Creep . The deformation of a metal part that is continually exposed to high centrifugal loads and temperatures.

Critical altitude . The altitude above which a reciprocating engine will no longer produce its rated horsepower with its throttle wide open.

Critical engine . The engine of a twin-engine airplane whose loss would cause the greatest yawing effect.

Critical Match number . The flight match number at which there is the first indication of air flowing over any part of the structure at a speed of Mach one, the local speed of sound.

CRT . Cathode ray tube. An electronic display tube in which a stream of electrons is attracted to the charged inner surface of the tube face. Acceleration grids and inner surface of the tube face. Acceleration grids and focusing grids speed the movement of the electrons and shape the beam to a pinpoint size. Electrostatic or electromagnetic forces caused by deflection plates or coils move the beam over the face of the tube. The inside of the tube face is treated with a phosphor material that emits light when the electrons strike it.

Cryogenic fluid . A liquid which boils at a temperature lower than about 110 ºK (-163 ºC) under normal atmospheric pressure.

CSD . Constant-speed drive. A component used with either aircraft gas turbine or reciprocating engines to drive AC generators. The speed of the output shaft of the CSD is held constant while the speed of its input shaft varies. The CSD holds the speed of the generator, and the frequency of the AC constant as the engine speed varies through its normal operating range.

CTF. Centrifugal twisting force . The force acting about the longitudinal axis of a propeller blade, and which tries to rotate the blade to a low-pitch angle. As the propeller rotates, centrifugal force tries to flatten the blade so all of its mass rotates in the same plane.

Curtiss Jenny (Curtiss JN4-D ). A World War I training airplane powered by a Curtiss OX-5 engine. It was widely available after the war and helped introduce aviation to the general public.

Customer bleed air . Air that is tapped off a turbine engine compressor and used for such airframe functions as the operation of air conditioning and pressurization systems.

Cylinder . The component of a reciprocating engine which houses the piston, valves, and spark plugs and forms the combustion chamber.

Data . The input for computer processing in the form of numerical information that represents characters or analog quantities.

Dataplate specifications . Specification of each gas turbine engine determined in the manufacturer’s test cell when the engine was calibrated. This data includes the engine serial number with the EPR that produced a specific RPM. The technician refers to this information when trimming the engine.

Dataplate performance . The performance specifications of a turbine engine observed and recorded by the engine manufacturer or overhauler and recorded on the engine dataplate. This data includes the engine speed at which a specified EPR is attained. When trimming the engine, the technician uses this data as the goal.

DC. Direct current . Electrical current in which the electrons always flow in the same direction.

Deaerator . A component in a turbine engine lubrication system that removes air from the scavenged oil before it is returned to the tank.

Deceleration . The amount the velocity of an object, measured in feet per second, is decreased by a force during each second it is acted upon by that force. Deceleration is usually expressed in terms of feet per second, per second (fps²).

DeHaviland DH-4 . An English designed observation airplane built in large quantities in the united States during World War I. After the war, surplus DH-4s were used for carrying the U.S. Mail.

Deicing . The removal of ice that has formed on a surface.