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. 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. 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. 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. 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. 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. 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. G-11