An Illustrated Guide to Reading Aircraft Torque Bolt Charts

There are three reasons why you might want to read an aircraft torque bolt chart. The first is, you’re in school. If that’s the case, congratulations! We hope you take the knowledge with you when you graduate. But learn your lessons well—the repercussions for misreading an aircraft torque bolt chart after you graduate are far more severe than any blown quiz. Once you’ve graduated, there are only two primary reasons to read an aircraft torque bolt chart: repair and design.

In the case of repair, you’ll look at a standard aircraft torque bolt chart to determine the appropriate level of torque to apply to a bolt you’re tightening or replacing. However, before you turn to a standard torque chart, you should first search for information in the Maintenance Instructions Manual and Illustrated Parts Breakdown that your aircraft manufacturer supplied with your aircraft. It is always preferable to use aircraft-specific torque information developed by the manufacturer over general or standard torque values. The only times you’ll use a standard aircraft torque bolt chart is if the manufacturer did not specify the torque for your bolt or if you do not have access to the manufacturer’s specifications.

When it comes to designing new aircrafts or aircraft components, you won’t be able to turn to manufacturer’s specifications, because you’ll be coming up with them. Without aircraft-specific torque specifications to follow, standard aircraft torque bolt charts will give you a good way to estimate the amount of torque you’ll need. These charts will help you get very close to proper torque for all your fasteners. However, the only way to truly determine the right amount of torque is to test it to failure, repeatedly. Knowing how to use an aircraft bolt chart, then, may help you reduce the amount of test cycles it takes to certify your torque specifications as airworthy.

Presumably, you searched for a guide to aircraft torque bolt charts with a sound understanding of proper torque in aviation. If not, however, the long and short of it is: torque matters. Without the proper torque, bolts or screws can fail. In aviation, if a bolt or screw fails, a safe flight can quickly be interrupted by an “unscheduled hard landing.”

Aviation bolts and screws without the proper torque can fail in a number of ways. First, if they have not received enough torque, they can loosen under vibration and come free. Not only do they then fail to hold their load, they also become foreign object debris and may cause significant damage as they rattle around. In fact, vibration is such an important factor in aviation that every bolt which is not fastened with a self-locking nut must also be safetied, usually with wire, to prevent it from vibrating loose.

Aviation bolts and screws can also fail from having received too much torque. Over-torquing can deform the fastener, the substrate or both. This will weaken the fastener or substrate, rendering it more likely to fail. Aircraft bolts are subject to multiple high stresses, including shear force, tension force, vibration, and expansion/contraction due to temperature changes. This makes them likely to fail when used in conditions beyond their rating, and this failure can happen in the blink of an eye.

Finally, aviation bolts are torqued specifically to distribute loads in a deliberate manner throughout the aircraft. This is done to combat the high levels of stress placed on each structural member. The faster an aircraft flies, the greater the stress. Naturally, in the space portion of the aerospace industry, the high speeds involved render this of supreme importance. Applying an incorrect amount of torque can unbalance the load distribution of the aircraft, placing more stress on certain structural members than they are rated to handle. Should this cause a structure to break down, the result can be cascading failure throughout the aircraft’s structure.

The golden rule when replacing aircraft bolts or screws is, “replace same with same.” The aircraft you’re working on has been tested and rated for the bolts and screws it carries—not for anything else. If you still have the bolt you removed from the aircraft or a supply of identical bolts, replacing it is simply a matter of ensuring the bolt is undamaged, then finding and applying the right torque. If you do not have the same bolt or a known identical copy, you’ll need to identify the type of bolt you need and procure it prior to installation. In addition to the bolt, you’ll need to procure identical copies of the nuts and washers which complete the bolt assembly. This chart will guide you through the process:

That’s it! While identifying your materials, fastener, and torque values can seem complex, it’s an unavoidable aspect of aviation maintenance and development. While this piece has primarily focused on the repair aspect of aircraft torque, the development side makes use of all the same information. It’s just as essential to identify the correct materials, fasteners, and torque values when creating a prototype aircraft as it is when repairing an aircraft in service. The major difference between the two applications is that when developing a new aircraft, engineers don’t have any manufacturer’s specifications to follow or existing bolts to look at. This can be overcome by consulting standard torque bolt charts, similar aircraft in service, and old-fashioned trial and error.

Source for all pictures not otherwise specified: AMT General Handbook.