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.  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.  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.  Clearance on request.  An IFR clearance not yet received  after filing a flight plan.  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.  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.  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.  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.  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.  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.  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.  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.  G-7 