Hardware

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The term aviation hardware refers to all the nuts, bolt, pins and fittings that hold your flying machine together.  Hardware connects your hoses and cables to the proper spots and generally holds everything else in-between together. It allows components to be attached rigidly or allow the free movement of parts.  The use of the proper hardware is critical in the construction of an aircraft.  Aircraft use a special type of hardware that is superior to the general nuts and bolts we have stored in the garage.  This hardware is made to military specifications and is certified for aviation use.  It is stronger and more corrosion resistant than common hardware.  It is specialized, just like the rest of your airplane.  You shouldn't skimp on your hardware any more than your engine or propeller.  This information is not intended to teach you everything, but to give you enough knowledge to recognize what you need, and go out and find it.  Aircraft grade bolts have designations.  AN hardware is manufactured to Air Force-Navy specifications, MS bolts are certified to a military standard and NAS bolts comply with National Aerospace Standards.  The most common hardware you will find at your aviation parts supplier are AN bolts, nuts, washers, pins etc.  These come in a variety of configurations to suit specific requirements.

Nuts and Bolts

Aircraft bolts are fabricated from heat treated corrosion resistant 4037 or 8740 steel producing a strength in excess of 125,000 psi and are cadmium plated. The low carbon steel hardware store variety of bolt has a 50,000 or 60,000 psi strength.  This strength discrepancy applies to most hardware. 

Bolts may have a hole drilled in the threads that is used for a cotter pin when a castle nut is used, or they may have a hole drilled in the head that is used to safety wire the bolt in place.  Other bolts use locknuts that have a fiber insert that gives a friction grip that locks onto the bolt.  These are also referred to as stop nuts.  These should not be reused after removal when you can turn the bolt by hand since their gripping power is diminished. The inserts in fiber insert locknuts can be compromised at high temperatures.  In high temperature situations all metal locknuts are used.  All these options are designed to reduce the risk of a bolt loosening during the vibration created in flight.  Stainless hardware is also available.  

Choosing the right bolts is generally not that difficult.  You can look at your suppliers websites and learn a lot about the available hardware and its applications.  There are also publications that delve deep into the subject.  And most plans and kits will specify the proper bolts and size.

So remember that all nuts and bolts aren’t the same.  Don’t skimp when buying hardware for your aircraft project.  It only takes one broken bolt to ruin your day!

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Drilled AN bolt.

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AN locknuts.

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AN castle nuts.

Washers

There are three types of washers used on aircraft.  They are plain (or flat) washers, split lock washers and spring washers.  Most washers are made of cadmium plated steel but where dissimilar metals are a concern with aluminum or magnesium structural components, aluminum washers can be used. 

 

Plain flat washers can be used under the head or nuts of the bolts.  They are used under the head of bolts that are screwed into tapped holes in the structure.  These washers provide a smooth bearing surface and can act as a shim to get the correct grip length of the nut and bolt combination.  They may also be used under lock washers to prevent damage to the component surface. 

 

Both split and spring lock washers are used in applications where self locking or castile nuts are not appropriate.

Screws

There are three basic types of screws used in aircraft construction: structural screws, machine screws and self-tapping screws.  Structural screws are made of heat-treated alloy steel and may be used in place of aircraft bolts.  Unlike the other screws, these have an unthreaded portion of the shank.  They can have round, button or countersunk heads.  

Machine screws are made from low carbon steel or brass and come with round and countersunk heads.  They are not used in structural applications but more commonly are used attaching instruments to the instrument panel and on fairing and inspection covers.

 

Self-tapping, or PK screws, as the name implies tap their own threads when installed.  They also come with round or countersunk heads.  They can be used to attach inspection covers, fairings and other nonstructural parts.  They can be used in conjunction with a tinnerman nut for extra holding power, especially where the screw is removed and reinstalled with some frequency.

Again, these should be aviation grade screws and not the hardware store type.  
 

AN Fittings

AN fittings are aircraft grade fittings used to connect oil and fuel lines between various components of the engine and fuel systems.  They are made of aluminum or steel and are generally anodized blue.  Steel should be used for steel connections and aluminum for aluminum connections.  They use standard pipe  threads for the connections.  They are an intranigral part in setting up the fuel and oil systems, but may be needed for other aspects like setting up a smoke system or oil overflow lines.    

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The blue fittings in this photo are AN fittings used in various applications.

Aircraft Control Cables

Many aircraft use steel control cables to actuate the flight controls.  Control cables are made of preformed corrosion resistant steel that is made to military specifications and has a special lubricant applied.  The cables run from the control stick or yoke and rudder pedals, to the ailerons, rudder, elevator etc.  They are routed through the airframe by the use of pullies to change their direction or add tension in long runs.   The cables are routed through fairleads to keep the cable away from the structure and eliminate the cable rubbing on the airframe.  Cables come in sizes from 1/32 to 1/2 inches with most control cables being 1/8 to 1/4 inches. 

