Positive Capture

Many homebuilders are folks who have been working on cars, trucks, boats and home construction most of their lives. They bring a lot of real-world experience in mechanical work to the task, and for the most part, this is great! Learning good craftsmanship takes time and practice, and most of us that have been working at it for 50-plus years are still working at it, which is the mark of a true craftsman.

While I am a great proponent of bringing skills and knowledge from one activity into another, it is important to realize differences and where those differences lie. The most common expression you hear is that if your car engine quits, you coast over to the side of the road and say a few bad words. If your airplane engine quits, those bad words might be louder and considerably longer—assuming that you have found a good place to set down. So keeping the airplane engine running is considered by most to be a lot more critical.

That criticality in aviation mechanics is important because, for the most part, airplanes fly because they are lighter than cars. Sure, the wings help a lot, but the biggest difference in how the two different kinds of vehicles are built is in weight savings in both systems and structures. Unless you are building a race car, weight is generally a lower priority than strength or reliability, so structure and fittings are less critical. But one other area where aviation places an emphasis is in systems and structural security. In short, we need to make sure that connections (structural, electrical or fluid) stay connected. Because if they don’t, we don’t have much backup to keep us in the air.

Bolt It Down

For this reason, aircraft fasteners that are critical almost always have a positive capture design or mechanism. A good example is drilled bolts with a castellated nut and a cotter pin. Safety wire is another—it keeps things from turning once they are fastened in place. Ahhh…. You say, but isn’t the proper torque value supposed to keep things in place? Well, yes, it is, but the safety wire ensures that if the torque method fails, the item won’t loosen and come off. That is the “positive capture” idea. The best thing about positive capture is that you can verify—by visual inspection—that the item has been fully and safely fastened.

Now there are many fasteners on airplanes that do not have a positive capture device—these are mostly in areas that either aren’t critical or are not routinely unfastened. Think nutplates and nylon nuts. In both these cases, friction is added to the equation to keep screws or bolts from loosening up. Most wing-attach bolts are fastened with nylon nuts, and many trim or floor panels are held on with screws in nutplates. The wing bolts are rarely removed and are torqued to specific values, with the nylon “drag” added to keep the fastener from moving if the torque fails. In most cases, those bolts are loaded in shear, so if the nut loosened, the fastener would still take its load anyway.

For screws holding on access panels, floor panels or trim, the added drag from a pinched nutplate keeps the fastener from loosening—a reason why it is poor practice (in most cases) to run a tap through the nutplate to “make it easier to put screws in.” Yeah, it does make it easier—it also makes it easier for them to loosen with vibration. That’s the point—making it hard for the screw to back out—so before you go and modify your nutplates, think about what can go wrong if the fastener loosens up in flight (say with vibration). If it makes no difference, tap away—but in most exterior cases, you really don’t want things falling off your airplane.

Liquid Lock

Circling back to those skills imported from other arenas—let’s talk about the use of thread locking compounds (Loctite is the most common brand name that is bandied about, just so you know what we’re talking about). There is a reason that you will find virtually no mention of it in manuals for certified aircraft or in FAA advisory circulars aimed at mechanics and maintenance personnel. The problem is not that it doesn’t work—it has been proven to work for generations in vehicles and machinery. The problem is that there is no way to verify that it has been used on a fastener once the fastener has been installed. You can’t generally look at a bolt and say, “Yup, that was Loctited,” because the thread locker is hidden from sight in the engaged threads.

It’s easy for a mechanic to visually inspect for cotter pins, safety wire, or lock tabs—but impossible to know if the past person got distracted putting in a bunch of screws and forgot the Loctite—or didn’t read the instructions and realize that it was needed. So Loctite is generally not regarded as a reliable method of making sure that fasteners won’t come out—at least in aviation circles. That doesn’t mean that you can’t use it in non-critical places—but if it is non-critical, why worry about it in the first place? But the inability to verify that a fastener has been positively captured using thread-locking compound is the reason you don’t see it specified in aviation maintenance manuals.

Now let’s not confuse thread locker with torque seal paint. Torque seal doesn’t do anything at all, except to serve as an indicator that a bolt or nut has not moved since someone applied the torque seal. Note that I didn’t say “since the fastener was torqued,” because, honestly, you don’t know if it was torqued properly before the torque seal was applied. Personally, I use torque seal on an aircraft that I am building/maintaining, and I do it to tell myself that I have finished a joint and torqued to final values. But if I open up someone else’s airplane or one I didn’t build, I can’t necessarily trust the torque seal. If it is critical, it needs to be checked.

Because of the critical nature of aircraft fasteners, positive capture needs to be assured and needs to be verifiable (in the field) by those who maintain flying machines. That is why you see so many drilled bolts, castellated nuts and cotter pins—as well as lots and lots of safety wire. Make sure that you can visually check your fasteners—or put a wrench on them to make sure they are tight. And don’t do something on an airplane “just because I do it that way on my car.”