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Pilot's Handbook of Aeronautical Knowledge

Chapter 15: Airspace

The two categories of airspace are regulatory and nonregulatory.

Within these two categories, there are four types of airspace: controlled, uncontrolled, special use, and other airspace.

Controlled Airspace

Airspace profile.

Controlled airspace covers airspace within which Air Traffic Control (ATC) service is provided.

Aircraft must establish two-way radio communications with the ATC facility providing air traffic services prior to entering Class B, C, or D airspace. Aircraft must maintain radio communication with ATC while operating in the airspace.

Class A airspace

Class A airspace extends from 18,000 feet MSL up to and including Flight Level 600 (60,000 feet). This includes airspace overlying water within 12 nautical miles of the coast of the 48 contiguous states and Alaska. All flight operations in Class A airspace are conducted under Instrument Flight Rules (IFR).

Class B airspace

cClass B airspace extends from the surface to 10,000 feet MSL surrounding the nation's busiest airports. The airspace includes a central area that starts at the surface, at the location of the primary air terminal. Additional layers begin at higher altitudes with wider diameters. Each Class B airspace has customized dimensions. They often are described as resembling an "upside-down wedding cake."

Each Class B airspace is designed to is contain all published instrument procedures once an aircraft enters the airspace. ATC clearance is required for all aircraft to operate in the area, and participation in aircraft separation service is required.

Aircraft must be granted specific permission from ATC to enter Class B airspace, e.g. "Cessna 172SP, you are approved to enter Class B airspace."

Class C airspace

Class C airspace extends from the surface to 4,000 feet MSL surrounding airports with a certain number of annual IFR or passenger operations. Typically, these are large regional airports that don't meet the requirements for Class B airspace.

The airspace includes a surface area with a five NM radius. An outer circle with a ten NM radius extends from 1,200 to 4,000 feet AGL, above the airport's field elevation.

Radio contact is sufficient to enter Class C airspace. At a minimum, this requires ATC responds to an initial contact by stating the aircraft's call sign. "Cessna 172SP, go ahead" and "Cessna 172SP, stand by" are examples of radio contact.

Class D airspace

Class D airspace extends from the surface to 2,500 feet AGL, above the airport's field elevation. These are smaller airports that don't meet the requirements for Class B or C airspace. The airspace is designed to contain IFR procedures, while arrival extensions for Instrument Approach Procedures (IAPs) may be Class D or Class E airspace.

Radio contact is sufficient to enter Class D airspace.

Class E airspace

Class E airspace is all controlled airspace not classified as Class A, B, C, or D. This includes most of the airspace in the United States below 18,000 feet (Flight Level 180). Class E airspace provides sufficient airspace for the safe control and separation of aircraft during IFR operations.

In most areas, the Class E airspace base is 1,200 feet AGL. In many other areas, the Class E airspace base is either the surface or 700 feet AGL. Some Class E airspace begins at an MSL altitude depicted on the charts, instead of an AGL altitude.

All airspace above FL 600 is Class E airspace.

Uncontrolled Airspace

Class G airspace, often called "uncontrolled airspace," is the portion of the airspace that has not been designated as Class A, B, C, D, or E. Class G extends from the surface to the base of the overlying Class E airspace.

ATC has no authority or responsibility to control air traffic in Class G airspace. However, visual flight rules (VFR) minimums apply to Class G airspace.

Class G airspace cannot extend above 14,500 feet. Above this is Class E airspace, extending to 17,999 feet (unless otherwise designated).

(Note that "Continental Control Airspace" used to refer to all airspace above 14,500 MSL, which simply designated a layer of controlled airspace across the entire United States. The term "Continental Control" was abandoned when all U.S. airspace was reorganized in 1993. Unless otherwise designated, all airspace above 14,500 is Class E.)

Special Use Airspace

Special use airspace or Special Area of Operation (SAO) is airspace in which certain flight activities must be confined. It also is airspace where limitations may be imposed on aircraft that are not part of those activities.

