Capillary tube.  A soft copper tube with a small inside diameter. The capillary tube used with vapor-pressure thermometer connects the temperature sensing bulb to the Bourdon tube. The capillary tube is protected from physical damage by enclosing it in a braided metal wire jacket. 

Carbon monoxide detector.  A packet of chemical crystals mounted in the aircraft cockpit or cabin where they are easily visible. The crystals change their color from yellow to green when they are exposed to carbon monoxide. 

Carbon-pile voltage regulator.  A type of voltage regulator used with high-output DC generators. Field current is controlled by varying the resistance of a stack of thin carbon disks. This resistance is varied by controlling the amount the stack is compressed by a spring whose force is opposed by the pull of an electromagnet. The electromagnet’s strength is proportional to the generator’s output voltage. 

Carburizing flame.  An oxyacetylene flame produced by an excess of acetylene. This flame is identified by a feather around the inner cone. A carburizing flame is also called a reducing flame. 

Carcass (tire component).  The layers of rubberized fabric that make up the body of an aircraft tire. 

Case pressure.  A low pressure that is maintained inside the case of a hydraulic pump. If a seal becomes damaged, hydraulic fluid will be forced out of the pump rather than allowing air to be drawn into the pump. 

Cathode-ray tube (CRT). A display tube used for oscilloscopes and computer video displays. An electron gun emits a stream of electrons that is attracted to a positively charged inner surface of the face of the tube. Acceleration and focusing grids speed the movement of the electrons and shape the beam into a pinpoint size. Electrostatic or electromagnetic forces caused by deflection plates or coils move the beam over the face of the tube. The inside surface of the face of the tube is treated with a phosphor material that emits light when the beam of electrons strikes it. 

Cavitation.  A condition that exist in a hydraulic pump when there is not enough pressure in the reservoir to force fluid to the inlet of the pump. The pump picks up air instead of fluid. 

CDI.  Course deviation indicator. 

CDU.  Control display unit. 

Center of gravity. The location on an aircraft about which the force of gravity is concentrated. 

Center of lift.  The location of the chord line of an airfoil at which all the lift forces produced by the airfoil are considered to be concentrated. 

Center of pressure. The point on the chord line of an airfoil where all of the aerodynamic forces are considered to be concentrated. 

Centering cam.  A cam in the nose-gear shock strut that causes the piston to center when the strut fully extends. When the aircraft takes off and the strut extends, the wheel is straightened in its fore-and-aft position so it can be retracted into the wheel well. 

Charging stand (air conditioning service equipment). A handy and compact arrangement of air conditioning servicing equipment. A charging stand contains a vacuum pump, a manifold gauge set, and a method of measuring and dispensing the refrigerant. 

Chatter.  A type of rapid vibration of a hydraulic pump caused by the pump taking in some air along with the hydraulic fluid. 

Check (wood defect).  Longitudinal cracks that extend across a log’s annual rings. 

Check valve. A hydraulic or pneumatic system component that allows full flow of fluid in one direction but blocks all flow in the opposite direction. 

Chemical oxygen candle system.  An oxygen system used for emergency or backup use. Solid blocks of material that release oxygen when they are burned are carried in special fireproof fixtures. When oxygen is needed, the candles are ignited with an integral igniter, and oxygen flows into the tubing leading to the masks. 

Chevron seal. A form of one-way seal used in some fluid- power actuators. A chevron seal is made of a resilient material whose cross section is in the shape of the letter V. The pressure being sealed must be applied to the open side of the V. 

Chromel.  An alloy of nickel and chromium used as the positive element in a thermocouple for measuring exhaust gas temperature. 

Circle.  A closed plane figure with every point an equal distance from the center. A circle has the greatest area for its circumference of any enclosed shape. 

