Hardware
INFORMATION: Hardware
Bolts and screws (fig. 3-128) are similar in that
both have a head at one end and a screw thread
at the other. However, there are several
differences between them. The threaded end of
a bolt is always relatively blunt. A screw may
be either blunt or pointed. Bolts and screws
may have right- or left-hand threads. A righthand
thread advances into engagement when
turned clockwise. A left-hand thread advances
into engagement when turned
counterclockwise. It is essential to always use
the correct hardware for the safe and efficient
operation of your aircraft. Worn, improperly
installed, loose, or missing items of hardware greatly affect the safety of your aircraft. A
sample of the hardware items you will use include bolts, washers, nuts, screws, rivets, cotter
pins, tubing, safety wire, clips, and clamps.
Purpose
Aircraft hardware is the term used to describe the various types of fasteners and
miscellaneous small items used in manufacturing and repairing aircraft. In its simplest form,
the purpose of hardware is to hold the airplane together.
Types of Hardware
Bolts
Aircraft use many types of bolts and each type used is for fastening something in place.
When manufacturing aircraft bolts either for general purpose or for specific use, companies
conform them to a rigid set of standards. Manufacturers create bolts of different dimensions
using various strengths of materials. The key to replacing aircraft hardware items is always
replace hardware items with the item called for in the aircraft’s -4 Illustrated Parts
Breakdown. In selecting a bolt, you should know the bolt’s dimensions.
Parts of a Bolt
The three principal parts of a bolt are the head, thread, and
grip (the unthreaded part) (fig. 3-129). Two main parts make
up a bolt, the head and the shank (threads plus grip). The most
common type of head is the hex head. This type of head may
be thick for greater strength or relatively thin in order to fit in
places having limited clearance. In addition, the head may be
common or drilled to safety wire the bolt. Some examples of
bolt head types include the following:
• Eyebolt (often used in flight control systems)
• Internal-wrenching bolt (tightened and loosened by
using a hex wrench)
• Clevis bolt (characteristic round head)
• External-wrenching head (12-point head gives a
greater gripping surface)
• Hi-torque style (installed with a special hi-torque
adapter)
Regardless, all bolts have a shank portion. The shank (fig. 3-130)
includes the grip length and the threaded area. The grip is the
unthreaded part of the bolt shaft. It extends from the threads to
the bottom of the bolt head. If a bolt is too long or too short, its
grip is usually the wrong length. When using a bolt, the grip
length should be approximately the same as the material’s
thickness. If the grip is too short, the threads of the bolt will
extend into the bolt’s hole and may act like a reamer when the
material is vibrating. To prevent reaming, no more than two
threads should extend into the bolt’s hole. If the grip is too long,
the nut will run out of threads before it tightens. In this event, use
a bolt with a shorter grip, or if the bolt grip extends only a short
distance through the hole, a washer (shim) may be used. Correct
length selection is indicated when the bolt extends through the
nut at least two full threads.
The shank’s remaining length has threads. The diameter of the bolt shank determines its size.
The results of using a wrong diameter bolt should be obvious. If the bolt is too big, it cannot
enter the bolt’s hole. If the diameter is too small, the bolt has too much play in the bolt’s hole,
and it is likely not as strong as the correct bolt. The threads come in one of two types: Coarse
and Fine (fig. 3-131). The two are not interchangeable. For any given size of bolt, there are a
different number of coarse and fine threads per inch. For instance, consider the 1/4-inch bolt.
Some are 1/4-28 bolts because they have 28 fine threads per inch. Others have only 20 coarse
threads per inch and are 1/4-20 bolts. To force one size of threads into another size, even
though both are 1/4 of an inch, can strip the finer threads of softer metal. The same result is
true concerning the other sizes of bolts; therefore, it is important to be certain that the bolts
you have selected have the correct type of threads.
Stresses
Primary factors in aircraft structures are strength, weight, and reliability. Numerous forces and
structural stresses act on an aircraft when it is flying and when it is static. Understanding the
stresses that your aircraft’s structure encounters help you in understanding why aircraft are
built the way they are. Stresses occur in any material that is subject to a load or any applied
force. When your aircraft is static, the force of gravity produces weight, which the landing
gear supports. The landing gear also absorbs the forces imposed during takeoffs and landings.
