Chordline . An imaginary line, passing through a propeller blade, joining the leading and trailing edges.

Cigarette . A commonly used name for a spark plug terminal connector used with a shielded spark plug.

Circular magnetism . A method of magnetizing a part for magnetic particle inspection. Current is passed through the part, and the lines of magnetic flux surround it. Circular magnetism makes it possible to detect faults that extend lengthwise through the part.

Circumferential coil spring (garter spring ). A coil spring formed into a ring. This type of spring is used to hold segmented ring-type carbon seals tightly against a rotating shaft.

Claret red . A dark purplish pink to a dark gray purplish red color.

Class A fire . A fire with solid combustible materials such as wood, paper, and cloth as its fuel.

Class B fire . A fire that has combustible liquids as its fuel.

Class C fire . A fire which involves energized electrical equipment.

Class D fire . A fire in which a metal such as magnesium burns.

Closed-loop control . A type of control in which part of the output is fed back to the input. This allows the input to continually compare the command signals with the output to determine the extent to which the commands have been complied with.

Coke . The solid carbon residue left when all volatile parts of a mineral oil have been evaporated by heat.

Cold-cranking simulation . A method used for specifying the characteristics of a lubricating oil at low temperature. Oils rated by this test have the letter W (standing for Winter) in their designation. For example, SAE 15W50.

Cold section . The portion of a gas turbine engine ahead of the combustion section. The cold section includes the inlet, compressor, and diffuser.

Cold-tank lubrication system . A turbine engine lubricating system in which the oil cooler is in the scavenge subsystem.

Collector ring . A ring made of thin corrosion-resistant steel tubing that encircles a radial engine and collects exhaust gases from each cylinder. The ring ends with a connection to the exhaust tail pipe.

Combustor (combustion chamber ). The section of a gas turbine engine in which fuel is injected. This fuel mixes with air from the compressor and burns. The intense heat from the combustion expands the air flowing through the combustor and directs it our through the turbine. Combustors are also called burners.

Commutator . A mechanical rectifier mounted on the armature shaft of a DC generator or motor. It consists of a cylindrical arrangement of insulated copper bars connected to the armature coils. Carbon brushes ride on the copper bars to carry current into or out of the commutator, providing a unidirectional current from a generator or a reversal of current in the motor coils.

Compensating winding . A series winding in a compoundwound DC generator. The compensating windings are embedded in the faces of the field poles and their varying magnetic field works with the fields from the interpoles to effectively cancel the field distortion caused by armature current.

Composite propeller blade . A propeller blade made from several materials such as metal, graphite, glass or aramid fibers, and foam.

Compression ratio (reciprocating engine ). The ratio of the volume of a cylinder of a reciprocating engine with the piston at the bottom of its stroke engine to the volume of the cylinder with the piston at the top of its stroke.