There are three basic types of cables which are used for different applications.  They are: nonflexible, flexible and extra flexible.    Nonflexible cable is used for straight runs where it does not change directions over pulleys.  It comes in 7 or 19 strands (1 x 7, 1 x 19).  Each strand is a single wire.  Flexible cable is made of seven strands.  Each of the seven strands is made up of 7 wires.  These cables can be bent around pulleys to some extent but should not be subject extreme bending or flexing.   They are used for engine controls and other applications where straighter runs are possible.  Extra flexible cable is made of seven strands each of which contain 19 wires (7 x 19).  These are the cables used in primary flight controls and can change direction over pulleys without damage (within reason). 

Control cables are made to length for the application and come with one of several types of swaged ends.  These ends slip over the cable and a rotary swaging tool is used to compress the end which grips the cable to create a very strong bond.

The common cable ends are: plain ball, ball and shank, fork end, eye end, stud ends with right or left threads, turnbuckles with right or left threads and copper or zinc sleeves.  I am sure there are other ends that you could find for your own special application.

Each end has a specific purpose and attachment process.  For example, turnbuckles and studs are generally used to adjust cable tension and studs can be used with threaded clevis terminals.  Forks are used with clevis bolts to allow a free moving attachment to the controls and control surfaces. 

Many cable runs are made up of two cables.  Each end may have a fork - either threaded or swaged - with the other end of each being studs that connect together with a turnbuckle that is used to adjust the tension of the cable.

As stated in other hardware information, aircraft control cables are not the hardware store steel cable variety, they are specialized and made to military specifications, as are the ends.  While legally you could use hardware store cable, why risk it!

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7 x 19 control cable.

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Threaded fork end.

Rivets

 

Rivets are extensively used in the construction of aluminum aircraft.  They are the primary fastener that hold the structure together.  Solid aluminum alloy rivets are the main rivets used, but blind rivets are used in some applications.  Rivets are not just used on aluminum and steel, but can be used with composite materials, fabric and more.

Solid rivets are made from several different aluminum alloys to suit certain needs.  Most have a unique mark on the head that denotes the material composition of the rivet.

 

 

 

 

 

 

 

Modern rivets use one of two head types: countersunk or universal.  Universal rivets have a rounded head that protrudes above the surface of the material being joined and countersunk heads are flush with the surface.

Common rivets suitable for aviation applications come in hard and soft forms. Soft rivets are generally made of 1100 aluminum and have a material code of A. These rivets are made out of pure aluminum and have a low tensile strength (usually 16,000 psi).  Soft rivets are used with softer aluminum alloys such as 3003 or 5052.  Soft rivets are not used in structural components, but are suitable where strength is not a consideration.  They have no marking on the head.

Hard, or structural rivets, are made from 2117T aluminum and have the material code AD.  They have a tensile strength of 38,000 PSI and are highly resistant to corrosion.  They have a single dimple marking on the head. These should be used for all structural applications.

An aircraft will require many different sizes of rivets during the construction process.  Choosing the right rivet for the job is critical.  If the rivet is too long or short the “shop head” created on the opposite side of the head will not form properly.  When the rivets are properly squeezed or driven the shop head should be 1.5 times the rivet shaft diameter in width and ½ the rivet diameter in height.  These are the ideal dimensions of a shop head but are not required for every rivet.  There are allowable variations in the shop head.  You can purchase a shop head gauge to check if your rivets fall in the allowable range, but it becomes pretty easy to eyeball with a little practice.  The proper size drilled hole is also very important.

 

Solid rivets are usually either squeezed with a hand rivet squeezer or driven with a pneumatic rivet gun and bucking bar.  There are other options that may need to be employed in certain situations. 

Blind rivets are used when there is only access to one side of the rivet. They are similar to the basic pop rivets we all know, but in aviation they are specialized hardware.  Cherry rivets are a self-plugging friction lock rivet that can be used in some structural applications.  The head on the blind side is formed by pulling the tapered stem into the hollow shank. This swells the shank and clamps the skins tightly together then the stem pulls in two. The stem wont fracture flush with the rivet head so must be filed flush with the head.  These rivets are sensitive to vibrations because of the friction-locking stem. They should be inspected and loose rivets replaced.  Removal consists of punching out the friction locked stem and then drilling it out.

Mechanical-lock rivets, or rivet nuts, have a device on the puller, or rivet head which locks the center stem into place when installed. These rivets are not susceptible to the stem failing out like with cherry rivets.

To find more in-depth information on rivets I strongly advise looking at the FAA Advisory Circular AC43.13-1B page 4-17.  You can also view a video by Homebuilt Help here  Working with Solid Rivets on Homebuilt Aircraft

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