Torque is vital in the maintenance of aircraft. Torque is the measurement of the tension that is applied to a bolt. If too little torque is applied to a bolt, it can becomes loose and lead to failure of the joint.
If too much torque is applied to a bolt, it can stretch the steel of the bolt, which can reduce the life of the bolt in cycles of tension. Because of the risks of failure caused by incorrect application of torque, maintenance workers must follow the specific torque values of each joint on the aircraft. Bolts are categorized in two ways: tension bolt and shear bolts.
Aircraft Bolts and Torque Basics
Tension bolts are those that take the force of the aircraft’s components attempting to pull the component apart. Tension bolts typically has a part number stamped on the head of the bolt. Shear bolts are those that are design to take the side loads on a component, which can lead to the joint being sheared apart.
Shear bolts have slimmer profiles than tension bolts. Shear bolts must not be used in place of a tension bolt, as the shear bolt will fail under the stresses of the aircrafts flight. Torque specifications for aircraft components must come from official sources, such as AC 43.13-1B.
For instance, the specification for a quarter-inch AN4 bolt is 50 to 70 inch-pounds. If the bolt is lubricated with an approved lubricant, reduce the torque by 20%. Do not use lubrication on aluminum-to-aluminum joints, as this can lead to galvanic corrosion.
Do not use anti-seize on airframes.
nuts must match the bolt that is to be secured. Castle nuts have drilled shanks and cotter pins, and are used on primary structures. Self-locking nuts have deformations on the bolt head, which grips on the bolt by friction.
However, do not use self-locking nuts on aircraft structures beyond 15 cycles of use; the self-locking properties fails after 15 cycles. Fiber lock nuts can only withstand 250 degrees Fahrenheit; do not use fiber lock on components near the exhaust. All-metal high-temperature nuts can withstand much higher temperatures and must be used near the exhaust.
Washers can be used to distribute the bolt’s clamping force over the structure. Hardened steel washers prevent the nut from embedding itself into the component. Oversized washers are used with composite and wood aircraft components.
Do not use split lock washers on critical components; they can crush under the bolt and fail to maintain the joint. Use cadmium-plated steel washers with steel bolts. Use aluminum washers on delicate aircraft components.
The installation process for bolts follows a specific sequence. First, inspect the bolt for any damaged threads or cracked plating. Second, ensure that the grip length of the bolt is appropriate for the hole in the component.
Third, hand-tighten the bolt to ensure that the threads are properly engaged. Fourth, use a calibrated click wrench to tighten the bolt to the proper torque. Fifth, secure the bolt with a cotter pin.
Finally, apply orange lacquer to the nut and bolt. Tools used to install bolts must be precise in their application of torque. The micrometer click wrench is the most common tool used to install aircraft components.
However, the technician must calibrate the micrometer click wrench once per year. Beam wrenches do not drift when tightening bolts. Grip gauges ensure that the bolt has the proper fit in the component.
Never use an impact gun to install bolts; it can damage the threads. Head markings indicate what type of bolt is used on an aircraft component. A single dash indicates that the bolt is made of standard cadmium steel.
A double dash indicates that the bolt is made of corrosion-resistant stainless steel. Triangles indicate that it is a NAS bolt; close-tolerance specifications. Crosses indicate a heat-treated bolt made of alloy metal.
The bolt size specifications use a scale of numbers. For instance, AN3 means that the bolt is three-sixteenths of an inch in diameter; AN8 means that the bolt is one-half inch in diameter. Many errors can occur during the installation of aircraft components and bolts.
For instance, an individual could use a shear bolt instead of a tension bolt. Another error is the use of locknuts beyond 15 cycles of use. Additionally, an individual might forget to apply torque seal to bolted joints; this will allow tampering with the structure of the aircraft to go unnoticed.
Finally, applying anti-seize to aluminum-to-metal joints will cause stress corrosion in the aluminum.
