Expansion wave.  The change in pressure and velocity of supersonic air as it passes over a surface that drops away from the flow. As the surface drops away, the air tries to follow it, and in changing its direction, the air speeds up to a higher supersonic speed, and its static pressure decreases. There is no change in the total amount of energy as air passes through an expansion wave. 

External-combustion engine. A form of heat engine in which the fuel releases its energy outside of the engine. This released heat expands air which is used to perform useful work. Steam engines are a popular type of external combustion engine. 

Extreme pressure (EP) lubricant.  A lubricant that reacts with iron to form iron chlorides, sulfides, or phosphides on the surface of a steel part. These compounds reduce wear and damage to surfaces in heavy rubbing contact. EP lubricants are specially suited for lubricating gear trains. 

FAA Form 337. The Major Repair and Alteration  form that must be completed when an FAA-certificated aircraft or engine has been given a major repair or major alteration. 

Face (propeller nomenclature).  The flat surface of a propeller that strikes the air as the propeller rotates. The face of a propeller corresponds to the bottom of an airplane wing. 

FADEC. Full-authority digital electronic control. A digital electronic fuel control for a gas turbine engine that is functioning during all engine operations, hence full authority. It includes the EEC (see EEC) and functions with the flight management computer. FADEC schedules the fuel to the nozzles in such a way that prevents overshooting power changes and over-temperature conditions. FADEC furnishes information to the EICAS (engine indication and crew alerting system). 

Fan pressure ratio. The ratio of the fan-discharge pressure to the fan inlet pressure. 

Feathering propeller. A controllable-pitch propeller whose blades can be moved into a high pitch angle of approximately 90º. Feathering the propeller of an inoperative engine prevents it from wind-milling and greatly decreases drag. 

Feeler gages. A type of measuring tool consisting of strips of precision-ground steel of accurately measured thickness. Feeler gages are used to measure the distance between close- fitting parts, such as the clearances of a mechanical system or the distance by which moving contacts are separated. 

FHP. Friction horsepower. The amount of horsepower used to turn the crankshaft, pistons, gears, and accessories in a reciprocating engine and to compress the air inside the cylinders. 

Fiber optics. The technique of transmitting light or images through long, thin, flexible fibers of plastic or glass. Bundles of fibers are used to transmit complete images. 

Fire sleeve. A covering of fire-resistant fabric used to protect flexible fluid lines that are routed through areas subject to high temperature. 

Flame tubes. Small-diameter metal tubes that connect can- type combustors in a turbine engine to carry the ignition flame to all of the combustion chambers. The British call combustion liners flame tubes. 

Flameout. A condition of turbine engine operation when the fire unintentionally goes out. Improper fuel-air mixture or interruption of the air flow through the engine can cause a flameout. 

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

Flashing the field. A maintenance procedure for a DC generator that restores residual magnetism to the field frame, A pulse of current from a battery is sent through the field coils in the direction in which current normally flows. The magnetic field produced by this current magnetizes the steel frame of the generator. 

Flashover. An ignition system malfunction in which the high voltage in the magneto distributor jumps to the wrong terminal. Flashover causes the wrong spark plug to fire. This reduces the engine power and produces vibration and excessive heat. 

Flat-rated engine. A turboprop engine whose allowable output power is less than the engine is physically capable of producing. 

Float carburetor.  A fuel metering device that uses a float- actuated needle valve to maintain fuel level slightly below the edge of the discharge nozzle. 

Flock.  Pulverized wood or cotton fibers mixed with an adhesive. Flock, attached to a wire screen, acts as an effective induction air filter for small reciprocating engines. 

