counterelectromotive force. See CEMF.
counter-rotating propellers. Propellers on wing-mounted engines that turn in opposite directions. When flying at a high angle of attack, the descending propeller blade has a higher angle of attack than the ascending blade and the half of the propeller disk with the descending blade produces more thrust than the half with the ascending blade.
On an airplane equipped with counter-rotating propellers the descending blades on both engines are near the aircraft centerline and neither engine is a critical engine. See critical engine.
countersink (metalworking tool). A metal-cutting tool used to chamfer (bevel) the edges of a hole so the head of a countersunk rivet will be flush, or even, with the surface of the metal.
countersink (verb). To prepare a rivet hole for a flush rivet by beveling the edges of the holes with a cutter of the correct angle.
countersunk-head rivet. A rivet whose cone-shaped head fits into a beveled hole in the metal. When the rivet is driven, the top of the manufactured head is flush with the surface of the metal.

counterweight. A concentrated mass used to balance a weight or a load. Counterweights are used on the
movable control surfaces of an airplane to prevent the surface from fluttering in high-speed flight.
counterweight (reciprocating engine crankshaft). A heavy weight fastened to one of the throws of a crankshaft to balance the assembly of the crankshaft, connecting rods, and pistons.
coupled approach (air traffic control). An instrument approach performed by the aircraft autopilot which is receiving position information and/or steering commands from on-board navigation equipment.
In general, coupled nonprecision approaches must be discontinued and flown manually at altitudes lower than 50 feet below the minimum descent altitude, and coupled precision approaches must be flown manually below 50 feet AGL.
course (flight planning). The intended direction of flight, measured in the horizontal plane in degrees clockwise from north.

Course Deviation Indicator. See CDI.
covalent bond. A type of linkage between atoms in a chemical compound in which an electron is shared by two atoms. The sharing of the electron holds the atoms together.