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area of the head and in the center of the barrel.
choo-choo (gas turbine engine operating condition). A mild compressor surge caused by an obstruction
of the airflow through the compressor. The name comes from the sound made by the engine.
chopper (electrical component). An electronic or electromagnetic device used to interrupt DC or
low-frequency AC at regular intervals to produce a pulsating voltage whose frequency is high enough that
it can be amplified by an AC amplifier.
chord of a circle. A straight line across a circle that touches the circumference at two points. The chord
passing through the center of a circle is called the diameter.
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chord of an airfoil. An imaginary line drawn through an airfoil from its leading edge to its trailing edge.
The chord, or chord line, is used as a reference, or datum, for laying out the curve of the airfoil.
chromel. An alloy of nickel and chromium. Chromel is highly resistant to oxidation and has a high
electrical resistance. It is one of the metals used in a thermocouple to measure the exhaust gas temperature
in both reciprocating and gas turbine engines.
chromel-alumel thermocouple. A thermocouple used to measure the temperature of the exhaust gas in
either reciprocating or gas turbine engines. It can be used for measuring temperatures up to 1,000°C.
Chromel is the positive lead, and it is normally enclosed in a white insulation for color coding.
Alumel is the negative lead, and it is normally enclosed in a green insulation. The alumel lead can also be
distinguished from the chromel lead because alumel is magnetic.
chrome molybdenum steel. A high-strength alloy steel in which chromium and molybdenum are the chief
alloying agents. It is easy to machine and weld, and has high strength for its weight. Most welded tubular
steel aircraft structure is made of chrome molybdenum tubing. Chrome molybdenum steels are the SAE
41xx series.
chrome nickel steel. An alloy of steel used for making many of the fittings and fasteners used in aircraft
structure. Chrome nickel steels are the SAE 3xxx series.
chrome pickling. A method of forming a hard oxide film on the surface of a piece of magnesium to protect
it from corrosion. To form this film, the magnesium is soaked in a solution of potassium dichromate.
Chrome pickling is also called dichromate conversion. The surface of the metal is converted into a
dense oxide film.
chrome plating. An electroplating process in which a thin film of chromium is plated on the surface of a
steel part.
There are two types of chrome plating: hard chrome and decorative chrome. Hard chrome is used
to plate the inside walls of an engine cylinder to form a wear-resistant surface. Decorative chrome is used
on such components as automobile bumpers to provide a durable, weather-resistant, and attractive surface.
chrome vanadium steel. A very tough alloy of steel with chromium and vanadium. Chrome vanadium
steel is used for making wrenches and other tools that require high strength and good impact resistance.
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chromic acid. An acid used for etching aluminum alloys to prepare them for painting. Chromic acid reacts
with the aluminum to form a durable film on the surface that keeps oxygen away from the metal and
inhibits the formation of corrosion.
chromium. A lustrous, hard, steel-gray, metallic chemical element. Chromium’s symbol is Cr, its atomic
number is 24, and its atomic weight is 51.996. Chromium is used to harden alloy steels and as an alloying
agent to make corrosion-resistant steels.
chronometric tachometer. An instrument used to measure the speed in revolutions per minute of the
crankshaft of an aircraft reciprocating engine. A chronometric tachometer repeatedly counts the number of
revolutions in a given period of time and displays the average speed on its dial.
chuck (machine tool component). The device on a lathe or drill press used to hold either the material
being worked or the machine tool used to work on the material. Chucks have three or more jaws to clamp
and hold the material or the tool.
chugging (gas turbine engine operating condition). A noise heard in a gas turbine engine caused by
unstable airflow in the engine. The chugging noise is produced by a mild compressor stall.
chute. A sloping channel used to allow objects or materials to slide from one level to a lower level.
cigarette (ignition system component). A ceramic or synthetic rubber insulator used on the end of an
ignition lead that attaches to a shielded spark plug. A steel coil spring on the end of the cigarette conducts
the high voltage from the lead wire to the center electrode of the spark plug.
　
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