Screws

Screws are the most commonly used threaded fastener in aircraft construction. They differ from bolts in that they are generally made from lower-strength materials. They can be fastened by a variety of tools, including screwdrivers, spanners and Allen keys. Most screws are threaded along their complete length, whilst some have a plain portion for part of their length.

There are a number of different types of screw, which can be used for a wide range of tasks. It is common sense that great care must be taken to replace screws with the correct items by using the markings on the screw, the IPC and any other systems in current use within the supply department, to protect against incorrect screws being installed.

Another point, requiring care, is the difference in terminology between the British and American names for screw heads. What the British refer to as a ‘countersunk-headed’ screw, the Americans call a ‘flat-head’ or ‘flush’ screw. Similarly, ‘mushroom-headed’ screws are known as ‘truss-heads’ in the USA.

Machine Screws

Machine screws (refer to figure below) are used extensively for attaching fairings, inspection plates, fluid line clamps and other light structural parts. The main difference between aircraft bolts and machine screws is that the threads of a machine screw usually run the length of the shank, whereas bolts usually have an unthreaded grip length.

The most common machine screw used in aviation is the fillister-head screw, which can be wire-locked using the drilled hole in the head. The flat-head (countersunk-head) screw is available with single or cross-point slotted heads. The round-head screw and the truss-head (mushroom-head) screw provide good holding properties on thin metal sheets.

FIGURE

Structural Screws

Structural screws (refer to figure below) are made of alloy steel are heat-treated and can be used in many structural situations. They have a definite grip and the same shear strength as a bolt of the same size. They are available with fillister, flat or washer heads. The washer head screw has a washer formed into its head to increase its holding ability with thin materials, much like the truss or mushroom head.

FIGURE

Self-Tapping Screws

Self-tapping screws (refer to figure below) have coarse threads and are used to hold thin sheets of metal, plastic and plywood together. The type A screw has a gimlet (sharp) point and the type B has a blunt point with threads that are slightly finer than the type A.

There are four types of head in normal use:

• Round head
• Countersunk oval-head
• Truss or mushroom-head
• Flat countersunk-head

FIGURE