Aircraft Fasteners

How to select the correct type of fastener


In the world of aircraft construction and repair, aircraft fasteners are utilized to assemble detail parts that are combined together with other parts into assemblies, which are assembled into installations that finally end up as a complete aircraft.

An interesting statistic is aircraft fasteners make up half of the 6,000,000 parts used to fabricate a Boeing 747-800 aircraft.

The purpose of the fastener is to connect all the different parts together in primary structural areas, secondary structure, pressurized and nonpressurized applications, and to transfer loads from one part to another in both production and repair applications.

Fastener information may be found in various sources which may include the specific aircraft maintenance manual chapter 20, the specific aircraft structural repair manual, or aircraft production and repair drawings. Advisory Circular 43.13-1B and the Aircraft General Handbook which can be found online at the FAA web site http://www.faa.gov also contain fastener information.

Fastener utilization and types

When engineers design an aircraft many things are considered when choosing the correct type of fastener. The type of joint the fastener will be exposed to in its application; shear or tension. What types of loads will be transferred through the joint. Aircraft loads may include those experienced during towing, normal flight operations, wind-gusts, pressurization, engine-out operations, landing, and more. All of this will determine how thick or thin the structure will have to be, the material type of the original structure, and the associated fasteners.

Fasteners must be able to achieve the transfer of load from one part to another. An example of this is the load transferred from an engine to a pylon, the pylon load to the wing, and the wing to fuselage. Fastener numbers and diameter are calculated to transfer this load. Other criteria are also needed to select the best fastener for the installation. This could include weight, inspect ability, tooling requirements, aerodynamic smoothness, access, corrosion protection, and of course cost.

Fasteners can be placed into many groupings which may be used as structural fasteners that take aircraft loads, to nonstructural fasteners that connect nonload bearing parts. There are restricted access applications or blind fasteners such a Huck Lock Bolts, Composi-Locks Fasteners, and CherryMAX fasteners.

For areas with access to both sides, standard rivets, structural bolts, and Hi-Lok fastening system fasteners are used. Materials for these fasteners include aluminum, steel, and titanium and are coated to prevent dissimilar metal corrosion. Styles of fastener heads vary from countersunk to protruding head based on the aerodynamic requirements of the aircraft.

Fastener codes and orientation

Fastener coding can be designated by the fastener manufacturer such as a CR3233 CherryMAX rivet, by an industry standard such an AN4 bolt, or by the airframe manufacturer as in a BACR15CE5D3 rivet from Boeing. Coding descriptions can be found in various fastener books, on the repair drawing, or on the production blueprint. Also fastener codes may be used to simplify repair and productions drawings. These fastener codes may be found in the upper-left part of the drawing quadrant.

It is standard for the head of a fastener to be installed head-up, or head-forward. However, the blueprints and repair drawing will give proper orientation in the fastener quadrant, normally the upper-right corner of the fastener quadrant, and will call-out the fastener head near or far based on the view of the drawing.

Some aircraft doublers applications can be creative, such as requiring a thin titanium doubler installed on the inside of the aircraft skin. In this specific example, the head of the rivet is on the inside of the fuselage and the outside skin is countersunk 82 degrees with the rivets bucked into the countersunk then shaved to flush for smoothness.

Joint design

This content continues onto the next page...

We Recommend