Prohibited areas are defined areas within which the flight of aircraft is prohibited for purposes of national security or other reasons. The area is charted as a "P" followed by a number. The National Mall in Washington D.C. is an example of a prohibited area.

Restricted areas are areas where operations are hazardous to nonparticipating aircraft. Flight of non-participating aircraft is not wholly prohibited, but it is subject to restrictions. Hazards such as artillery firing, aerial gunnery, or guided missiles may exist in restricted areas. Aircraft on IFR flight plans may be permitted by ATC to enter a restricted area when it is not active.

Restricted areas are charted with an "R" followed by a number.

Warning areas are similar in nature to restricted areas. However, the United States government does not have sole jurisdiction over the airspace. Warning areas extend three (3) NM outward from the coast of the United States. A warning area may be located over domestic waters, international waters, or both.

A Military Operation Area (MOA) is airspace established for the purpose of separating military training activities from IFR traffic. When "hot," ATC reroutes or restricts nonparticipating IFR traffic. If the MOA is not in use, IFR traffic may be cleared through. MOAs are depicted on aviation charts. They do not have numbers. The back of the chart inclues times of operation, altitudes affected, and the controlling agency.

Alert areas may contain a high volume of pilot training or an unusual type of aerial activity. They are depicted on aeronautical charts with an "A" followed by a number. Pilots should exercise caution in alert areas. Pilots of participating aircraft, as well as pilots transiting the area, are equally responsible for collision avoidance.

A Controlled Flying Area (CFA) contain activities that, if not conducted in a controlled environment, could be hazardous to nonparticipating aircraft. Within a CFA, activities must be suspended when a spotter aircraft, radar, or ground lookout position indicates an aircraft might be approaching the area. CFAs are not depicted on aviation charts.

Other Airspace Areas

"Other airspace" refers to various other types of specified airspace:

Local Airport Advisory (LAA) is a service provided by a Flight Service Station (FSS) that is located at the airport. When the tower is closed, this can include local airport advisories, weather reporting, and other observations.

Military Training Routes (MTRs) are used by military aircraft to maintain proficiency in tactical flying. MTRs are usually established below 10,000 feet MSL for operations at speeds in excess of 250 knots. They are identified as IFR (IR), and VFR (VR), followed by a number.

MTRs with no segment above 1,500 feet AGL are identified by four-number characters. MTRs that include one or more segments above 1,500 feet AGL are identified by three-number characters.

IFR low altitude en route charts depict all IR routes and all VR routes that operate above 1,500 feet. VFR sectional charts depict IR and VR routes.

Temporary Flight Restrictions (TFRs) are issued for several reasons. These can include:

A flight data center (FDC) Notice to Airmen (NOTAM) is issued to designate a TFR. The NOTAM begins with the phrase "FLIGHT RESTRICTIONS" followed by the location of the temporary restriction, effective time period, area defined in statute miles, and altitudes affected. The NOTAM also contains the FAA coordination facility.

Parachute jump aircraft operations are published in the Chart Supplement U.S and depicted on aviation charts.

Published VFR routes are for transitioning around, under, or through some complex airspace. These are variously referred to as VFR flyways, VFR corridors, Class B airspace VFR transition routes, and terminal area VFR routes. They are depicted on VFR Terminal Area Planning (TAC) charts.

Terminal Radar Service Areas (TRSAs) are areas where participating pilots can receive additional radar services, which provides separation between all IFR operations and participating VFR aircraft.

The primary airport(s) within the TRSA become(s) Class D airspace. The remaining portion of the TRSA overlies other controlled airspace, which is normally Class E airspace beginning at 700 or 1,200 feet and established to transition to/ from the en route/terminal environment.

TRSAs are depicted on VFR sectional charts and TACs with a solid black line and altitudes for each segment. The Class D portion is charted with a blue segmented line. Participation in TRSA services is voluntary, but pilots operating under VFR are encouraged to contact the radar approach control and take advantage of the service.

