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Say "sectional" to a pilot and the first thought isn't a piece of furniture. Instead, they will immediately think of the sectional aeronautical chart used in aviation. One special group of shaded lines and circles on the sectional is especially important because each of these indicates a designated controlled airspace.


In the United States there are six distinct classes of airspace, A, B, C, D, E, and G, assigned by the Federal Aviation Administration (FAA). There is no Class F airspace in the U.S. Each airspace has its own set of rules and restrictions. These regulations are designed to reduce the chance of a mid-air collision between aircraft.


To a new aviation student, learning how to recognize the different airspaces on the sectional and the rules associated with each can seem overwhelming, but, like most complicated tasks, once you break the airspaces down into individual pieces, understanding the structure becomes easier.



Class A

The most logical starting point for studying U.S. Airspace is Class A, not because it is first alphabetically, but because it sits on top of all the other airspaces. Starting at 18,000 feet above sea level, or "MSL" (Mean Sea Level), and extending up to 60,000 feet MSL, Class A airspace is generally the domain of larger, pressurized aircraft, although there are exceptions. All flights in Class A airspace are IFR, or Instrument Flight Rules, and pilots must be instrument rated. IFR allows pilots to fly in clouds or other reduced visibility situations. In addition, the FAA requires pilots to file a flight plan before every flight in this airspace.


Pilots who are not instrument rated, or choose not to fly IFR, fly VFR, or Visual Flight Rules, which means just what it sounds like-flying by recognizing landmarks and features on the ground. When flying VFR, pilots are responsible for avoiding other aircraft by using the "see and be seen" method. Using "see and be seen" in class A airspace would not be practical. Imagine two airliners flying toward each other at 500 miles per hour. Their closing rate would be 1000 mph. By the time the pilots saw each other, it might be too late to avoid a collision. Thus, the requirement to always file a flight plan and fly IFR in Class A airspace.


Every aircraft in Class A airspace must have a working mode C transponder on board. This device provides Air Traffic Control (ATC) with information about the plane's position. With this information, ATC can assist in maintaining aircraft separation.


Since Class A airspace overlays the entire U.S., starting uniformly at 18,000 feet MSL, there isn't a need to show it on sectionals.



Class E

Class E airspace lies just below Class A airspace, usually beginning at 1,200 feet above ground level (AGL), but not always, and extending up to, but not including the floor of Class A, or 18,000 feet MSL. Some pilots think of the "E" as meaning "everywhere else," because the other types of airspace are usually confined around the location of an airport or, as in the case of Class G airspace, exist generally at low altitude.


Class E airspace is divided into two sections, one above 10,000 feet MSL, and one below it. The only differences between the two sections are the visual flight rules pilots are required to follow. When flying above 10,000 feet, visibility must be at least 5 miles, and pilots must keep a separation of 1,000 feet above and below, and 1 mile horizontally from all clouds. Below 10,000 feet, the requirements are three miles visibility, and 1,000 feet above, 500 feet below, and 2,000 feet horizontally from all clouds. Any pilot with at least a student pilot certificate can fly in Class E airspace.


On a sectional, Class E airspace is usually only indicated when it begins at an altitude other than 1,200 AGL. For example, over or near many airports Class E airspace begins at 700 feet AGL. When it does, it is indicated on the sectional by a magenta colored band that fades toward the interior of the airspace. In other cases, Class E can drop all the way to the surface, in which case, it is marked by a dashed magenta line on the sectional. The purpose of these lower altitude Class E airspaces is to help keep IFR and VFR landing and takeoff pattern traffic separated.



Class G

In the absence of any other class of airspace, Class G airspace begins at the surface and extends upward to the floor of Class E. It covers most of the surface of the U.S., and is the only airspace that's considered "uncontrolled." That doesn't mean there are no rules, though. Because there is very little air traffic at low altitudes, they are just more lax.


The rules for flying in Class G airspace below 1,200 feet AGL are fairly simple. In the daytime, visibility must be at least 1 mile, the cloud ceiling must be at least 1,000 feet AGL, and pilots must remain clear of clouds. When Class G extends higher than 1,200 feet, usually in mountainous areas, the rules are the same as in Class E with the exception of visibility, which only has to be 1 mile. At night, however, the rules for Class G require a visibility of at least 3 miles.


