How to Select and Use Aircraft Screws

How to Select and Use Aircraft Screws

By Robert G. Lock

This piece, which ran most recently in the “How To” section of the September/October 2019 issue of Vintage Airplane magazine, is part of an ongoing series of reprints of timeless tips from the late Robert G. Lock.

There are several types of screws available for aircraft use; however, two of the most popular are the AN525 washer head and the AN526 screw. Both are protruding-head screws and can be purchased as either slotted or Phillips head. All the older aircraft used slotted-head screws because the Phillips head was not invented yet.

The AN525 is a structural screw, being made from 2330 nickel steel and then heat-treated, just like the bolts. After heat-treating, the screws are plated with cadmium for corrosion protection. Common sizes are the No. 8 and No. 10.

The AN526 is a nonstructural, truss-head screw made from low-carbon (1020) steel. It is not hardened by heat-treating because there is not enough carbon in the alloy. These screws are commonly used for attaching a cowling, fairings, etc. Do not substitute these screws in any structural application. Figure 1 (above) is a sketch of these screws.

When driving screws into structure it is wise to use a washer under the head to protect a painted surface or to keep from scratching polished aluminum. For the size 10 screws you can use an AN90-10L steel washer or a No. 10 fiber washer. Either is commonly used.

Screw code sizes are difficult to remember, so I always refer to a catalog or my trusty Standard Aircraft Handbook for reference.

The standard No. 10 screw will take an AN365-1032A fiber lock nut, which is normally used. Or an AN366 winged nut plate can be riveted to the structure to make the removal and installation job easier. Figure 2 shows the AN366 nut plate.

Figure 2.

The Phillips-head screws are easier to install and remove because the screwdriver fits well into the slots, so when using a slotted-head screw, be careful and hold the screwdriver firmly to keep it from slipping out of the slot and damaging the finish.

Sheet metal screws may be used for attaching nonstructural parts on the aircraft. These screws come in truss heads similar to the above machine screws and can be purchased in either Phillips or slotted heads. Type A screws have a point on the end of the threads, while type B screws are flat on the bottom of the threads. They are normally secured by Tinnerman nut plates that carry the NAS395 code and come in various types from A-E and H. The Tinnerman nut plates come in screw sizes No. 6, No. 8, and No. 10.

Figure 3 is a Tinnerman Type NAS 144 nut plate that can be flush riveted to a structure. This type of nut plate readily will accept any coarse-threaded sheet metal screws — just secure the correct size when ordering.

Figure 3.

Brass screws are used for installing instruments because they are nonmagnetic. The common screw to use is the AN520B round-head screw that comes in sizes 6-32, 8-32, and 10-32. Head types are either slotted or Phillips (recessed). Figure 4 shows a brass screw.

Figure 4.

Special nut plates are available that fit into instrument holes and make mounting much easier. You must order these special mounting nuts by paying attention to the length of the ears that insert into the instrument. When using this type of mounting nut, the only thread size available is 6-32; therefore, the only brass screw that can be used would be a 6-32. Figure 5 is a sketch of an instrument mounting nut plate.

Figure 5.

There are flush-head countersunk machine screws available for aircraft use. These screws carry code AN510 and have an 82-degree included angle between the taper. Like most all other machine screws, they come in slotted and recessed-head styles.

Finally, there are fillister-head screws that have a hole for safety wire drilled through the head. These screws carry the code AN502. They are manufactured from 2330 nickel steel and are heat-treated and cadmium-plated.

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