cloud (meteorology). A collection of tiny droplets of water suspended, or floating, in the air. A cloud forms when invisible water vapor in the air condenses and becomes droplets of visible liquid water.
cloud bank (meteorology). A generally well-defined mass of cloud observed at a distance. It covers an appreciable portion of the sky on the horizon, but does not extend overhead.
cloudburst (meteorology). A term used to describe an extremely heavy rain.
cloud detection radar (meteorology). A vertically directed radar used to detect cloud bases and tops.
cloud point (petroleum specification). The temperature to which a petroleum product must be cooled for the wax it contains to begin to solidify and separate out into tiny crystals. These tiny crystals cause the material to appear cloudy.
clove hitch. A knot used to make individual spot ties for securing a bundle of electrical wires in an aircraft. A clove hitch is actually two half hitches around the wire bundle secured with a square knot.
club propeller. A short, stubby propeller used for testing an aircraft engine after it has been overhauled. Club propellers impose the correct load on the engine, and they move the maximum volume of cooling air through the cylinder fins.
cluster weld (aircraft construction). A welded joint in an aircraft steel-tube fuselage made where a number of tubes meet at a common point.
clutch. A component in a machine that connects or disconnects parts while they are moving. Clutches may be actuated by mechanical levers, hydraulic actuators, centrifugal force, or electromagnetism.
clutter (radar operation). The reception and visual display of radar returns caused by precipitation, chaff, terrain, numerous aircraft targets, or other phenomena.
coalesce. To come together, or unite, to form a whole unit. In a water separator, such as used in an air-cycle air conditioning system, tiny droplets of water suspended in the air are forced to coalesce, or join together, to form large drops of water that separate from the air.
coalescent bag (air conditioning system component). A porous bag in the water separator of an air-cycle air conditioning system which traps water from the air.
coastal fix. A navigation aid or intersection where an aircraft transitions between the domestic route structure and the ocean route structure.
coast-down check (gas turbine engine maintenance check). A check of the length of time needed for a gas turbine engine to coast to a complete stop after the fuel is shut off when the engine is operating at its idle speed.
The measured coast-down time is compared with the time specified in the engine operations manual to determine the condition of the engine.
coated cathode (electron tube component). A cathode of an electron tube which is covered with a material that emits a great many electrons when it is heated.
coaxial. Rotating about the same axis. Coaxial rotors of a helicopter are mounted on concentric shafts in such a way that they turn in opposite directions to cancel torque.