During flight, any maneuver that causes acceleration or deceleration increases the forces and
stresses on the wings and fuselage. The empennage, or tail section, absorbs the same stresses
and transmits them to the fuselage structure. Therefore, the loads experienced by your aircraft
include tension, compression, shearing, bending, and torsion. Even though there are many
types of stresses, only two main types will be covered: “Tension” (pulling) and “Shear”
(cutting).
Tension stress (fig. 3-132) is the force of stretching an object or pulling at its ends. Tension is
produced by two forces pulling in opposite directions along the same straight line. For
example, tensile strength indicates the ability of the bolt to withstand the stretching force of
tension that occurs when the bolt and nut are tightened or torqued. Tension stress failure is a
pulling force that separates the bolt head or threads from the non-threaded area (grip).
Shear stress (fig. 3-133) is the force that tends to cut or break a bolt across its grip length by
sliding one part over the other in opposite directions. Cutting a piece of paper with a pair of
scissors is an example of shearing action. Shear strength refers to the hardness or resistance to
the cutting action that can occur if the joined materials shift sideways in opposite directions.
Bolt Types and Uses
Most aircraft structural bolts are general-purpose hexagon head bolts, internal wrenching
bolts, or close tolerance bolts. You need to have a good foundational knowledge about bolts
and their intended uses. The following material only represents a small portion of the
available variations.
General-Purpose Bolts
The hex head aircraft bolt is an all-purpose structural bolt used for general applications
involving tension or shear loads where a light drive fit is permissible. Light drive fits are those
requiring light assembly pressures and produce permanent assemblies.
Drilled Head/Shank Bolts
Drilled head bolts (fig. 3-134)
differ from the standard
hexagon head bolts. The
drilled head bolt has a hole
drilled in the head for
securing with safety wire. A
drilled hole is in each face of
the bolt head. Drilled shank
bolts have a hole drilled through the shank of the bolt for a cotter pin or safety wire and is
commonly used with a castellated nut.
Close-Tolerance Bolts
Machined more accurately during manufacturing than general-purpose bolts. Made to precise
measurements for use when a tight drive fit is required due to load reversals and vibrations.
Use close-tolerance type bolts in areas such as engine mounts, landing gear, and wing mounts,
to name a few. To prevent sideways motion that could elongate the hole and/or shear the bolt
you must use close-tolerance bolts.
Internal/External Wrenching Bolts (fig. 3-135)
These bolts, fabricated from high strength steel, are suitable for use in both tension, and shear
applications. Therefore, both bolts are used primarily in high-tension stress areas. The head of
the internal wrenching bolt is recessed and designed to allow the insertion of an internal
wrench (HEX-wrench) when installing or removing the bolt. The twelve-point external
wrench bolt design accepts a 12-point socket for removal or installation.
Washers
Different types of washers are used in aircraft structures; they are plain, lockwashers, and
special washers. Be sure to use washers under the heads of both bolts and nuts unless
specifically omitted by the TO. A washer guards against physical damage to the material
secured and prevents corrosion of the structural members. Use aluminum alloy washers under
the head and nut of a steel bolt securing aluminum alloy or magnesium alloy components.
Corrosion will attack the washer rather than the components themselves. Use steel washers
when joining steel members with steel bolts.
Plain
Use the plain washer (fig. 3-136) under hexagon nuts to
provide a smooth load-bearing surface. A plain washer
can act as a shim in obtaining the correct relationship
between the threads of the bolt and the nut. The general
rule is to use one washer under each nut. However, if one
washer fails to cover the under-thread grip or fails to
place the nut in the proper position for cotter pins, then
one or more washers may be added. Change the bolt
length if you need more than two washers. Another function for the plain washer is for it to
protect soft surfaces from damage caused by the spring action of a lockwasher
Lockwashers
Use lockwashers (fig. 3-137) whenever the self-locking
or castellated type nut is not used. They provide
sufficient friction from the spring action of the washer to
prevent loosening of the nut because of vibration.
Special Washers
Special applications have special washers designed for them. There are many types of special
washers used in a wide variety of applications as specified in the applicable technical orders.
