Class A airspace.  Airspace from 18,000 feet MSL up to and including FL 600, including the airspace overlying the waters within 12 NM of the coast of the 48 contiguous states and Alaska; and designated international airspace beyond 12 NM of the coast of the 48 contiguous states and Alaska within areas of domestic radio navigational signal or ATC radar coverage, and within which domestic procedures are applied. 

Class B airspace.  Airspace from the surface to 10,000 feet MSL surrounding the nation’s busiest airports in terms of IFR operations or passenger numbers. The configuration of each Class B airspace is individually tailored and consists of a surface area and two or more layers, and is designed to contain all published instrument procedures once an aircraft enters the airspace. For all aircraft, an ATC clearance is required to operate in the area, and aircraft so cleared receive separation services within the airspace. 

Class C airspace.  Airspace from the surface to 4,000 feet above the airport elevation (charted in MSL) surrounding those airports having an operational control tower, serviced by radar approach control, and having a certain number of IFR operations or passenger numbers. Although the configuration of each Class C airspace area is individually tailored, the airspace usually consists of a 5 NM radius core surface area that extends from the surface up to 4,000 feet above the airport elevation, and a 10 NM radius shelf area that extends from 1,200 feet to 4,000 feet above the airport elevation. 

Class D airspace.  Airspace from the surface to 2,500 feet above the airport elevation (charted in MSL) surrounding those airports that have an operational control tower. The configuration of each Class D airspace area is individually tailored, and when instrument procedures are published, the airspace is normally designed to contain the procedures. 

Class E airspace.  Airspace that is not Class A, Class B, Class C, or Class D, and is controlled airspace. 

Class G airspace.  Airspace that is uncontrolled, except when associated with a temporary control tower, and has not been designated as Class A, Class B, Class C, Class D, or Class E airspace. 

Clean configuration.  A configuration in which all flight control surfaces have been placed to create minimum drag. In most aircraft this means flaps and gear retracted. 

Clearance.  ATC permission for an aircraft to proceed under specified traffic conditions within controlled airspace, for the purpose of providing separation between known aircraft. 

Clearance delivery.  Control tower position responsible for transmitting departure clearances to IFR flights. 

Clearance limit. The fix, point, or location to which an aircraft is cleared when issued an air traffic clearance. 

Clearance on request.  An IFR clearance not yet received after filing a flight plan. 

Clearance void time. Used by ATC, the time at which the departure clearance is automatically canceled if takeoff has not been made. The pilot must obtain a new clearance or cancel the IFR flight plan if not off by the specified time. 

Clear ice.  Glossy, clear, or translucent ice formed by the relatively slow freezing of large, supercooled water droplets. 

Coefficient of lift (C$_{L}$). The ratio between lift pressure and dynamic pressure. 

Cold front. The boundary between two air masses where cold air is replacing warm air. 

Compass course.  A true course corrected for variation and deviation errors. 

Compass locator.  A low-power, low- or medium-frequency (L/MF) radio beacon installed at the site of the outer or middle marker of an ILS. 

Compass rose.  A small circle graduated in 360° increments, to show direction expressed in degrees. 

Complex aircraft. An aircraft with retractable landing gear, flaps, and a controllable-pitch propeller. 

Compressor pressure ratio. The ratio of compressor discharge pressure to compressor inlet pressure. 

Compressor stall. In gas turbine engines, a condition in an axial-flow compressor in which one or more stages of rotor blades fail to pass air smoothly to the succeeding stages. A stall condition is caused by a pressure ratio that is incompatible with the engine rpm. Compressor stall will be indicated by a rise in exhaust temperature or rpm fluctuation, and if allowed to continue, may result in flameout and physical damage to the engine. 

Computer navigation fix.  A point used to define a navigation track for an airborne computer system such as GPS or FMS. 