National Security Areas (NSAs) are established at locations of increased security and safety of ground facilities. Flight in NSAs may be temporarily prohibited, which would be disseminated via NOTAM. Pilots are requested to voluntarily avoid flying through these depicted areas.

VFR Minimums

Visual Flight Rule weater minimums.

No pilot may operate an aircraft under basic VFR when the flight visibility is less, or at a distance from clouds that is less, than that prescribed for the corresponding altitude and class of airspace.

Airspace questions can seem tricky on knowledge tests, since they require memorization.

There aren't any VFR minimums in Class A airspace, because VFR traffic is not permitted.

Three (3) statute miles of visibility is the most common requirement, which applies to class B, C, D, E, and G — always, or at least at times.

The number "3-152" is a common mnemonic. In addition to three miles of visibility, aircraft must remain 1,000 above clouds, 500 feet below clouds, and 2,000 feet horizontal from clouds. This applies to class C, D, and and E airspace.

Class B has one variable: There are no cloud requirements, other than that VFR traffic must remain "clear of clouds."

Class E has one variable: Above 10,000 feet, minimum visibility increases from three (3) to five (5) miles, and the cloud-distance requirements are "111" — 1,000 feet above, 1,000 feet below, and one (1) mile horizontal. These increases account for aircraft traveling at higher speeds when at higher altitudes.

Therefore, practically speaking, "3-152" and "5-111" cover the most common types of airspace.

Class G Airspace (and its exceptions…)

Which brings us to Class G — airspace that does not have any controlling authority, but that does have VFR minimums, and it has six variables. FAA tests often include Class G questions because of this complexity.

Class G also is the only class of airspace with separate day and night VFR minimums.

The rules covering Class G minimums are more complex than the rules for all other airspaces combined. Examining specific use-cases is the best way to retain the information for testing purposes. After the test, you can always consult FAR 91.155.

Class G airspace isn't depicted on a VFR sectional the way other airspaces are. Instead, we tend to see it by knowing where it is not. This leaves us with two questions:

  • Where does Class G originate at the surface?
  • Under what circumstances does Class G terminate?

Class G starts everywhere it can start, and it terminates:

  • The moment it encounters airspace that's required for ATC services, typically, separation of IFR traffic;
  • At 1,200 feet AGL, unless designated otherwise.

Therefore, Class G can never originate at an airport where instrument approaches exist. Class B and C airspaces are easy to spot on VFR sectionals. Class D airspace is depicted with a dashed blue line, while Class E Surface Airspace is depicted with a dashed magenta line.

Per the Aeronautical Chart User's Handbook, "Class E airspace exists at 1,200 AGL unless designated otherwise." A common example of "otherwise" is class E airspace that begins at 700 AGL adjacent to airports with instrument approaches. This floor is depicted on the sectional with a magenta vignette. (Floors other than 700 AGL abutting Class G airspace have a blue vignette, but these aren't a common sight on sectionals.)

VFR Sectional KOTH.

Southwest Oregon Regional (KOTH) (Klamath Falls Sectional) offers a look at these varying airspaces. Class D originates at the surface, in the immediate vicinity of the airport. Class E Surface Airspace is established to the east and west as arrival extensions, which support instrument approaches. And Class E with a floor of 700 AGL is established in the airport's general vicinity.

Thus, Class G exists where these airspaces do not exist: below 700 AGL within the magenta vignette — but not in the airspaces that originate at the surface. External to the vignette, Class G extends vertically to 1,200 AGL.

Have a look at the triple-line over the Pacific Ocean. The serrated magenta line is the boundary of a Military Operation Area (MOA), while the colocated blue serrated line is the boundary of a Warning Area. The uneven blue line between them marks a boundary between where Class G has a ceiling of 1,200 AGL (east) and a ceiling of 5,500 MSL (west). That blue line runs the length of the west coast, which essentially declares all airspace as Class G to 1,200 AGL — unless otherwise designated. (A similar line exists on the east coast.) This is why we are most likely to see Class G airspace by seeing where it is not.

