filler rods (welding). Long pieces of wire that are melted into a weld to provide additional material to strengthen the weld. Filler rods are usually called welding rods.
filler valve (oxygen system component). A valve in an oxygen system through which oxygen from the storage bottles is put into the system. Filler valves normally incorporate a restriction that prevents the system from being filled too quickly. If they were filled too quickly, enough heat could be built up to cause an explosion.
fillet (aircraft structure). A fairing used to give shape, but not strength, to an object. A fillet produces a smooth junction where two surfaces meet. Fillets are often used to produce a smooth, aerodynamically clean junction between the wing and the fuselage of an airplane.
filling (meteorology). An increase in the central pressure of a meteorological pressure system. Filling, which is the opposite of deepening, is more commonly applied to a low, rather than a high.
fillister-head screw. A machine screw whose head has a rounded top, straight sides, and a flat bearing surface. Fillister-head screws often have holes drilled through the head so they can be safetied with safety wire.

fill threads (aircraft fabric). Threads woven across the width of a piece of fabric, from one selvage edge to the other. Fill threads are also called woof, or weft, threads.

film strength (lubricant characteristic). The measure of the ability of a lubricant to maintain a continuous film without breaking down under mechanical pressure.
filter (electrical circuit). A tuned electrical circuit that will pass certain frequencies or bands of frequencies while attenuating others. See band-pass filter, band-reject filter, high-pass filter, and low-pass filter.

filter (fluid filter). A component in a system that traps and holds contaminants carried in the fluid flowing through the system. Aircraft fuel systems, lubrication systems, and hydraulic systems all use filters.
filter capacitor (electronic component). An electrolytic capacitor connected in parallel with the output of a DC power supply. The capacitor has a low impedance for any ripple that may be superimposed on the DC output, and the ripple is passed to ground, while the DC is not affected.
filter choke (electronic component). An iron-core inductor installed in series with the output of a DC power supply. The filter choke impedes, or opposes, any ripples or portions of alternating current that are superimposed on the DC output.
fin (aircraft structural component). A fixed vertical aerodynamic surface used to provide directional stability to an aircraft.

final approach (aircraft flight operation). The portion of an aircraft flight just before landing. The aircraft is aligned with the runway and is on a descending flight path.