by Todd Cox


To introduce basic airspace classifications that will assist pilots while flying online VATSIM.

When we discuss airspace, there are many rules and procedures that are involved, for both the U.S. and ICAO nations. This
lesson is designed to present key concepts regarding airspace within the domestic U.S. It is presented in the very basic of terms, as the scope of airspace in general is very technical and can cause confusion. ICAO airspace concepts are presented in a separate lesson.

Guiding Documents
There are many real-world documents that cover airspace. Most of these documents can be found online, and are presented here as a guide for reading off-line. The regulatory guiding document for US Domestic airspace is Title 14, Code of Federal Regulations∞, Aeronautics and Airspace, subchapters D through G. Basically, these subchapters are the FARS that we hear about. There are many FARs associated with airspace and, for the sake of brevity, I will list some of them along with some other documents you can review at your leisure.




Section 91.126: Operating on or in the vicinity of an airport in Class G airspace.
Section 91.127: Operating on or in the vicinity of an airport in Class E airspace.
Section 91.129: Operations in Class D airspace.
Section 91.130: Operations in Class C airspace.
Section 91.131: Operations in Class B airspace.
Section 91.133: Restricted and prohibited areas.
Section 91.135: Operations in Class A airspace.

Other Documents:

Airspace 101
Without getting to in depth, in the U.S. there are two categories of airspace: regulatory and nonregulatory. Within these two categories there are four general airspace types. They are Controlled, Uncontrolled, Special Use, and Other areas. For the remainder of this lesson, we will concentrate on controlled and uncontrolled airspace. There are six classes of controlled and uncontrolled airspace, which is explained below.

Controlled Airspace
Controlled Airspace is defined as 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. 

  • Controlled airspace is a generic term that covers Class A, Class B, Class C, Class D, and Class E airspace.
  • Controlled airspace is also that airspace within which all aircraft operators are subject to certain pilot qualifications, operating rules, and equipment requirements in FAR 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.

Uncontrolled Airspace
Known as Class G airspace. Class G is airspace not designated as Class A, B, C, D, or E. Basically, NO separation services provided by ATC, and all aircraft - VFR or IFR - are required to provide their own separation.

The basic differences between airspace types are:

  1. ATC services available.
  2. Weather minima for VFR operations.
  3. Type of operations allowed.

Airspace Mnemonics
Many of us are taught mnemonics (pronounced nimonics) to memorize various items in our day-to-day activities. Here is a way to keep airspace classes straight while flying:

  • Six classes of Airspace:
    • A (Always Above and Across the US, Absolute Control)
    • B (Busiest and Biggest Airports)
    • C (Communicate, Comply, Mode-C)
    • D (Double-Duty, Designation changes)
    • E (Everything else where there is control)
    • G (Ground hopping my way!)

But what happened to Class F? Class F- exists, but is not used in the U.S.

Breaking Down the Airspace

  • 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 (NM) of the coast of the 48 contiguous States and Alaska. Unless otherwise authorized, all persons must operate their aircraft under IFR.
    What this basically means is that all operations must be conducted under instrument flight rules (IFR) and are subject to ATC clearances and instructions. ATC separation is provided to all aircraft. Why is this? Well, think for a second...where do Jet Routes start in the U.S.? They start at 18000 feet, as does our transition level, where we set out altimeters to 29.92 start. Also, Class A airspace is not specifically charted.


  • Class B - Generally, that airspace from the surface to 10,000 feet mean sea level (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, and is designed to contain all published instrument procedures.
    • 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."

Pilots operating on a IFR flight plan do not specifically need to request permission to enter Class B airspace. However, if you are flying VFR, you must request and receive permission to fly into Class B airspace. Just contacting ATC does not meet this clearance requirement. Keep in mind that IFR aircraft have priority over VFR aircraft in the airspace. Once in the airspace, VFR pilots must comply with ATC instructions. Also, there is what is known as a MODE C Veil, which is from the surface to 10,000 msl and is within 30nm of the primary Class B airport where a operating Mode C transponder (Mode C is altitude encoding) is required.

If you would like to take a look at some of the Class B airspace areas, download the Airspace Designations and Reporting Points∞ manual. 

  • 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 airspace area is individually tailored, the airspace usually consists of:
    • a surface area with a 5NM radius.
    • an outer circle with a 10 NM radius that extends from 1,200 feet to 4,000 feet above the airport elevation.
    • An Outer Area, 20 nm radius, from lower limits of radar coverage to ceiling of approach control airspace (not charted).

Pilots must establish communications with air traffic control prior to entering the airspace. Notice that this does not mean you need a specific clearance as you would with Class B. Now before you get confused, here is the difference. In Class B, establishing radio contact does not construe a specific clearance to enter Class B airspace. The controller must specifically clear you to enter the airspace, and the pilot cannot enter the Class B airspace until that permission is received. ATC can also deny permission to enter Class B airspace, based on a number of factors.

However, with Class C airspace, the pilot must establish communications with ATC prior to entering. Unless the pilot is specifically told to remain clear, the establishment of communication authorizes pilot entry into Class C airspace. Just like in Class B, VFR aircraft must comply with instructions issued by ATC and have a operating Mode C Transponder. VFR aircraft are only separated from IFR aircraft within the airspace.

  • 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.
    • Airspace reverts to Class E or G if Tower is closed.


  • Class E - Generally, that airspace that is not Class A, Class B, Class C, or Class D, and is controlled airspace. Class E airspace has many configurations and many be difficult for some to comprehend. The basic purpose of Class E airspace is to ensure that aircraft operating on IFR flight plans can remain in controlled airspace during the entire flight, and to provide controlled airspace for terminal operations where a control tower is not in operation. Class E airspace extend upward from the surface to a designated altitude; or to the adjacent or overlaying controlled airspace.

There are seven forms of Class E airspace. One of the most important forms is Federal Airways, or VICTOR Routes. VICTOR Routes extend upward from 1200 feet to, but not including 18000 feet MSL. These airways include colored airways for NDB routes and VOR airways.

  • Class G - Generally, that portion of airspace that has not been designated as Class A, Class B, Class C, Class D, or Class E airspace. No ATC Separation Services provided unless an emergency exists. Pilots are not required to file a flight plan. Now before you go, "AHA", the majority of Class G airspace in the U.S. is located below 1200 AGL. Also, there are areas where contact with ATC is mandatory. There are specific weather minima and rules that must be adhered to when flying in Class G airspace and pilots, weather VFR or IFR assume responsibility for all air traffic separation and terrain avoidance.

What if No ATC is Online?
While this doesn't happen much in the real world, in VATSIM it happens all the time that you may be approaching a Class B airport with no ATC online. You should treat your approach or take off from that airport as you would an uncontrolled airport and announce your intentions on the UNICOM channel.

This lesson covers the basic classifications of airspace used in the U.S. Specific requirements for operating IFR/VFR within these airspace areas, procedural rules, and specific pilot requirements will be covered in later lessons.