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inspection.
garnet paper. An abrasive and polishing paper made by coating one side of a sheet of flexible paper with a
layer of crushed garnet.
gas. The physical state of matter in which a material takes the shape of its container and expands to fill the
entire container. Oxygen and nitrogen are two chemical elements that are gases at normal room temperature
and pressure. The air we breathe is a physical mixture of gases, primarily nitrogen and oxygen.
gas discharge tube (electron tube). A gas-filled diode electron tube used in a high energy
capacitor-discharge ignition system for a turbine engine. The discharge tube blocks all electron flow until
the voltage across its electrodes builds up to the ionization voltage of the gas inside the tube. When this
voltage is reached, the gas ionizes and the tube conducts, discharging the capacitor through the
high-voltage transformer.
gas-filled tube (electron tube). An electron tube whose envelope is filled with an inert gas, such as argon.
The inert gas prevents the heated elements from oxidizing.
gas generator (gas turbine engine). The basic gas turbine engine consisting of the compressor, diffuser,
combustor, and turbine. The gas generator, also called the core engine, is the part of a turbine engine that
produces the hot, high-velocity gases. The gas generator does not include the inlet duct, fan section, free
power turbines, or the tail pipe.
gasket. A seal between two stationary objects. A gasket is usually made of a soft material so it can conform
to the surfaces of the two parts and form a leakproof seal. A gasket is similar to a packing, except a packing
is used when there is relative motion between the parts being sealed.
gasoline. A liquid hydrocarbon fuel obtained from the distillation of crude oil. Gasoline is the most
commonly used fuel for aircraft reciprocating engines and automobile engines.
gassing (battery condition). A condition that occurs near the end of the charging cycle of lead-acid storage
batteries. Hydrogen and oxygen are released as free gases, hydrogen at the negative plate, and oxygen at
the positive plate.
gas tungsten arc welding. See GTAW.
gas turbine engine. An internal combustion engine in which energy is released from the fuel in a
constant-pressure cycle. Air is taken into the engine and compressed by either an axial-flow or a centrifugal
compressor. The compressed air then passes into the combustion chambers where fuel is sprayed into a
continuously burning fire. The burning fuel heats and expands the air which then flows through a turbine,
where some of the energy is extracted to turn the compressor. The energy remaining in the gases leaving
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
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the engine causes the exhaust velocity to be greater than that of the air entering the engine, and this
acceleration produces thrust.
Some gas turbine engines use additional stages of turbines to extract energy from the hot gases to
drive such devices as fans, propellers, rotors, generators, or pumps. Gas turbine engines are far superior to
reciprocating engines, with regard to their weight-to-horsepower ratio.
gas welding. A method of joining metals by melting them and allowing the molten metal from the two
pieces to flow together. The heat used to melt the metal is provided by burning a mixture of fuel gas and
oxygen. The two most generally used fuel gases are acetylene and hydrogen.
gate (electrode of a semiconductor device). The electrode in a silicon controlled rectifier (SCR), triac, or
field effect transistor (FET), to which trigger pulses of electrical energy are applied to cause a flow of
electrons between the other electrodes.
gate hold procedures (aircraft operation). Procedures at selected airports to hold aircraft at the gate or
other ground locations whenever departure delays exceed or are anticipated to exceed five minutes.
gate-type check valve. A type of one-way valve that uses a swinging gate, or flapper, to control the flow of
fluid. Gate-type check valves are used in multiengine aircraft vacuum systems to isolate one of the pumps
if it should fail.
gauss. The unit of magnetic flux density in the centimeter-gram-second system of metric measurement.
One gauss is a flux density of one line of flux (one maxwell) per square centimeter.
Gay Lussac’s law. See Charles’s law.
GCA (ground controlled approach) (air traffic control). A radar approach system operated from the
ground by air traffic control personnel transmitting instruction to the pilot by radio. The approach may be
conducted with surveillance radar (ASR) only or with both surveillance and precision approach radar
(PAR).
Usage of the term GCA by pilots is discouraged except when referring to a GCA facility. Pilots
　
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