Tab and Keyed
Tab and keyed washer (fig. 3-138) use are in situations where a need occurs such as in helping
to hold items in place. A common example found is under the retaining nuts of switches. The
key of the washer engages a groove on the switch shaft and the tab lines up with a hole on the
mounting surface. These two features work together to keep the switch body from spinning in
its mounting location.
Countersunk
Countersunk washers (fig. 3-138),
commonly called dimpled washers; used
under the heads of bolts having a radius
(curved portion) between the head and
shank. The countersink feature of the
washer provides relief for bolt head to
shank radius, allowing a smooth seating
surface for the bolt. Install the washer with
the countersunk side toward the bolt head.
Nuts
Aircraft nuts come in a variety of shapes and sizes. Despite the many and varied differences,
they all fall under one of two general groups: self-locking and non-self-locking.
Purpose
The purpose of a nut is to properly load a bolted assembly (threaded fastener), such as a bolt,
screw, or stud. To do this there must be a mating condition of the thread and a correct torque
applied when tightening. A minimum of two threads must extend beyond the nut for proper
loading and locking. This applies to both self-locking and plain nuts. Use thick nuts for
tension stress and thin nuts for shear stress.
Types
Self-locking and non-self-locking aircraft nuts differ in design and material, just as bolts do,
because their design is to do a specific job with the bolt. For instance, some of the nuts are
made of cadmium-plated carbon steel, stainless steel, brass, or aluminum alloy. Markings on
the nuts themselves, does not identify the metal used in their manufacturing process. Instead,
the recognition of the material used must come from the luster of the metal.
Self-Locking Nuts
Self-locking nuts (fig. 3-139), also referred to as prevailing torque nuts provide tight
connections that will not loosen under vibrations. The locking action of these nuts eliminates
the need for external locking methods such as safety wire or cotter pins. Self-locking nuts
approved for use on aircraft meet critical strength, corrosion-resistance, and temperature
specifications. Provided there is no damage to the threads or no permanent distortion, selflocking
nuts are generally suitable for reuse in noncritical applications. NOTE: If any doubt
exists about the condition of a nut, use a new one! However, new self-locking nuts must be
used each time components are installed in critical areas throughout the entire aircraft,
including flight, engine, and fuel control linkage and attachments.
• Metallic Self-Locking—these may be used in areas where the temperature will rise
above 250° F. The locking action achieved is by metal-to-metal friction, usually a
deformed thread pattern or by spring tension at the top of the nut.
• Non-Metallic Self-Locking—these are constructed with a nonmetallic (nylon) insert,
which is designed to lock the nut in place. The insert is unthreaded and has a smaller
diameter than the inside diameter of the nut. This nylon insert develops a compressive
locking force on the bolt threads. Because of the nylon insert, these nuts use must
NOT be in an area subjected to temperatures in excess of 250° F.
Non-Self-Locking Nuts
The most common of the non-self-locking nuts (fig. 3-140), also referred to as free-spinning
nuts are the castle nut, the plain hex nut, the castellated shear nut, and the wing nut. Non-selflocking
nuts require the use of a separate external locking device for installation. There are
several types of locking devices (cotter pin, check nut, lockwasher, or safety wire) and since
there is no single locking device to use with all types of non-self-locking nuts, you will select
the one that is most suitable for the type of nut you use.
• Plain Hex Nuts have limited use on aircraft structures. When non-self-locking plain
hex nuts are used, they require an auxiliary locking device such as a check nut or a
lockwasher.
• Castellated Nuts are used with drilled shank hex-head bolts, clevis bolts, eyebolts, and
drilled-head studs. To secure these nuts it will be with a cotter pin or safety wire
because of their design.
• Wing Nuts are used where the removal of the assembly is frequent, and the desired
tightness is using your fingers. They tighten without using a wrench and sometimes
have a drilled hole in one wing for external securing with safety wire as needed.
Plate Nuts
Plate Nuts (fig. 3-141) are used for blind mounting in inaccessible locations and for easier
maintenance. They are available in a wide range of sizes and shapes. Floating plate nuts
provide a controlled amount of nut movement to compensate for subsequent misalignment.
They can be either self-locking or plain. You will find these nuts in locations where access is
limited, such as your aircraft panels and fairings.
