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Anchor Nuts . . . Why? Where? How?

Sport Aviation By Tony Bingelis
More than 15 years ago I wrote a chapter titled "Where and How to Install Nut Plates" for my book "The Sportplane Builder," (it's the blue one). Since that time I have learned a few more things about installing anchor nuts.
For some reason I like to call them nut plates. Be that as it may, I still don't like those pesky little things but, boy, are they important! As the name implies, anchor nuts are captive nuts you can attach permanently to the blind side (backside usually) of an assembly. Often this is a removable accessory, component part, or cover, of some sort.
These anchor nuts, or nut plates, as they are often called, are usually installed where access to a regular hex nut with a wrench would be difficult, if not impossible. With anchor nuts installed, it becomes possible to insert and torque your machine screws or bolts from one side. Anchor nuts eliminate the need for groping around under an assembly in an attempt to install an ordinary hex nut.

Where Do You Use Them?

You will find that anchor nuts are just about the only way you can make some installations. The most frequently affected problem locations are the following:
1. Inspection access covers and hatches.
2. Cowling attachment.
3. Installation of accessories on the firewall.
4. The installation of antennas.
5. Propeller spinner/bulkhead installation.
6. The attachment of wing root fairings.
7. Instrument panel installation.
If you think about it you will realize that the applications listed have this in common. Each is, for all practical purposes, a so-called "blind installation." That is, the part generally has to be secured with little or no access to the opposite side for installing and torquing ordinary nuts.
Substituting anchor nuts in these locations, therefore, not only makes the installation possible, they will also permit you to remove the part later for replacement or maintenance.
Keep this thought in mind. During the construction of your airplane you will enjoy much easier access to everything than will be possible later after the airplane is finished.
For example, after you complete your airplane, you will find it impossible to remove an accessory like the voltage regulator, gascolator, or air/oil separator from the firewall, single-handedly, if the unit was installed with ordinary hex nuts.
Obviously, without the aid of a second person inside the airplane manipulating a wrench on the opposite side, such a simple task is almost impossible.
But, even if you have somebody to help, what do you do if you have a big fuselage fuel tank up front? You may still lack wrench access to the firewall unless the tank is removed first!
Of course, you would have missed all that fun had you anticipated the need and installed the accessory with anchor nuts.
The same problem can confront you should you have to remove an antenna. It would be simple enough to accomplish the task single-handedly if anchor nuts are installed . . . if not, you will need somebody outside the airplane with a back-up wrench.
In short, consider installing anchor nuts anywhere you find it impossible, or impractical, to install standard aircraft hex nuts.
Naturally, quite a bit of extra work (and cost) is involved in installing anchor nuts, so it is not likely you will consider installing them just for the fun of it . . . not without a good reason.

Selecting the Right Kind of Anchor Nut

This is no big problem because almost any type of anchor nut (nut plate) you can get will most likely be suitable for the job. Assuming, of course, it is the correct size for the bolt or machine screw you intend to install.
Incidentally, many a builder has, on occasion, installed a wrong size anchor nut, simply because he failed to check it before installing it. That always means more unnecessary work as they have to be drilled out and replaced.
Here are a few thoughts you can mull over. It is not at all unusual for the typical all-metal homebuilt to have more than two or three hundred anchor nuts installed.
Two of the most commonly used anchor nut varieties include:
1. The two lug nylon insert locking type (AN366F).
2. The two lug all-metal anchor nut (K1000).
Other less frequently used types include the all-metal floating anchor nuts, one lug nuts, corner nuts, and even miniature nuts for limited space installations.
NOTE: Check your homebuilt supplier catalogs for the options and sizes they normally carry. Don't be confused by the specification numbers (AN, MS, NA, etc.). Look at the illustrations and select the bolt/screw size you need accordingly.
Cost is very definitely a factor to consider when selecting the type of anchor nuts you will use.
For example, according to one of my catalogs, I can see why the all-metal K1000 two lug anchor nut is so popular. For one thing, it is quite inexpensive (about 17 cents each). Besides that, it is very light, is lubricated lightly with a dry lubricant, has a self-locking capability, and can tolerate temperatures as high as 450 degrees F.
In contrast, the listed, nylon locking insert anchor nut (AN366F) is a heavier, bulkier, anchor nut that can only tolerate a maximum of 250 degrees F. And, in spite of these shortcomings, retails between $1.10 and $2.98 each. (In 1994, more than ever before, it pays to shop around.)
My little RV-3 has over 345 anchor nuts installed as of this date. Guess which variety of anchor nuts I am using? It is the 17 cent variety, of course. If I had to install the expensive AN366F elastic-insert type, I would have had to obtain a bank loan by now.
Incidentally, I know you won't be able to resist the temptation to figure out how much 345 of the higher priced nylon locking insert anchor nuts would have cost me, so, go ahead . . . the numbers will shake you up.
Sometimes the space remaining for installation of an anchor nut is so limited that neither a standard two lug anchor nut or a miniature anchor nut will fit. Often a corner nut or a one lug anchor nut has to be substituted in these locations.
In other locations it may be necessary to install a right-angle nut, but these are rare exceptions.
Anchor nuts are also made with a floating captive nut which supposedly allows easier alignment on assembly. I find these to be conducive to sloppy workmanship and, therefore, merit little consideration for use in the average homebuilt. Besides, they are more expensive and more difficult to rivet in place.

Tips For Installing Anchor Nuts

Although anchor nuts are most frequently used to attach access plates, fairings and the like, they can also be used in structural applications with bolts.
Normal belief has it that the only way to attach anchor nuts is by riveting them to aluminum surfaces. This is not so.
You can also rivet these nut plates to plywood surfaces and fiberglass components. For that matter, anchor nuts may even be tack welded to steel parts.
On wood surfaces, anchor nuts are secured with nails or screws as well as by riveting. It all depends on the thickness of the surface material. In either case, it would be prudent to add a dab of epoxy adhesive between the nut plate and the wood surface.
The minimum acceptable diameter for aircraft structural bolts is 3/16" (AN3 bolts).
It is well to mention that larger anchor nuts are available and they are suitable for use in highly stressed locations.
The only noticeable difference in the anchor nuts used with larger bolts (3/8" dia. [AN6] and up) is that they are secured with larger rivets (1/8" rivets instead of the smaller 3/32" rivets) to better resist the higher torque limits imposed by such bolts during installation.
The most popular size anchor nut is the one that takes 832 machine screws. This size is used almost everywhere you would need to install an anchor nut.
Fortunately, this is also the easiest of the anchor nuts to install. The reason being that a 1/8" Cleco can be used to temporarily clamp it in place while you drill the 3/32" attachment rivet holes.