In the design of a fastener joint the fastener must remain intact and not fail based on the ultimate load design of the joint. If the fasteners were to fail, the joint would simply come apart. In an ultimate joint failure the idea is for the joint to tear apart. If an improper joint is designed, the result could be a material shear-out failure, material tension failure, or fastener shear failure which is generally caused by the wrong diameter or material fastener, or insufficient fastener edge-margin.
Requirements for edge-margin, the distance from the center of the fastener to the edge of the part, and the distance from the center of a fastener to the center of the next fastener must be observed to ensure a proper joint. During aircraft fatigue testing it has been found that an equally spaced fastener joint will outperform a staggered fastener joint design. However, a staggered fastener joint design is favored in fuel tank applications as they provide a greater faying surface sealing area.
Adherence to proper process is critical for the installation of aircraft fasteners. Like the aviation saying “Inspection cannot build quality into a part, it merely verifies its presence” applies to aircraft fasteners. Once a solid rivet is bucked, the Hi-Lok collar is tightened, or the blind-rivet is pulled, the quality of both the hole and countersink are not visible.
Traditionally when countersinking, it is acceptable to have the fastener a little above flush rather than below flush after installation. The only exception would be near an area having specific aerodynamic smoothness requirements such as near the reduced vertical separation minimums (RVSM) critical part areas. Countersinking below flush can create a knife-edge condition between the layers of materials and can eventually cause a crack emanation or shearing of the fastener head.
Proper fastener-hole size and finish must be maintained based on the fastener type and material the fastener is going through. Rivets are traditionally installed in a clearance-fit whereas the hole is larger than the fastener. Bolts and Hi-Loks are installed in a transition-fit hole where the fastener and structure are roughly the same size. In some cases a fastener being installed into steel or titanium requires a close-ream hole to prevent the removal of protective coating on the fastener during installation. To get an exact and quality size and finish hole the use of reamers and core drills are required. The key is to have all of the fasteners in an area or repair to “load-up” together.
Criteria for inspection, removal, and substitution of aircraft fasteners must be closely followed in order to carry the load through an aircraft structure. The criteria may be found in the previously mentioned references. If removal of a fastener is required for additional access or because of improper installation, care must be observed to prevent damage to surround structure and the fastener hole itself. Some fastener substitutions are allowed per the airframe manufacturer if the replacement fastener meets or exceeds the original strength, diameter, and corrosion protection characteristics.
After the aircraft, engine, or component manufacturer, or the repair engineer designs an assembly or repair, adherence to the maintenance and repair process is critical for loading and unloading of the fastener joint throughout its service life.
Marty Holzer is a certified AMT currently working as a line maintenance technician for Delta Airlines. Over the last 25 years he has worked in aircraft manufacturing, MRO, line maintenance, and as a technical instructor. Holzer has developed and taught structural repair courses which including reading and interpreting drawings for structural repair on commercial airline aircraft. He can be reached at email email@example.com.
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