In some western areas of the U.S., the ceiling of Class G airspace can reach 14,500 feet MSL. Why that height? Well, the highest mountain in the contiguous U.S. is Mount Whitney at a height of 14,491 feet MSL. For pilots flying IFR or wanting to use beacon navigation systems, mountains interfere with the required radio signals. To avoid this problem, the controlled airspace is raised to above the height of the mountains that may interfere.



Class D

At an airport with an operational control tower where the amount of aviation traffic is relatively heavy but most of the traffic consists of smaller private planes, the FAA creates a Class D airspace around it. The purpose of this airspace is to control the flow of traffic into and out of the airport so that the chance of a collision is lessened.


Class D airspace is a single cylinder that starts at the surface and usually extends upward to about 2,500 feet AGL. Its diameter averages about 10 miles. On a sectional, it is shown as a blue dashed line with it's ceiling altitude shown in brackets.


To take off from or land at an airport in Class D airspace, or even fly through it, you must establish two-way radio communication with the control tower. Some airports only have an operating control tower during certain hours. When the tower is not in operation, the airspace usually reverts to Class E airspace.


The weather minimums for Class D airspace are the same as for Class E airspace below 10,000 feet MSL.



Class C

At a relatively busy airport that handles small jets as well as smaller private aircraft, Class C airspace is established. This airspace consists of two cylinders, with the first starting at the surface and extending up to 1,200 feet AGL and having a diameter of approximately 10 miles from the center of the primary airport it surrounds. The second cylinder sits directly on top of the first and extends upward to 4,000 feet AGL with a diameter of about 20 miles. On a sectional, it is indicated by two solid magenta rings, one ring for each cylinder, around the primary airport.


As with Class D airspace, flying in Class C airspace requires contacting the control tower at the primary airport. But in addition, all aircraft flying in Class C airspace, or above it to an altitude of 10,000 feet MSL, must have a working Mode C transponder.


The weather minimums for Class C airspace are the same as for Class E airspace below 10,000 feet MSL.



Class B

The best way to think of Class B airspace is that the "B" stands for busy. This is the busiest airspace and surrounds only the largest airports. Where Class C airspace consists of two cylinders with the larger sitting atop the smaller, Class B airspace consists of three cylinders with the very largest sitting on the next largest, and the smallest at the bottom. It is often described as an upside-down wedding cake. It is indicated on a sectional by solid blue lines for each of the sections or rings.


This airspace starts at the surface and usually extends up to about 10,000 feet MSL with an average diameter of the upper cylinder of 30 to 60 miles. The lower and middle rings vary in diameter, height, and shape, depending on local traffic patterns and geography. For example, the middle ring of Class B airspace at a given airport may have a floor that starts at 4,000 feet AGL on one side of the airport, and a floor that starts at 3,000 feet on the other side. These altitudes are indicated in blue on the sectional by placing the ceiling height over the floor height with a line between them with the last two digits left off.


The complexity of this airspace demands additional regulations to reduce the chance of a midair collision. One such restriction is that student and sport pilots are not allowed to fly in Class B airspace unless they have been given additional training and have a special endorsement in their logbook from an instructor. A second additional restriction is that a pilot wishing to enter this airspace must be granted a clearance from ATC before entering. This differs from Class C and Class D airspace in that with C and D, the requirement is only to establish two-way communication with ATC before entering. With Class B, the pilot must be told specifically that they are cleared to enter the airspace. A third additional restriction is that all aircraft entering this airspace must have a working Mode C transponder.


While the requirements for operating in Class B are more stringent overall, there is one rule that is less so; the VFR weather minimums. Since ATC is controlling all aircraft movement in this airspace, the requirement of other airspaces to remain a certain distance from clouds, for example, 500 below/1000 above/2000 laterally, is changed to simply that pilots must remain "clear of clouds" in Class B.


In addition to these six classes of airspace, there are many special use, restricted, and prohibited airspaces, as well as temporary flight restrictions, Air Defense Identification Zones (ADIZ), such as the one over the White House, and military operations areas that pilots need to understand. The best way to gain the required knowledge is to learn the basics first, though, and build on that foundation.



References:


Rod Machado's Private Pilot Handbook, Second Edition



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