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  Monday 22 October 2018 04:38 GMT  

Aviation Theory

Airspace Classifications

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pages: 1: Airspace types, Class A | 2: Class B to E | 3: Special Airspace Use


Within the United States, airspace is classified as either controlled or uncontrolled. Special use airspace and other airspace areas also exist as additional classifications and may include both controlled and uncontrolled segments. As pilots in command and controllers, you need to know which flight restrictions or aircraft equipment requirements are applicable in these different airspace classifications. There are naturally differences between the rules and capabilities for the simulator world and the real world. In order for the simulator experience to be as realistic as possible, certain concessions will have to be made. The following discussion is based on real world flying and the applicable FARs (Federal Aviation Regulations), also known as 14 CFR. Where possible, the same rules will be used in the simulator environment.


Controlled airspace means a section of airspace of defined dimensions within which air traffic control is provided to IFR (Instrument Flight Rules) flights and to VFR (Visual Flight Rules) flights. The level of service depends on the airspace classification. Controlled airspace is a generic term and covers Class A, Class B, Class C, Class D, and Class E airspace in the United States. Internationally, there is also Class F. When you are operating under IFR, your flight must conform with ATC (Air Traffic Control) clearances from takeoff to touchdown and your transponder must be on, including Mode C if installed. During this time, ATC provides separation between your aircraft and all other IFR flights. If workload permits, ATC also provides traffic advisories for VFR operations. When flying in the simulator environment, it is very important for pilots to remember that controllers are separating you from other traffic, even though it is not visible to you and in fact there would be no midair, other than on the radar, even if you occupy the same piece of air as another aircraft. except under multiplayer situations.

When operating in VFR conditions under an IFR flight plan, it is important for pilots to remember that ATC does not have a responsibility to separate you from VFR traffic, although in most cases, they will provide this service. Figure 1-1 shows the VFR flight requirements for the various classes of controlled airspace.

Airspace Flight Visibility Distance From Clouds
Class A N/A N/A
Class B 3 statute miles Clear of Clouds
Class C and Class D 3 statute miles 500 feet below
1,000 feet above
2,000 feet horizontal
Class E:
Less than 10,000 feet MSL


At or above 10,000 feet MSL

3 statute miles

5 statute miles

500 feet below
1,000 feet above
2,000 feet horizontal

1,000 feet below
1,000 feet above
1 statute mile horizontal

[Figure 1-1]



Within the conterminous United States, Class A airspace extends from 18,000 feet MSL Mean Sea Level) up to and including FL 600. FL stands for Flight Level and is used to indicate altitudes above 18,000' MSL. To obtain the Flight Level, the altimeter is set at the ISA (International Standard Atmoshere) altimeter setting of 29.92" Hg or 1013.2 hPa (hectoPascals, also known as millibars). The altitude read is prefaced by the letters FL and the last two zeros are omitted. For example, 25,000 feet is referenced as FL 250. Instrument high altitude enroute charts are used for flights in Class A airspace.

Because aircraft in Class A airspace operate at high speeds, it is impractical for pilots to reset their altimeters every 100 nm. By using flight levels, all pilots are maintaining their assigned altitudes using the same altimeter reference. In addition:

To fly in Class A airspace, you must adhere to the following guidelines:

  1. If acting as pilot in command, you must be rated and current for instrument flight.
  1. You must operate under an IFR flight plan and in accordance with an ATC clearance at specified flight levels.
  1. Your aircraft must be equipped with instruments and equipment required for IFR operations, including an encoding altimeter and transponder. You are also required to have a radio providing direct pilot/controller communications on the frequency specified by ATC for the area concerned. In addition, you must have navigation equipment appropriate to the ground facilities to be used.
  1. When VOR equipment is required for navigation, your aircraft must also be equipped with DME (distance measuring equipment) if the flight is conducted at or above 24,000 feet MSL. If the DME fails in flight, you must immediately notify ATC. Then, you may continue to operate at or above 24,000 feet MSL and proceed to the next airport of intended landing where repairs can be made.

To fly in controlled airspace within the contiguous United States, your aircraft must meet certain equipment requirements. For example, if your aircraft has a functioning transponder which has been properly inspected, you must operate it at all times during flight within all controlled an uncontrolled airspace. In addition, if your aircraft is equipped with a properly inspected encoding altimeter, you must operate it as well. Mode C is required in all airspace (controlled or uncontrolled) within the 48 contiguous states and the District of Columbia at and above 10,000 feet MSL, excluding the airspace at and below 2,500 feet AGL. For flights under IFR, you must operate Mode C at all times unless ATC directs otherwise. Now, let's look at some specific classes of airspace which are described in the following paragraphs and graphically illustrated in Figure 1-2.

Illustration showing a schematic overview of US airspace classification
[Figure 1-2]


... page 2: Class B to E

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