3/21/24 

Pilot/Controller Glossary 

controller workload. Pilot communications during operations within a sector are normally maintained on 

discrete frequencies assigned to the sector. 

(See DISCRETE FREQUENCY.) 

CONTROL SLASH

−

 A radar beacon slash representing the actual position of the associated aircraft. Normally, 

the control slash is the one closest to the interrogating radar beacon site. When ARTCC radar is operating in 

narrowband (digitized) mode, the control slash is converted to a target symbol. 
CONTROLLED AIRSPACE

−

 An airspace of defined dimensions within which air traffic control service is 

provided to IFR flights and to VFR flights in accordance with the airspace classification. 

a. 

Controlled airspace is a generic term that covers Class A, Class B, Class C, Class D, and Class E airspace. 

b. 

Controlled airspace is also that airspace within which all aircraft operators are subject to certain pilot 

qualifications,  operating rules, and equipment requirements  in 14 CFR  Part 91 (for  specific operating 

requirements, please refer to 14 CFR Part 91). For IFR operations in any class of controlled airspace, a pilot must 

file an IFR flight plan and receive an appropriate ATC clearance. Each Class B, Class C, and Class D airspace 

area designated for an airport contains at least one primary airport around which the airspace is designated (for 

specific designations and descriptions of the airspace classes, please refer to 14 CFR Part 71). 

c. 

Controlled airspace in the United States is designated as follows: 

1. 

CLASS A

−

 

Generally, that airspace from 18,000 feet MSL up to and including FL 600, including the 

airspace overlying the waters within 12 nautical miles of the coast of the 48 contiguous States and Alaska. Unless 

otherwise authorized, all persons must operate their aircraft under IFR. 

2. 

CLASS B

−

 Generally, that airspace from the surface to 10,000 feet MSL surrounding the nation’s busiest 

airports in terms of airport operations or passenger enplanements. The configuration of each Class B airspace 

area is individually tailored and consists of a surface area and two or more layers (some Class B airspace areas 

resemble upside-down wedding cakes), and is designed to contain all published instrument procedures once an 

aircraft enters the airspace. An ATC clearance is required for all aircraft to operate in the area, and all aircraft 

that are so cleared receive separation services within the airspace. The cloud clearance requirement for VFR 

operations is “clear of clouds.” 

3. 

CLASS C

−

 Generally, that airspace from the surface to 4,000 feet above the airport elevation (charted 

in MSL) surrounding those airports that have an operational control tower, are serviced by a radar approach 

control, and that have a certain number of IFR operations or passenger enplanements. Although the configuration 

of each Class C area is individually tailored, the airspace usually consists of a surface area with a 5 NM radius, 

a circle with a 10 NM radius that extends no lower than 1,200 feet up to 4,000 feet above the airport elevation, 

and an outer area that is not charted. Each person must establish two-way radio communications with the ATC 

facility providing air traffic services prior to entering the airspace and thereafter maintain those communications 

while within the airspace. VFR aircraft are only separated from IFR aircraft within the airspace. 

(See OUTER AREA.) 

4. 

CLASS D

−

 Generally, that 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 will normally 

be designed to contain the procedures. Arrival extensions for instrument approach procedures may be Class D 

or Class E airspace. Unless otherwise authorized, each person must establish two-way radio communications 

with the ATC facility providing air traffic services prior to entering the airspace and thereafter maintain those 

communications while in the airspace. No separation services are provided to VFR aircraft. 

5. 

CLASS E

−

 

Generally, if the airspace is not Class A, Class B, Class C, or Class D, and it is controlled 

airspace, it is Class E airspace. Class E airspace extends upward from either the surface or a designated altitude 

to the overlying or adjacent controlled airspace. When designated as a surface area, the airspace will be 

configured to contain all instrument procedures. Also in this class are Federal airways, airspace beginning at 

either 700 or 1,200 feet AGL used to transition to/from the terminal or en route environment, en route domestic, 

and offshore airspace areas designated below 18,000 feet MSL. Unless designated at a lower altitude, Class E 

airspace begins at 14,500 MSL over the United States, including that airspace overlying the waters within 12 

PCG C

−

9