Just to the south of the controlled airspaces associated with KOTH is a smaller airport called Gederos, which is private. This is the sort of airport where Class G minimums come into effect. Gederos has a field elevation of 280 MSL, which means that Class G extends to 1,480 MSL (1,200 AGL) when calculated in the vicinity of the airport.

Rules & Exceptions

Let's return to "3-152," because it's the most common visibility and cloud-distance requirement in Class G regulations. Unless an exception is identified, Class G is 3-152.

Daylight Benefit: Only one (1) mile of visibility is required for daytime operations when operating below 10,000 MSL. At night below 10,000 MSL, Class G is 3-152.
   Mnemonic: "It's easy to see in daylight (below 10,000 feet), so only one mile of visibility is necessary."

Low-Altitude Benefit: Aircraft conducting daytime operations at or below 1,200 AGL must remain "clear of clouds," just as in Class B airspace. Again, at night below 10,000 MSL, Class G is 3-152.
   Mnemonic: "It's easy to remain clear of clouds below 1,200 AGL because there aren't many clouds there anyway on VFR days."

Traffic Pattern Benefit (Night): FAR 91.155 states that if visibility is less than three (3) statute miles but not less than one (1) statute mile during night hours, an airplane in an airport traffic pattern within 1/2 mile of the runway may operate clear of clouds. Thus, when at traffic-pattern distance and altitude, the Daylight Benefit (one mile visibility) and Low-Altitude Benefit (clear of clouds) are in effect during night operations.
   Mnemonic: "The airport's beacon at night is artificial sunlight. Within 1/2 mile of the beacon at night, I'm only required to have one (1) mile of visibility and remain clear of clouds."

Other than 1,200 feet AGL: While Class G airspace terminates at 1,200 AGL unless otherwise designated, some Class G ceilings can be higher than 1,200. In these cases, Class G conforms to Class E: It's 3-152 from 1,200 < > 10,000, and then 5-111 above 10,000. However, the Daylight Benefit is good below 10,000 MSL, so daytime operations are 1-152.

Practically speaking…

In reality, it's unlikely that you will ever operate an aircraft in Class G airspace and not use "one-and-clear" during daylight hours. Knowledge tests can include questions about Class G airspace that is more than 1,200 AGL, but in fact virtually of of this airspace is over oceanic waters. Variable Class G ceilings over land have been phased out over the past two decades.

Class G test questions also like to mix the 1,200 and 10,000 feet values to trip you up. You can get a questions on required visibility above 1,200 feet, or cloud clearance requirements below 10,000 feet. The 10,000 figure is for daylight visibility (1 mile), whereas the 1,200 figure is for clouds (remain clear).

All traffic patterns at Class G airports use the "one-and-clear" minimums for both day and night operations. This appears to apply to both piston and turbine aircraft. The regulations suggest that larger, faster aircraft can pull "one-and-clear" up to as high as 1,500 AGL.

What if the airport traffic pattern is in Class E airspace that has a 700 AGL floor? Inevitably, the airport will have Class G airspace from the surface to 700 AGL, which means the complete traffic pattern exists in two types of airspace. The regulations state that one-and-clear operations "may be conducted in Class G airspace below 1,200 feet above the surface." Thus, Class E airspace minimums override Class G traffic pattern benefits. If you come across this scenario on a test question, don't consider the airport to be a Class G field.

Not including airport traffic patterns, night cancels Class G exceptions. It's as simple as that. At night, Class G does not become Class E, but it conforms to Class E — 3-152 becoming 5-111 at 10,000 MSL.