The most common threaded fastener used in aircraft construction is the screw. The three most
used types of screws are the machine screw, structural screw, and the self-tapping screw. The
three head slots of screws include the straight (common), Phillips, and Reed and Prince.
Structural screws differ from structural bolts only in the type of head. Structural screws have a
definite grip length and the same shear and tensile strengths as the equivalent size bolt.
Structural screws are available in countersunk head and round head types. The commonly
used machine screws (fig. 3-142) are the flat head, round head, fillister head, pan head, truss
head, and socket head types.
Head styles vary depending on the application.
• Flat countersunk-head screws—used in countersunk holes where a flush surface is
necessary.
• Round/Pan/Truss head screws– used where the raised head height is not detrimental.
• Setscrews (fig.3-143)—used to position and hold components in place, such as gears
on a shaft.
• Fillister head screws (fig. 3-144)— have a hole drilled through the head for safety
wire.
A glance at any aircraft will show you the thousands of rivets in the outer skin alone, which is
just an indication of how important they are. Besides the riveted skin, rivets used are also for
joining spar sections, holding rib sections in place, securing fittings to various parts of the
aircraft, and for fastening bracing members and other parts together. It is important for you to
know how to inspect rivets. Most rivets used in aircraft construction are made of aluminum
alloy. Rivets (fig. 3-145) are metal pins used to join two or more parts together and installed
through drilled holes. When inspecting rivets, look for signs of failure like tipped heads,
looseness, and chipped/cracked paint. If the tipped heads are in the same direction and rivets
are loose in consecutive groups, the joint has undergone excessive load. Tipped rivet heads
that are in different directions and are not in groups are usually from improper installation.
With chipped/cracked paint, it may be necessary to remove paint to check the true condition
of the rivets. The presence of tipped heads, looseness, and chipped/cracked paint indicates a
failure of the rivet.
Pins
Pins used are in areas that carry shear loads (loads attempting to cut the pins). Three types of
pins used are taper, flat, and cotter pins.
Taper Pins
Taper pin (fig. 3-146) are used in joints that carry a shear load and where it is important that
no free motion (play) exists. The taper of the pin matches up to the tapered socket structure of
the component and the when the pin tightens it draws into the joint to keep the joint from
moving.
Flat-Head Pins
Flat-Head Pin (fig. 3-147) use is with rod end terminals or secondary flight control rods,
which do not operate continuously. The flat-head pin is secured with a cotter pin. The pin
normally installs with the head up to ensure the flat-head pin remains in the installed position
(does not drop out) in case of cotter pin failure.
Cotter Pins
Cotter pins are used to secure bolts, screws, nuts, pins, and in applications where such
securing is required. Cotter pins will not be re-used on aircrafts (will discuss more later in this
section).
Panel and Quick-Release Fasteners (fig. 3-148)
Fasteners are used to secure cowlings, plates, doors, and panels that require frequent removal
for inspection and servicing. They are generally locked and unlocked with only one-quarter
turn of the fastener. The Quick-Release fasteners design does not allow them to carry primary
structural loads. These fasteners are available in several different styles and usually referred to
by the manufacturer’s trade name like the Camloc, Airloc, and Dzus.
Cable, Terminal and Turnbuckle
Use these to connect various controls in the control cabin or flight deck to various operating
systems such as flight controls and engine controls.
Cable (flexible)
A cable (fig. 3-149) is a group of wires or a group of strands of wires twisted together into a
strong wire rope. Cables may be designated as 7x7 or 7x19 according to their construction.
A 7x7 cable consists of six strands of seven wires each, laid around a center (core) strand of
seven wires. A 7x19 cable consists of six strands of 19 wires, laid around a 19-wire central
strand. Most aircraft cable is either 7x7 or 7x19. The size of cable given is in terms of
diameter measurement. A 1/8-inch cable or a
5/16-inch cable means that the cable
measures 1/8-inch or 5/16-inch in diameter.
Aircraft control cables vary in diameters,
ranging from 1/16-inch to 3/8-inch.
NOTE: Cables are inspected for broken wires
by running a cloth over the length of the
cable, NEVER a hand. If the cloth snags, a
broken wire is indicated.
