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AIRFRAME TECHNOLOGY Traditionally when countersink- 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 fas- tener head near or far based on the view of the drawing. Some aircraft doublers appli- cations can be creative, such as requiring a thin titanium doubler installed on the inside of the air- craft skin. In this specific example, the head of the rivet is on the inside of the fuselage and the out- side skin is countersunk 82 degrees with the rivets bucked into the countersunk then shaved to flush for smoothness. Joint design 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 fas- teners 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 www.AMTSociety.org • www.AMTonline.com • Aircraft Maintenance Technology September 2011 57 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. Process driven 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 veri- fies its presence" applies to aircraft fasteners. Once a solid rivet is bucked, the Hi-Lok collar is tight- ened, or the blind-rivet is pulled, the quality of both the hole and countersink are not visible. ing, 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 aerody- namic 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 mate- rial 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 fas- tener 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