But if 10,000 MSL is within 1,200 feet of the surface, such as in the vicinity of Lake County, Colo. (LXV, 9,934 feet), then 5-111 is deferred to 1,200 above the surface (11,134 MSL). Aircraft that are above 10,000 MSL but below 1,200 AGL, but not in a traffic pattern, must have 1-152 (day) or 3-152 (night). And aircraft within LXV's traffic pattern are required have have one mile of visibility and remain clear of clouds, day or night. (LXV is the highest airport in the conterminous United States and one of the few locations where these types of exceptions will apply.)

Commercial Pilot & Flight Instructor Test Questions

Class A

Within the contiguous United States, the floor of Class A airspace is 18,000 feet MSL.

In which type of airspace are VFR flights prohibited? Class A. (91.135)

Class B

When operating VFR in Class B airspace, what are the visibility and cloud clearance requirements? 3 SM visibility and clear of clouds. (91.155)

Which is true regarding VFR operations in Class B airspace? Solo student pilots are authorized to fly in Class B airspace if they meet certain requirements. (91.131)

Which equipment is required when operating an aircraft within Class B airspace? Two-way radio communications, a transponder with encoding altimeter, and ADS-B Out equipment. (91.131)

The minimum avionics equipment necessary to operate in Class B airspace is two-way radio communications equipment, transponder, and encoding altimeter.

When flying beneath the lateral limits of Class B airspace, the maximum indicated airspeed authorized is 200 knots.
— No person may operate an aircraft in the airspace underlying a Class B airspace area designated for an airport, or in a VFR corridor designated through such a Class B airspace area, at an indicated airspeed of greater than 200 knots. (230 MPH).

Unless otherwise authorized, what is the maximum indicated airspeed at which an aircraft may be flown in a satellite airport traffic pattern located within Class B airspace? 250 knots. (91.117)
— Unless otherwise authorized, no person may operate an aircraft within Class B airspace at an indicated airspeed of more than 250 kts.

While on a flight, you contact Approach Control and request clearance through the Class B airspace. The controller states "Radar contact, standby." What are you authorized to do? You may not enter the airspace until you have received authorization from ATC.

An airport without a control tower lies within the controlled airspace of an airport with an operating tower. According to regulations, two-way radio communications with ATC are required for landing clearance at the tower-controlled airport only, as well as to fly through the area. (91.129)
— This applies to Class B, C, and D airspace.

Class C

To operate an aircraft within Class C airspace from a satellite airport without an operating control tower, a pilot must contact ATC as soon as practicable after takeoff..

What minimum avionics equipment is required for operation within Class C airspace? Two-way communications and transponder with ADS-B.
— The original version of this answer is "transponder with automatic altitude reporting capability."

All operations within Class C airspace must be in an aircraft equipped with a transponder with ADS-B and in communication with the responsible ATC facility.
— The original version of this answer is "transponder with automatic altitude reporting capability."

What are the requirements, if any, to overfly Class C airspace? Transponder with ADS-B is required above the airspace ceiling and upward to 10,000 feet. (91.215)
— The original version of this answer is "transponder with automatic altitude reporting capability."

Which is true regarding flight operations to or from a satellite airport, without an operating control tower, within the Class C airspace area? Prior to entering that airspace, a pilot must establish and maintain communication with the ATC serving facility.
— The question refers to the Class C airspace area (30 nm), not the airspace itself (20 nm).

How long before the proposed operation should a request be submitted to the controlling ATC facility to operate in Class C airspace without the required altitude reporting transponder? 1 hour.

Class D

The vertical limit of Class D airspace will normally be designated up to, and including, 2,500 feet AGL.

When a control tower located on an airport within Class D airspace ceases operation for the day, waht happens to the airspace designation? The airspace reverts to Class E or a combination of Class E and G airspace during the hours the tower is not in operation.

When operating an airplane within Class D airspace under special VFR, the flight visibility required is 1 SM. (91.157)

No person may operate an airplane within Class D and E airspace between sunset and sunrise under special VFR unless the airplane is equipped for instrument flight. (91.157)
— The pilot also must meet the legal requirements to operate in IMC (rating, currency).