Terminals (fig. 3-150)
Cable terminals are swaged (tightly compressed) onto the cable, and the joint formed is
normally stronger than the cable itself. Styles include fork, eye, ball, and threaded end
allowing several installation options.
Turnbuckle
A turnbuckle (fig. 3-151) is a mechanical screw device consisting of two threaded terminals
and a threaded barrel. Turnbuckles are in the cable assembly for making minor adjustments in
cable length and for adjusting cable tension. One of the terminals has right-hand threads and
the other has left-handed threads. The barrel has matching right- and left-hand threads
internally. The opposite styles of threads (right- and left-handed) allow you to adjust cable
tension by turning the barrel assembly. After adjusting a turnbuckle properly, you must secure
it with safety wire or safety clips (fig. 3-152).
Tubing
Tubing assemblies transport liquids or gases (usually under pressure) between various
components of the aircraft system. Tube assemblies are used in aircraft for fuel, oil,
hydraulics, breathing oxygen, vent lines, and bleed air. Modern aircraft use corrosion-resistant
steel, titanium alloy, and aluminum alloy tubing. Corrosion-resistant steel tubing is used in
high-pressure hydraulic systems (3,000 psi and above) such as landing gear, wing flaps, and
brakes. Aluminum alloy tubing used is for both high-pressure and general-purpose lines.
Tubing installation starts with the correct size cushion clamps (fig. 3-153). Cushion clamps,
that are too small, can deform the mounting foot of the clamp, too large will allow the tube to
move and debris to get in between the clamp and tubing. The clamps supported must be at 15-
to-20-inch intervals and close to tubing bends as possible. Flexible grommets (fig. 3-153)
provide protection of tubing where the tubing passes through bulkheads (walls). Properly
installed grommets prevent chaffing (rubbing) between the tubing and the bulkhead material.
Fluid line identification color-coded bands (fig. 3-153) used are to indicate the functions,
contents, hazards, direction of flow, and pressure in fluid lines.
Hoses
There are two basic types of hoses (fig. 3-154) used in military aircraft. They are synthetic
rubber and polytetrafluoroethylene, commonly known as Teflon. Hose assemblies are used to
connect moving parts with stationary parts (allows for vibration and relative motion between
parts) and in locations where flexibility is needed. The indicator stripe (also called the lay
line) is a series of dots or dashes use in determining the straightness or lie of a hose (fig. 3-
159). Hose sizes are determined by the inside diameter and external wire braiding added to
hoses are used in high-pressure applications.
Proper installation prevents premature failure of the hose and protects it against damage from
chafing (rubbing). Clamping also prevents deflection of rigid (solid) connections between
hoses and tubing. Clamps (fig. 3-159) shall support hose assemblies at 24-inch intervals and
fit snugly around outside diameter of the hose. Grommets are used to protect hoses passing
through walls like the method used for tubing. In addition, a hose installs in a manner that
prevents excessive bending, crimping, compression or clamping of the hose. The indicator
stripe should not indicate any twisting in the hose.
Fittings
Fittings for tube connections are made in many configurations and styles out of materials such
as aluminum alloy, titanium alloy, corrosion-resistant steel, brass, and bronze. Use fittings to
connect tubing and hoses to each other and necessary components. For quick identification
purposes, a color code system (fig. 3-155) is used.
Blue—aluminum alloy
Black—carbon steel
Bright/Shiny Surface—stainless steel
Securing Devices
There are many different types of securing devices to stop rotation and other movement of
fasteners, bolts, screws, and pins that may come loose due to vibration in the aircraft. But the
three most common types of securing devices are safety wire, cotter pins, and check nuts.
Safety Wire
Safety wiring is the most positive and satisfactory method of securing. It is a method of
wiring together two or more units. The tightening of the wire counteracts any tendency of a
unit trying to loosen. Safety wire (fig. 3-156) comes in many types and sizes. The zinc coated
soft steel wire comes in thickness of .020 inch, .032 inch, and .041 inch diameters.
Single Wire Method
The single wire method (fig. 3-157) may be used on small screws in a closely spaced
geometrical pattern (triangle, square, circle, or rectangle). Secure the wire when both ends of
the wire meet at a fastener.
