army/Navy
3
Repair rivets
The most frequently used repair rivet is the AD rivet because it can be installed in the received condition.
Some rivet alloys, such as DD rivets (alloy 2024-T4), are too hard to drive in the received condition and
must be annealed before they can be installed. Typically, these rivets are annealed and stored in a
freezer to retard hardening, which has led to the nickname “ice box rivets.” They are removed from the
freezer just prior to use. Most DD rivets have been replaced by E-type rivets which can be installed in the
received condition.
Sizing
The proper sized rivets to use for any repair can also be determined by referring to the rivets
(used by the manufacturer) in the next parallel row inboard on the wing or forward on the
fuselage.
Another method of determining the size of rivets to be used is to multiply the skin’s thickness by
3 and use the next larger size rivet corresponding to that figure.
For example, if the skin is 0.040-inch-thick, multiply 0.040 inch by 3 to get 0.120 inch and use
the next larger size of rivet, 1 ⁄8-inch (0.125 inch).
Installation of Rivet
Repair layout involves determining the number of rivets required, the proper size and style of rivets to be
used, their material, temper condition and strength, the size of the holes, the distances between the holes,
and the distance between the holes and the edges of the patch. Distances are measured in terms of rivet
diameter.
Rivet Length
To determine the total length of a rivet to be installed, the combined thickness of the materials to be
joined must first be known. This measurement is known as the grip length.
The total length of the rivet equals the grip length plus the amount of rivet shank needed to form a
proper shop head.
The later (shop head) equals (11/2) one and a half times the diameter of the rivet shank. Where A is
total rivet length, B is grip length, and C is the length of the material needed to form a shop head, this
formula can be represented as A = B + C. [Fig. 4-76]
4
Rivet Spacing
Rivet spacing is measured between the centerlines of rivets in the same row. The minimum spacing
between protruding head rivets shall not be less than 31 ⁄2 times the rivet diameter.(31/2D). The
minimum spacing between flush head rivets shall not be less than 4 times the diameter of the rivet. These
dimensions may be used as the minimum spacing except when specified differently in a specific repair
procedure or when replacing existing rivets.
Edge Distance
Edge distance, also called edge margin by some manufacturers, is the distance from the center of
the first rivet to the edge of the sheet.
It should not be less than 2 or more than 4 rivet diameters and the recommended edge distance is
about 21⁄2 rivet diameters.
The minimum edge distance for universal rivets is 2 times the diameter of the rivet; the minimum
edge distance for countersunk rivets is 21 ⁄2 times the diameter of the rivet.
If rivets are placed too close to the edge of the sheet, the sheet may crack or pull away from the rivets. If
they are spaced too far from the edge, the sheet is likely to turn up at the edges. [Fig. 4-79]
It is good practice to lay out the rivets a little further from the edge so that the rivet holes can be oversized
without violating
5
Rivet Pitch
Rivet pitch is the distance between the centers of neighboring rivets in the same row. The smallest
allowable rivet pitch is 3 rivet diameters. The average rivet pitch usually ranges from 4 to 6 rivet
diameters, although in some instances rivet pitch could be as large as 10 rivet diameters. Rivet spacing on
parts that are subjected to bending moments is often closer to the minimum spacing to prevent buckling of
the skin between the rivets. The minimum pitch also depends on the number of rows of rivets.
One-and three-row layouts have a minimum pitch of 3 rivet diameters,
a two-row layout has a minimum pitch of 4 rivet diameters.
The pitch for countersunk rivets is larger than for universal head rivets.
If the rivet spacing is made at least 1⁄16-inch larger than the minimum, the rivet hole can be oversized
without violating the minimum rivet spacing requirement. [Fig. 4-80]
SPECIAL FASTENERS.
When solid shank rivets become impractical to use, then special fasteners are used. These, you will
remember, are of two types; special and blind fasteners.
The term ‘Special Fasteners’ refers first to their job requirement and second to the tooling needed for the
installation. In certain locations, aircraft require strength that cannot be produced by a solid shank rivet,
so a special high strength fastener is used.
For example, if high shear strength is required, then special High Shear rivets are used. These are usually
installed with special tools and will be discussed later.
Blind Fasteners
There are several different types of blind fasteners which can be hollow or self-sealing. They include the
following types, all of which can be installed from one side of the work.
Chobert
Avdel
Tucker/Pop
Cherry
Note: It is most important that the correct tools are always used with the types of rivets mentioned above.
Chobert Rivets
These are available with a snap (round) head or a countersink head and are closed by forcibly pulling a
mandrel through the bore of the rivet. This closes the 'tail' and expands the rivet tightly into the hole. To
seal Chobert rivets, a separate sealing pin is driven into the hollow bore of the rivet.
6
Fig. 33 Chobert Rivet
Cherry Rivets
These rivets, of American manufacture, are similar to Avdel rivets, except that the stem is positively
locked in the rivet bore. During final forming, a locking collar is forced into a groove in the stem,
preventing further movement. After the closing operation, the remainder of the stem is milled flush with
the skin.
There are many different types of Cherry rivets, two of the most popular being the Cherry Lock and the
Cherry Max. The Cherry Lock, however, requires a range of closing tools for different sized rivets, whilst
the Cherry Max series can all be closed with a single tool.
