Fishmouth splice.  A type of splice used in a welded tubular structure in which the end of the 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 a fishmouth, and is slipped over the smaller tube welded. A fishmouth splice has more weld area than a butt splice and allows the stresses from one tube to transfer into the other tube gradually. 

Fire pull handle.  The handle in an aircraft flight deck that is pulled at the first indication of an engine fire. Pulling this handle removes the generator from the electrical system, shuts off the fuel and hydraulic fluid to the engine, and closes the compressor bleed air valve. The fire extinguisher agent discharge switch is uncovered, but it is not automatically closed. 

Fire zone.  A portion of an aircraft designated by the manufacturer to require fire-detection and/or fire-extinguishing equipment and a high degree of inherent fire resistance. 

Fitting.  An attachment device that is used to connect components to an aircraft structure. 

Fixed fire-extinguishing system.  A fire-extinguishing system installed in an aircraft. 

Flameout. A condition in the operation of a gas turbine engine in which the fire in the engine unintentionally goes out. 

Flap (aircraft control).  A secondary control on an airplane wing that changes its camber to increase both its lift and its drag. 

Flap (helicopter rotor blade movement).  Up-and-down movement of the tip of a helicopter rotor blade. 

Flap overload valve.  A valve in the flap system of an airplane that prevents the flaps being lowered at an airspeed which could cause structural damage. If the pilot tries to extend the flaps when the airspeed is too high, the opposition caused by the air flow will open the overload valve and return the fluid to the reservoir. 

Flash point.  The temperature to which a material must be raised for it to ignite, but not continue to burn, when a flame is passed above it. 

Flat pattern layout.  The pattern for a sheet metal part that has the material used for each flat surface, and for all of the bends, marked out with bend-tangent lines drawn between the flats and bend allowances. 

Flight controller.  The component in an autopilot system that allows the pilot to maneuver the aircraft manually when the autopilot is engaged. 

Fluid.  A form of material whose molecules are able to flow past one another without destroying the material. Gases and liquids are both fluids. 

Fluid power.  The transmission of force by the movement of a fluid. The most familiar examples of fluid power systems are hydraulic and pneumatic systems. 

Flutter.  Rapid and uncontrolled oscillation of a flight control surface on an aircraft that is caused by a dynamically unbalanced condition. 

Fly-by-wire.  A method of control used by some modern aircraft in which control movement or pressures exerted by the pilot are directed into a digital computer where they are input into a program tailored to the flight characteristics of the aircraft. The computer output signal is sent to actuators at the control surfaces to move them the optimum amount for the desired maneuver. 

Flying boat.  An airplane whose fuselage is built in the form of a boat hull to allow it to land and takeoff from water. In the past, flying boats were a popular form of large airplane. 

Flying wing.  A type of heavier-than-air aircraft that has no fuselage or separate tail surfaces. The engines and useful load are carried inside the wing, and movable control surfaces on the trailing edge provide both pitch and roll control. 

Foot-pound.  A measure of work accomplished when a force of 1 pound moves an object a distance of 1 foot. 

Force.  Energy brought to bear on an object that tends to cause motion or to change motion. 

Forehand welding.  Welding in which the torch is pointed in the direction the weld is progressing. 

Form drag.  Parasite drag caused by the form of the object passing through the air. 

Former. An aircraft structural member used to give a fuselage its shape. 

FMC. Flight Management Computer. 

Forward bias.  A condition of operation of a semiconductor device such as a diode or transistor in which a positive voltage is connected to the P-type material and a negative voltage to the N-type material. 

FPD.  Freezing point depressant. 