Double Twist Method
The double twist method (fig. 3-158) is the most common type of securing method used to
secure hardware. When you are safety wiring closely spaced bolts, the number that can be
safety wired by a 24-inch length of wire is the maximum in a series. The wire’s arrangement
is in such a manner that if the bolt or screw begins to loosen, the force applied to the wire is in
the tightening direction. When you are safety wiring widely spaced bolts by the double-twist
method, a group of three should be the maximum number in a series.
Safety Cable
Safety cable (fig. 3-159) use may be as a substitute for safety wire to prevent loosening.
Safety cable is available in sizes of .020 inch, .032 inch, and .041 inch. When safety cable
substitutes for safety wire in an existing installation (maintenance rework, etc.), use the
equivalent diameter of safety cable to that of the safety wire. This method of securing cannot
be used for any shear or break way applications.
NOTE: Minimize the mixing of safety wire and safety cable.
Cotter Pin
Cotter pins (fig. 3-160) are used to secure castellated nuts on
drilled shank bolts and screws. Some cotter pins are utilized to
secure flat head straight pins in push pull rods. Regardless of
shape or material, all cotter pins are used for the same general
purpose—securing. The cotter pin should fit neatly into the
hole with very little side or free play. Do not bend the prong
over the bolt end beyond the bolt diameter (cut it off if
necessary). Do not bend the prong down against the surface of
the washer. (Again, cut if off, if necessary). Do not extend the
prongs outward from the sides of the nut if you use the optional
wraparound method. NEVER reuse cotter pins.
Check-Nut
Use these nuts (fig. 3-161) as locking devices for non-selflocking
plain hex nuts, setscrews, and threaded rod ends.
This method consists of using two plain hex nuts: install
one on the bolt and torque, and then thread the second nut
down onto the first nut and torque. The friction of the two
nuts locked against each other prevents them from
loosening.
Lockwasher
Lockwasher (fig. 3-162) are used to prevent threaded
fasteners from loosening. The most common lockwasher
used is the helical spring washer (split ring). The
lockwasher flattens when torquing the bolt down. When
torqued, it acts as a flat washer contributing normal
friction for locking the screw or bolt and the working
surface.
Safety Clip
The Air Force has adopted only two methods for securing
turnbuckles as standard procedures: the safety clip locking
(preferred) method (fig. 3-163) and the wire wrapping
(safety wire) method as illustrated in (fig. 3-164). You must
examine safety clips after assembly for proper engagement
of the hook lip in the turnbuckle barrel hole by the
application of slight pressure in the disengaging direction.
NEVER reuse safety clips, as removal of the clips from the
installed position severely damages them.
Electrical Connectors
An important part of aircraft electrical maintenance is determining the correct type of
electrical hardware for a given job. These maintenance functions normally require a joint
effort on the part of you, the crew chief, and the electrician (specialist). It is important to
become familiar with wire and cable, connectors, terminals, and bonding devices.
Purpose
Electrical connectors are devices attached to the ends of cables and sets of wires to make them
easier to connect and disconnect. Many of the components used in aircraft systems rely on
several wires to carry electrical signals. Using electrical connectors simplifies installation and
maintenance of electrical equipment by making it much easier to remove devices without
disconnecting each wire individually.
Parts
Each connector consists of a plug assembly and a receptacle assembly. The two assemblies
coupled are by means of a coupling nut. Usually mounted on a part of the equipment.
Receptacle (fig. 3-165),
The receptacle assembly is the half of the connector to which the plug is connected. It is
usually mounted (fixed) on a part of the equipment. An alignment key (fig. 3-166) on the
receptacle assembly provides the correct connection with the plug assembly’s notch for
proper electrical pin alignment.
Plug
The plug assembly attaches to the cable end and is the part of the connector on which the
coupling nut (fig. 3-166) is mounted. The plug assembly is the removable part of the
connector. A threaded sleeve provides for securing of the two parts of the connector assembly
(plug/receptacle) together. You can see the plug assembly’s notch in (fig. 3-166).
Back Shell
The back shell (fig. 3-167) is on the back of the plug body to secure the electrical wires going
into the plug assembly. It provides strain relief for the inserted wires. The back shell separates
into two parts and a threaded ring or screws hold it together.