convergent-divergent exhaust duct (gas turbine engine exhaust duct). An exhaust duct used on the engine of a supersonic aircraft. The cross-sectional area of the duct decreases in the direction of the gas flow until it is small enough that the gases passing through this area travel at the speed of sound. Beyond this point of minimum area, the duct enlarges so the gases can be further speeded up above the speed of sound.
convergent-divergent inlet duct (supersonic airplane component). A form of duct, or passage, used for the inlet air to supersonic airplane engines.
The forward section of the duct is convergent (it becomes smaller in the direction of the airflow). The air entering the duct at a supersonic speed is slowed to a speed of Mach one by a shock wave in the narrowest part of the duct.
Beyond this point, the area of the duct increases — becomes divergent. The air, now traveling slower than the speed of sound, is further slowed, and its pressure increases.
convergent duct. A duct, or passage, whose cross-sectional area decreases in the direction of fluid flow.

conversion coating (aircraft finishing system component). A chemical solution used to form an airtight oxide or phosphate film on the surface of aluminum or magnesium parts. The conversion coating prevents air reaching the metal and keeps it from corroding.
convertaplane (aircraft design). A type of heavier-than-air aircraft able to rise and descend vertically. The convertaplane rises vertically by using rotors similar to those on a helicopter. But, when it is in the air, the rotors can be tilted forward to produce thrust to pull the convertaplane through the air as a fixed-wing aircraft.
converter (electrical machine). A rotating electrical machine that changes, or converts, alternating current into direct current. A converter is usually a form of motor-generator, in which an AC motor drives a DC generator.
convex surface. A surface that is curved outward with its outer edges lower than the center.

coolant. A fluid used to remove heat from a component or a system. A mixture of ethylene glycol and water is normally used as a coolant in liquid-cooled aircraft engines.
cooling fins. Thin ribs that extend outward from a surface to carry heat from the surface into the air flowing through the fins.
The cylinders of air-cooled aircraft engines are finned to increase their surface area, so a maximum amount of heat can be transferred from the cylinder into the air.
coordinated universal time (UTC). Time corrected for the seasonal variations in the earth’s rotation about the sun. Coordinated universal time is also known as universal time coordinated (thus UTC), Greenwich mean time (GMT), and Zulu time.
coordinates (air traffic control). The intersection of lines of reference, usually expressed in degrees/minutes/seconds of latitude and longitude, used to determine a position or location.