Cherry Lock rivets are manufactured from 2017 or 5056 alloys, Monel metal or Stainless Steel, whereas
Cherry Max are made from 5056 alloy, Monel or Inconel 750. They are all available with either universal
or countersink heads and due to their positive locking method, can be installed in place of solid shank
rivets.
7
Cherry Lock Rivet
Figure 35
8
Special Fasteners
These can include Hi-Shear, Avdelock, Jo-Bolts, and Rivnuts. The first three are all formed by means of
a collar which is swaged into the grooves in fastener shank or expanded over the shank to form a blind
head. Rivnuts are formed using a similar method to cherry locks, but with a threaded mandrel screwed
into the Rivnut. The advantage of Rivnuts, (see Fig 38), is that after closing, a fixed nut is left behind
which may be used for the attachment of de-icing boots, floor coverings and other non-structural parts.
The dimensions required to identify a bolt are expressed in terms of the diameter of the shank and the
length from the bottom of the head to the end of the bolt. The grip length should be the same as the
thickness of the material being held together. This measurement can be found by reference to the
applicable charts. Bolt heads are made in a variety of shapes, with hexagonal being the most common.
9
General Purpose Bolts
All-purpose structural bolts used for both tension and shear loading is made under 'AN' standards from 3
to 20, the bolt diameter is specified by the AN number in 1/16"; for example:
AN3 = 3/16" diameter
AN11 = 11/16" diameter
The range is from AN3 to AN20 which have hexagon heads, are made from alloy steel and have UNF
(fine) threads.
The length of the bolt is expressed as a dash number. Bolts increase in length by 1/8" and the dash
number(s) will show the length.
For example:
AN3-7 = 7/8" long
AN3-15 = 1 5/8" long
Other markings will identify whether the bolt has a drilled shank, a drilled head for locking and indicate
what material the bolt is made from.
Clevis Bolts
These bolts (AN21 to 36) are designed for pure shear load applications such as control cables. The
slotted domed head, results in this bolt often being mistaken for a machine screw.
A clevis bolt has only a short portion of the shank threaded with a small notch between the threads and
the plain portion of the shank, which allows the bolt to rotate more freely in its hole.
Because the length of this bolt is more critical than normal bolts, its length is given in 1/16" increments.
Clevis Bolt Identification
Nuts
All nuts used on aircraft must have some sort of locking device to prevent them from loosening and
falling off. Many nuts are held in place on a bolt, by passing a split pin through a hole in the bolt shank
and through slots, or castellations in the nut. Others have some form of locking insert that grips the bolt's
thread, whilst others rely on the tension of a spring-type lock-washer to hold the nut tight enough against
the threads to prevent them from vibrating loose.
Sometimes, nuts that are plain with no locking devices are used and prevented from coming undone, once
they have been tightened, by the use of locking wire attached to an adjacent nut or to the aircraft structure.
There are two basic types of nuts, self-locking and non-self-locking. As the name implies,;
a self-locking nut locks onto a bolt with no external help, whilst
a non-self-locking nut relies on either a split pin, lock-nut, locking washer or locking wire, to stop
it from undoing.
10
Standard Nuts
Figure 41
Anchor nut.
Another type of nut in general use is the Anchor nut. These are permanently mounted on nut plates that
enable inspection panels and access doors to be easily removed and installed, without access being
required on the reverse side of the work. To make fitment of the panel easier when there is a large number
of screws, the nuts are often mounted 'floating' on their mounts, which allows for small differences in the
position of the attaching screws.
Tinnerman nuts
Although rarely used on large commercial airliners, Tinnerman nuts are manufactured from sheet steel
and are used mainly on light aircraft, for the fitting of instruments into the flight deck panels, the
attachment of inspection panels, etc. Some light aircraft engine cowlings have U-type tinnerman nuts
fitted over the inner edge of the cowling frame. When the retaining screws are tightened, spring action
holds them tightly and safely in place.
Examples of self-locking nuts, anchor nuts and U-type tinnerman nuts are shown in figures 42 and 43
above
11
ADHESIVE BONDED STRUCTURES
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.
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 place 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 44, 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 45) 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.
12
METHODS OF SURFACE PROTECTION
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, airtight 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
There are two metals most commonly alloyed with aluminium, to produce high strength skin and
component parts for aircraft manufacture. These are, Copper and Zinc. These alloys suffer extensively
from the effects of corrosion, so a cladding technique is used as a form of corrosion protection. ‘Alclad’
as it is termed is a soft, highly corrosion-resistant, pure aluminium skin, rolled onto the face of each
base alloy sheet, effectively sandwiching the alloy.
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).
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
13
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
14
15
2.7.12 CONSTRUCTION METHODS – ENGINE ATTACHMENTS
Engine mountings consist of the structure that transmits the thrust provided by either the propeller or
turbojet, to the airframe. The mounts can be constructed from welded alloy steel tubing, formed sheet
metal, forged alloy fittings or a combination of all three. Some typical examples are shown in Figures 27
to 29. All engine mounts are required to absorb not only the forward thrust during normal flight, but the
reduced force of reverse thrust and the vibrations produced by the particular engine/propeller
combination..
Fabricated Piston Engine Mounting
Figure 27
16
17