critical Mach number (aerodynamics). The flight Mach number at which there is the first indication of air flowing over any part of the structure at a speed of Mach one, the local speed of sound.
critical part. The part in an assembly or component whose failure would prevent the assembly or component functioning as it should.
critical pressure (reciprocating engine fuel). The highest pressure of the fuel-air mixture inside the cylinder of a reciprocating engine that allows the mixture to burn evenly, rather than explode, or detonate.
critical RPM range (engine operating condition). A range of engine RPM in which the natural vibratory frequency of the engine-propeller combination is excited. Continued operation in the critical range, which is marked on the tachometer dial with a red arc, can cause destructive vibration.
critical stress area (aircraft structure). An area in an aircraft structure that either has been exposed to an excess of stress or would cause the maximum amount of strength loss if it were damaged.
critical temperature (reciprocating engine fuel). The temperature inside the cylinder of a reciprocating engine that will cause the fuel to explode, rather than burn evenly when it is ignited.
critical temperature of a metal (heat treatment). The temperature at which the internal structure of a metal takes on a crystalline form.
crocus cloth. An abrasive cloth that has a fine, dark red iron oxide bonded to its surface. Crocus cloth is used for polishing hard metal surfaces.
cross (fix) at (altitude) (air traffic control). Instructions from ATC for an aircraft operating under instrument flight rules to cross a specified fix at a specified altitude. Instructions may also be given to cross the specified fix at or above a given altitude, or at or below a certain altitude.
cross-bleed (gas turbine engine air starting system). An air ducting system used in a multiengine turbojet airplane in which compressor bleed air from one engine can be directed to another engine where it can be used to operate its air starter.
cross coat (aircraft finishing system). A double coat of aircraft finishing material in which the second coat is sprayed over the first coat at right angles to it, before the solvents in the first coat have evaporated from it.
cross-country flying. Flying from one airport to another over a distance that is long enough to require the use of some form of navigation.
crosscut saw. A form of wood saw having short, sharp, knife-like teeth. A crosscut saw is used to cut wood across its grain.
cross-feed system (aircraft fuel system). An arrangement of the fuel system plumbing of a multiengine aircraft that allows any of the engines to operate from any of its fuel tanks.
The cross-feed system also allows fuel to be transferred from one side of the aircraft to the other side to maintain balance in flight.
cross-feed valve (fuel system component.) A valve in a fuel system that allows any engine of a multiengine aircraft to draw fuel from any fuel tank. Cross-feed systems are used to allow a multiengine aircraft to maintain a balanced fuel load.