Critical Mach number.  The flight Mach number at which there is the first indication of supersonic airflow over any part of the aircraft structure. 

Cross coat.  A double coat of aircraft finishing material in which the second coat is sprayed at right angles to the first coat, before the solvents have evaporated from the first coat. 

Cross-feed valve (fuel system component).  A valve in a fuel system that allows any of the engines of a multi-engine aircraft to draw fuel from any fuel tank. Cross-feed systems are used to allow a multi-engine aircraft to maintain a balanced fuel condition. 

Cross-flow valve.  An automatic flow-control valve installed between the gear-up and gear-down lines of the landing gear of some large airplanes. When the landing gear is released from its uplocks, its weight causes it to fall faster than the hydraulic system can supply fluid to the gear-down side of the actuation cylinder. The cross-flow valve opens and directs fluid from the gear-up side into the gear-down side. This allows the gear to move down with a smooth motion. 

CRT.  Cathode-ray tube. 

Cryogenic liquid. A liquid which boils at temperatures of less than about 110 °F (–163 °C) at normal atmospheric pressures. 

Cuno filter.  The registered trade name for a particular style of edge-type fluid filter. Cuno filters are made up of a stack of thin metal disks that are separated by thin scraper blades. Contaminants collect on the edge of the disks, and they are periodically scraped out and allowed to collect in the bottom of the filter case for future removal. 

Current.  A general term used for electrical flow. See conventional current. 

Current limiter.  An electrical component used to limit the amount of current a generator can produce. Some current limiters are a type of slow-blow fuse in the generator output. Other current limiters reduce the generator output voltage if the generator tries to put out more than its rated current. 

Cusp. A pointed end. 

Cyclic pitch control.  The helicopter control that allows the pilot to change the pitch of the rotor blades individually, at a specific point in their rotation. The cyclic pitch control allows the pilot to tilt the plane of rotation of the rotor disk to change the direction of lift produced by the rotor. 

Dacron.  The registered trade name for a cloth woven from polyester fibers. 

Damped oscillation.  Oscillation whose amplitude decreases with time. 

Database.  A body of information that is available on any particular subject. 

Data bus.  A wire or group of wires that are used to move data within a computer system. 

Debooster valve.  A valve in a power brake system between the power brake control valve and the wheel cylinder. This valve lowers the pressure of the fluid going to the brake and increases its volume. A debooster valve increases the smoothness of brake application and aids in rapid release of the brakes. 

Decay. The breakdown of the structure of wood fibers. Wood that shows any indication of decay must be rejected for use in aircraft structure. 

Decomposition.  The breakdown of the structure of wood fibers. Wood that shows any indication of decay must be rejected for use in aircraft structure. 

Deciduous.  A type of tree that sheds its foliage at the end of the growing season. Hardwoods come from deciduous trees. 

Dedicated computer.  A small digital computer, often built into an instrument or control device that contains a built-in program that causes it to perform a specific function. 

Deep-vacuum pump.  A vacuum pump capable of removing almost all of the air from a refrigeration system. A deep- vacuum pump can reduce the pressure inside the system to a few microns of pressure. 

Deflator cap.  A cap for a tire, strut, or accumulator air valve that, when screwed onto the valve, depresses the valve stem and allows the air to escape safely through a hole in the side of the cap. 

Deicer system. A system that removes ice after it has formed on an aircraft. 

Delamination.  The separation of the layers of a laminated material. 

Delivery air duct check valve.  An isolation valve at the discharge side of the air turbine that prevents the loss of pressurization through a disengaged cabin air compressor. 

