4.  
An adiabatic compression of the gas from D to A, returning it to its original conditions.

carriage bolt. A round-headed bolt with a square section of the shank just below the head. A carriage bolt is used to hold pieces of wood together. The square section of the shank embeds itself in the wood and keeps the bolt from turning when the nut is tightened on the threads.
carrier frequency (radio transmission). The frequency of the alternating current carrier that produces the electromagnetic waves which radiate from a radio transmitting antenna.
The audio-frequency signal from the microphone modulates (changes the amplitude or the frequency) of the carrier.
cartridge fuse (electrical component). A type of fuse used to protect an electrical circuit from an excess of current. The fusible link, the part of the fuse that melts when too much current flows through it, is held between metal ferrules, or caps, that screw onto each end of an insulating tube. The ferrules are held in special fuse clips inside the fuse box.
cartridge-pneumatic starter (gas turbine engine starter). A pneumatic starter for a gas turbine engine that uses gas pressure produced when powder in a cartridge burns in the breech of the starter.
The high-pressure gas produced by the burning powder spins a turbine geared to the engine compressor.
cartridge starter (reciprocating engine starter). A type of starter used on some large aircraft reciprocating engines. A cartridge, looking much like a shotgun cartridge, is fired in the breech of the starter. The gas produced by the burning powder pushes down on a piston inside a cylinder. The piston is attached to a helical spline, a spline cut in the shape of a spiral, in such a way that as the piston moves down, the spline twists. The twisting spline turns a gear that meshes with a gear on the engine crankshaft.
When the cartridge is fired and the piston is pushed down, the helical spline turns the gear and cranks the engine fast enough for it to start.
cartridge-type fluid filter (fuel and oil filter). A disposable filter element used in an aircraft fuel or lubrication system. These filters are of the depth type and are made of cellulose fibers that trap and hold most of the contaminants in the fuel or oil that passes through them.
CAS (calibrated airspeed). A measure of the speed at which an aircraft is moving through the air.
Calibrated airspeed is found by correcting the airspeed shown on the airspeed indicator, the indicated airspeed (IAS), for any errors in the instrument or those caused by its installation (installation, or position errors). See position error.
14 CFR Part 1: “The indicated airspeed of an aircraft, corrected for position and instrument error. Calibrated airspeed is equal to true airspeed in standard atmosphere at sea level.”
cascade (electrical circuits). A method of connecting multiple stages of electrical circuits so the output of one stage feeds the input of the next stage.