Aircraft bolts and their proper application are fundamental to aircraft design, construction, and maintenance. It’s imperative that anyone working with nuts and bolts understands them correctly because any work on your aircraft usually involves nut and bolt fasteners.

Let’s first discuss bolt quality. Use no bolts or nuts that are not AN/NAS/MS certified. No argument! SAE bolt specifications are excellent, but the enforcement of the specs is not, so you can never be sure of what you’re getting.

AN, MS, and NAS hardware offer a range of fasteners manufactured and inspected to stringent standards. Dimensions are closely controlled, surfaces are fully finished and true, and the strength and hardness are consistent and dependable. They’re not only strong, they’re very tough; these bolts will bend before they break.

The aircraft application of these bolts is governed by the FAA-published ADVISORY CIRCULAR AC 43.13-1B thru –2A Combined. This advisory circular (AC) contains methods, techniques, and practices acceptable to the regulating bodies for the inspection and repair of our civil aircraft, only when there are no applicable or superseding manufacturer’s repair or maintenance instructions.

When it comes time to install these bolts and tighten nuts onto them, there are two ways to load the bolt. There is shear, which is a load perpendicular to the length of the bolt; and there is tension, which is a load pulling along the length of the bolt. Therefore, you’ll see at least two tables in reference documents dealing with the limit loads available for a given bolt size and strength. Unless otherwise stated by the bolt manufacturer, consult the torque values for aircraft bolts listed on page 7-9 of AC 43.13-1B.

In the end, what keeps the bolted joint tight is the residual tensile stress established in the bolt during the tightening process. That’s just a fancy way to say that tightening the nut on a bolt stretches the bolt. It also locks the male and female threads together. In actual practice, most bolts are improperly tightened. Fortunately, there are ways to counter improper torque settings. We’ll discuss them shortly.

If a bolt is going to clamp parts together, the bolt must be internally stressed in tension; otherwise, the clamping force simply won’t exist. In tightening a bolt to a specified torque value, we’re actually stretching the bolt and loading it in tension to a predetermined level of stress. The level of installed tension (known as “preload”) is more important to the strength and fatigue resistance of the bolt than its ultimate strength.

A bolt that’s properly preloaded, or stretched to its designated tension load, will resist other cyclic loads, like vibration, much better. A bolt that’s installed in an under-stressed condition may loosen under load and possibly fail. On the other hand, a bolt that’s over-tightened (stretched beyond yield) will fail during installation or will be weakened to the point where it’ll fail prematurely in use from fatigue under cyclic stress (vibration). Another possibility is that over-torquing the nut and bolt will damage the parts they’re attached to. This can also have disastrous consequences.

In the real world, it’s not practical to properly clean and torque every nut and every bolt every time. Further, since most of our applications involve only shear loads, in many cases, it isn’t even necessary.

What we’re really looking for is a level of installed tensile stress that is just below the yield strength of the bolt material. Standard AN torque tables are usually compiled for plated fasteners without lubrication (dry threads).

There is also a shear stress developed in the bolt by the friction caused by tightening. But that stress goes away almost as soon as the act of tightening is completed. With no movement between the nut and bolt, there’s no friction and thus the shear stress evaporates. Naturally, the tension stress remains.

This is the reason to always torque your bolts in steps – usually three steps – rather than just tightening the bolt to its final torque all at once. It is also the reason why most mechanics click the torque wrench twice on the final tightening operation.

The use of a torque wrench is the most common way of ensuring a tight joint. I recommend investing in a good quality torque wrench and ensuring its calibration regularly. For more critical applications like connecting rod bolts or head bolts, they may specify actually measuring the stretch of the bolt (micrometer) or using a specified angle torque device. This is likely beyond the scope of most of us in our type of flying and maintenance, though.

Another critical aspect of nut and bolt use is making sure these fasteners are safety locked. You must lock or safety all bolts and/or nuts, except self-locking nuts. This is done in a number of ways. Parts subject to rotation, or in a spot where a self-locking nut might not be enough insurance, must be cotter-pinned.

Naturally, this requires the use of castle nuts, which have slots cut to match the hole in the bolt. An important note about using castle nuts: If you torque the nut to spec and the pin slots don’t match up with the pinhole in the bolt, never loosen the nut to insert the pin. It won’t hurt to tighten the nut that small amount beyond the specified torque so that they will line up.

Bolts without locking nuts must be safety wired, which is done at the bolt head. The safety wiring technique is a discussion for another day, but if you want to see an excellent example of safety wiring bolt heads, look at how prop bolts are safetied. Most of the planes we fly use prop bolts with drilled heads to allow safety wire through.

Recommended for further detailed reading on this subject is the “Motorbooks International NUTS, BOLTS, FASTENERS, and Plumbing Handbook” by Carroll Smith. It’s a very complete guide to hardware for race and street cars (which use a lot of aircraft-grade hardware and fittings).

If you are doing ANY work on your airplane, you really should have a copy of AC 43.13-1B / 2A Acceptable Methods of Aircraft Repair. This book is available from bookstores (recommend Calgary Pilot Supply) for approximately $30.00. You can also download the manual (in whole or in part) in PDF format from the FAA’s website in the online Regulatory and Guidance Library. Also, make use of the knowledge of your fellow club members who have practical experience in construction and maintenance matters.