firewall shutoff valve (aircraft fluid system component). A valve located on the airframe side of the engine firewall that will completely shut off the flow of fuel, oil, or hydraulic fluid to the engine in the case of an engine compartment fire.
fire zone. An area in an aircraft designated by the aircraft manufacturer as requiring a fire detection and/or fire extinguishing system.
firing order (reciprocating engine). The sequence in which the cylinders of a reciprocating engine fire in a normal cycle of operation.
firmer chisel (wood-cutting tool). A type of woodworking chisel having a thin, flat blade.
firmware (computer operations). A computer program stored in read-only memory (ROM). Programs are stored in ROM when they are used repeatedly; such as, for example, the operating procedures in dedicated computers.
first-class lever. A lever in which the fulcrum is located between the force and the weight. The force and the weight try to rotate the lever in opposite directions.
first gust (meteorology). The leading edge of a spreading downdraft, called a plow wind, in front of an approaching thunderstorm.
fir-tree attachment. A method of attaching turbine blades into the turbine wheel. The root of the blade is shaped like a notched-edge Christmas tree which fits loosely in a similar-shaped slot in the periphery of the wheel. When the engine is cold, the blades are loose, but as the engine heats up, the blades tighten in the wheel.
fisheyes (finishing system defect). A type of finishing system defect in which small, local areas of the finish do not cure properly. These small areas resemble the eyes of a fish.
Fisheyes are caused by small specks of wax or silicone material on the surface when the finish was sprayed on, or by contaminants that fell on the finish before it cured.
fishmouth splice (welded tubular structure). A type of splice used in a welded steel tubular structure, in which the end of a tube, whose inside diameter is the same as the outside diameter of the tube being spliced, is cut in the shape of a V, or fishmouth, and is slipped over the smaller tube and welded.
A fishmouth splice has more weld area than a butt splice, and it allows the stresses from one tube to transfer gradually into the other tube.
fission. See nuclear fission.
fishtail (aircraft flight maneuver). A method of decreasing the speed of an airplane during its approach for landing. Airplanes without flaps can be slowed down and kept on a relatively straight flight path by alternately skidding the airplane to the left and then to the right, by using the rudder while keeping the wings level with the control stick or wheel. Side and forward slips are used more often than fishtailing for slowing an airplane.
fitting. An attachment device that is used to connect components to an aircraft structure.
five-hour rating (battery capacity rating). The ampere-hour rating of a battery when current is taken from it at a rate that will discharge it in five hours. A 20-ampere-hour battery will supply four amps of current for five hours, but if it is discharged at a higher rate, its ampere-hour capacity will be less. The five-hour rating is the rating most generally used for aircraft storage batteries.
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