When operating an airplane for the purpose of takeoff or landing within Class D airspace under special VFR, what minimum distance from clouds and what visibility are required? Remain clear of clouds, and the ground visibility must be at least 1 SM.
— The regulations refer to ground visibility, as taken from an ATIS/ASOS, and not flight visibility.

Class E

The minimum visibility for VFR flight in Class E airspace increases from 3 to 5 SM beginning at an altitude of 10,000 feet. (91.155)

With certain exceptions, Class E airspace extends upward from either 700 feet or 1,200 feet AGL to, but does not include 18,000 feet.
— Not to be confused with Class G airspace, which terminates at 14,500 feet, becoming Class E.

Class E airspace within the contiguous United States extends upward from either 700 feet or 1,200 feet AGL to, but not including, the base of the overlying controlled airspace..
— Class E must terminate at 18,000 feet (Class A airspace), but it also must terminate if it reaches overlying Class B or Class C airspace.

While in Class E airspace in VFR conditions, what in-flight visibility is required when flying more than 1,200 feet AGL and at or above 10,000 feet MSL? 5 SM. (91.155)

Regulations state that, at an airport located within Class E airspace and at which ground visibility is not reported, takeoffs and landings of airplanes under special VFR are authorized if the flight visibility is at least 1 SM. (91.157)

Regulations stipulate that, at an airport located within Class E airspace and at which ground visibility is not reported, takeoffs and landings of airplanes under special VFR are authorized if the flight visibility is at least 1 SM.

During operations within controlled airspace at altitudes of more than 1,200 feet AGL, but less than 10,000 feet MSL, the minimum horizontal distance from clouds requirement for VFR flight is 2,000 feet. (91.155)

In the contiguous U.S., excluding the airspace at and below 2,500 feet AGL, an operable coded transponder equipped with Mode C capability is required in all airspace above 10,000 feet MSL.

Class G

While in Class G airspace under day VFR conditions, what in-flight visibility is required when flying more than 1,200 feet AGL and less than 10,000 feet MSL? 1 SM. (91.155)

While in Class G airspace in VFR conditions, what minimum distance from clouds should be maintained when flying more than 1,200 feet AGL, at or above 10,000 MSL? 1,000 feet below; 1,000 feet above; 1 mile horizontal. (91.155)
— True for both Class E and Class G airspace within these parameters.

An airplane may be operated in uncontrolled airspace at night below 1,200 feet above the surface under the following conditions: Less than 3 miles but more than 1 mile visibility in an airport traffic pattern and within one-half mile of the runway. (91.155)

Special Use Airspace (SUA)

Flight through a restricted area should not be accomplished unless the pilot has received prior authorization from the controlling agency.

A warning area is airspace of defined dimensions established over either domestic or international waters for the purpose of separating military from civilian aircraft.

Flight through a Military Operations Area (MOA) is permitted anytime, but caution should be exercised because of military activity.

A Military Operations Area (MOA) is airspace of defined vertical and lateral limits established for the purpose of separating certain military training activities from IFR traffic.
— The distractor "military services conducting VFR low altitude navigation, tactical training, and flight testing" appears valid, but this may reference a Military Training Route (MTR). Any use of weapons in the area would make the airspace Restricted or Warning, not an MOA.

When operating VFR in a Military Operations Area (MOA), a pilot should exercise extreme caution when military activity is being conducted.
— There is no restriction for VFR flight within a MOA. A clearance is not required to enter a MOA. MOAs are established to coordinate military and IFR traffic.

If a military training route has flights operating at or below 1,500 feet AGL, it will be designated by VR or IR and a four-digit number.
— Higher-altitude MTRs use three-digit numbers.

Robert Wederquist   CP-ASEL - AGI - IGI
Commercial Pilot • Instrument Pilot
Advanced Ground Instructor • Instrument Ground Instructor

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